-<!--
-$PostgreSQL: pgsql/doc/src/sgml/xfunc.sgml,v 1.104 2005/09/14 21:14:26 neilc Exp $
--->
+<!-- doc/src/sgml/xfunc.sgml -->
<sect1 id="xfunc">
- <title>User-Defined Functions</title>
+ <title>User-defined Functions</title>
<indexterm zone="xfunc">
<primary>function</primary>
<listitem>
<para>
procedural language functions (functions written in, for
- example, <application>PL/pgSQL</> or <application>PL/Tcl</>)
+ example, <application>PL/pgSQL</application> or <application>PL/Tcl</application>)
(<xref linkend="xfunc-pl">)
</para>
</listitem>
of function can take base types, composite types, or
combinations of these as arguments (parameters). In addition,
every kind of function can return a base type or
- a composite type. Functions may also be defined to return
+ a composite type. Functions can also be defined to return
sets of base or composite values.
</para>
<para>
Throughout this chapter, it can be useful to look at the reference
- page of the <xref linkend="sql-createfunction"
- endterm="sql-createfunction-title"> command to
+ page of the <xref linkend="sql-createfunction"> command to
understand the examples better. Some examples from this chapter
can be found in <filename>funcs.sql</filename> and
- <filename>funcs.c</filename> in the <filename>src/tutorial</>
+ <filename>funcs.c</filename> in the <filename>src/tutorial</filename>
directory in the <productname>PostgreSQL</productname> source
distribution.
</para>
In the simple (non-set)
case, the first row of the last query's result will be returned.
(Bear in mind that <quote>the first row</quote> of a multirow
- result is not well-defined unless you use <literal>ORDER BY</>.)
+ result is not well-defined unless you use <literal>ORDER BY</literal>.)
If the last query happens
to return no rows at all, the null value will be returned.
</para>
<para>
- <indexterm><primary>SETOF</><seealso>function</></> Alternatively,
- an SQL function may be declared to return a set, by specifying the
- function's return type as <literal>SETOF
- <replaceable>sometype</></literal>. In this case all rows of the
- last query's result are returned. Further details appear below.
+ Alternatively, an SQL function can be declared to return a set (that is,
+ multiple rows) by specifying the function's return type as <literal>SETOF
+ <replaceable>sometype</replaceable></literal>, or equivalently by declaring it as
+ <literal>RETURNS TABLE(<replaceable>columns</replaceable>)</literal>. In this case
+ all rows of the last query's result are returned. Further details appear
+ below.
</para>
<para>
The body of an SQL function must be a list of SQL
statements separated by semicolons. A semicolon after the last
statement is optional. Unless the function is declared to return
- <type>void</>, the last statement must be a <command>SELECT</>.
+ <type>void</type>, the last statement must be a <command>SELECT</command>,
+ or an <command>INSERT</command>, <command>UPDATE</command>, or <command>DELETE</command>
+ that has a <literal>RETURNING</literal> clause.
</para>
<para>
Besides <command>SELECT</command> queries, the commands can include data
modification queries (<command>INSERT</command>,
<command>UPDATE</command>, and <command>DELETE</command>), as well as
- other SQL commands. (The only exception is that you can't put
- <command>BEGIN</>, <command>COMMIT</>, <command>ROLLBACK</>, or
- <command>SAVEPOINT</> commands into a <acronym>SQL</acronym> function.)
- However, the final command
- must be a <command>SELECT</command> that returns whatever is
+ other SQL commands. (You cannot use transaction control commands, e.g.
+ <command>COMMIT</command>, <command>SAVEPOINT</command>, and some utility
+ commands, e.g. <literal>VACUUM</literal>, in <acronym>SQL</acronym> functions.)
+ However, the final command
+ must be a <command>SELECT</command> or have a <literal>RETURNING</literal>
+ clause that returns whatever is
specified as the function's return type. Alternatively, if you
want to define a SQL function that performs actions but has no
- useful value to return, you can define it as returning <type>void</>.
- In that case, the function body must not end with a <command>SELECT</command>.
+ useful value to return, you can define it as returning <type>void</type>.
For example, this function removes rows with negative salaries from
- the <literal>emp</> table:
+ the <literal>emp</literal> table:
<screen>
CREATE FUNCTION clean_emp() RETURNS void AS '
</screen>
</para>
+ <note>
+ <para>
+ The entire body of a SQL function is parsed before any of it is
+ executed. While a SQL function can contain commands that alter
+ the system catalogs (e.g., <command>CREATE TABLE</command>), the effects
+ of such commands will not be visible during parse analysis of
+ later commands in the function. Thus, for example,
+ <literal>CREATE TABLE foo (...); INSERT INTO foo VALUES(...);</literal>
+ will not work as desired if packaged up into a single SQL function,
+ since <structname>foo</structname> won't exist yet when the <command>INSERT</command>
+ command is parsed. It's recommended to use <application>PL/pgSQL</application>
+ instead of a SQL function in this type of situation.
+ </para>
+ </note>
+
<para>
The syntax of the <command>CREATE FUNCTION</command> command requires
the function body to be written as a string constant. It is usually
most convenient to use dollar quoting (see <xref
linkend="sql-syntax-dollar-quoting">) for the string constant.
If you choose to use regular single-quoted string constant syntax,
- you must escape single quote marks (<literal>'</>) and backslashes
- (<literal>\</>) used in the body of the function, typically by
- doubling them (see <xref linkend="sql-syntax-strings">).
+ you must double single quote marks (<literal>'</literal>) and backslashes
+ (<literal>\</literal>) (assuming escape string syntax) in the body of
+ the function (see <xref linkend="sql-syntax-strings">).
</para>
- <para>
- Arguments to the SQL function are referenced in the function
- body using the syntax <literal>$<replaceable>n</></>: <literal>$1</>
- refers to the first argument, <literal>$2</> to the second, and so on.
- If an argument is of a composite type, then the dot notation,
- e.g., <literal>$1.name</literal>, may be used to access attributes
- of the argument. The arguments can only be used as data values,
- not as identifiers. Thus for example this is reasonable:
+ <sect2 id="xfunc-sql-function-arguments">
+ <title>Arguments for <acronym>SQL</acronym> Functions</title>
+
+ <indexterm>
+ <primary>function</primary>
+ <secondary>named argument</secondary>
+ </indexterm>
+
+ <para>
+ Arguments of a SQL function can be referenced in the function
+ body using either names or numbers. Examples of both methods appear
+ below.
+ </para>
+
+ <para>
+ To use a name, declare the function argument as having a name, and
+ then just write that name in the function body. If the argument name
+ is the same as any column name in the current SQL command within the
+ function, the column name will take precedence. To override this,
+ qualify the argument name with the name of the function itself, that is
+ <literal><replaceable>function_name</replaceable>.<replaceable>argument_name</replaceable></literal>.
+ (If this would conflict with a qualified column name, again the column
+ name wins. You can avoid the ambiguity by choosing a different alias for
+ the table within the SQL command.)
+ </para>
+
+ <para>
+ In the older numeric approach, arguments are referenced using the syntax
+ <literal>$<replaceable>n</replaceable></literal>: <literal>$1</literal> refers to the first input
+ argument, <literal>$2</literal> to the second, and so on. This will work
+ whether or not the particular argument was declared with a name.
+ </para>
+
+ <para>
+ If an argument is of a composite type, then the dot notation,
+ e.g., <literal><replaceable>argname</replaceable>.<replaceable>fieldname</replaceable></literal> or
+ <literal>$1.<replaceable>fieldname</replaceable></literal>, can be used to access attributes of the
+ argument. Again, you might need to qualify the argument's name with the
+ function name to make the form with an argument name unambiguous.
+ </para>
+
+ <para>
+ SQL function arguments can only be used as data values,
+ not as identifiers. Thus for example this is reasonable:
<programlisting>
INSERT INTO mytable VALUES ($1);
</programlisting>
<programlisting>
INSERT INTO $1 VALUES (42);
</programlisting>
- </para>
+ </para>
+
+ <note>
+ <para>
+ The ability to use names to reference SQL function arguments was added
+ in <productname>PostgreSQL</productname> 9.2. Functions to be used in
+ older servers must use the <literal>$<replaceable>n</replaceable></literal> notation.
+ </para>
+ </note>
+ </sect2>
<sect2 id="xfunc-sql-base-functions">
<title><acronym>SQL</acronym> Functions on Base Types</title>
<para>
The simplest possible <acronym>SQL</acronym> function has no arguments and
simply returns a base type, such as <type>integer</type>:
-
+
<screen>
CREATE FUNCTION one() RETURNS integer AS $$
SELECT 1 AS result;
<para>
Notice that we defined a column alias within the function body for the result of the function
- (with the name <literal>result</>), but this column alias is not visible
- outside the function. Hence, the result is labeled <literal>one</>
- instead of <literal>result</>.
+ (with the name <literal>result</literal>), but this column alias is not visible
+ outside the function. Hence, the result is labeled <literal>one</literal>
+ instead of <literal>result</literal>.
</para>
<para>
- It is almost as easy to define <acronym>SQL</acronym> functions
- that take base types as arguments. In the example below, notice
- how we refer to the arguments within the function as <literal>$1</>
- and <literal>$2</>.
+ It is almost as easy to define <acronym>SQL</acronym> functions
+ that take base types as arguments:
+
+<screen>
+CREATE FUNCTION add_em(x integer, y integer) RETURNS integer AS $$
+ SELECT x + y;
+$$ LANGUAGE SQL;
+
+SELECT add_em(1, 2) AS answer;
+
+ answer
+--------
+ 3
+</screen>
+ </para>
+
+ <para>
+ Alternatively, we could dispense with names for the arguments and
+ use numbers:
<screen>
CREATE FUNCTION add_em(integer, integer) RETURNS integer AS $$
bank account:
<programlisting>
-CREATE FUNCTION tf1 (integer, numeric) RETURNS integer AS $$
- UPDATE bank
- SET balance = balance - $2
- WHERE accountno = $1;
+CREATE FUNCTION tf1 (accountno integer, debit numeric) RETURNS numeric AS $$
+ UPDATE bank
+ SET balance = balance - debit
+ WHERE accountno = tf1.accountno;
SELECT 1;
$$ LANGUAGE SQL;
</programlisting>
</programlisting>
</para>
+ <para>
+ In this example, we chose the name <literal>accountno</literal> for the first
+ argument, but this is the same as the name of a column in the
+ <literal>bank</literal> table. Within the <command>UPDATE</command> command,
+ <literal>accountno</literal> refers to the column <literal>bank.accountno</literal>,
+ so <literal>tf1.accountno</literal> must be used to refer to the argument.
+ We could of course avoid this by using a different name for the argument.
+ </para>
+
<para>
In practice one would probably like a more useful result from the
function than a constant 1, so a more likely definition
- is
+ is:
<programlisting>
-CREATE FUNCTION tf1 (integer, numeric) RETURNS numeric AS $$
- UPDATE bank
- SET balance = balance - $2
- WHERE accountno = $1;
- SELECT balance FROM bank WHERE accountno = $1;
+CREATE FUNCTION tf1 (accountno integer, debit numeric) RETURNS numeric AS $$
+ UPDATE bank
+ SET balance = balance - debit
+ WHERE accountno = tf1.accountno;
+ SELECT balance FROM bank WHERE accountno = tf1.accountno;
$$ LANGUAGE SQL;
</programlisting>
which adjusts the balance and returns the new balance.
+ The same thing could be done in one command using <literal>RETURNING</literal>:
+
+<programlisting>
+CREATE FUNCTION tf1 (accountno integer, debit numeric) RETURNS numeric AS $$
+ UPDATE bank
+ SET balance = balance - debit
+ WHERE accountno = tf1.accountno
+ RETURNING balance;
+$$ LANGUAGE SQL;
+</programlisting>
+ </para>
+
+ <para>
+ A <acronym>SQL</acronym> function must return exactly its declared
+ result type. This may require inserting an explicit cast.
+ For example, suppose we wanted the
+ previous <function>add_em</function> function to return
+ type <type>float8</type> instead. This won't work:
+
+<programlisting>
+CREATE FUNCTION add_em(integer, integer) RETURNS float8 AS $$
+ SELECT $1 + $2;
+$$ LANGUAGE SQL;
+</programlisting>
+
+ even though in other contexts <productname>PostgreSQL</productname>
+ would be willing to insert an implicit cast to
+ convert <type>integer</type> to <type>float8</type>.
+ We need to write it as
+
+<programlisting>
+CREATE FUNCTION add_em(integer, integer) RETURNS float8 AS $$
+ SELECT ($1 + $2)::float8;
+$$ LANGUAGE SQL;
+</programlisting>
</para>
</sect2>
- <sect2>
+ <sect2 id="xfunc-sql-composite-functions">
<title><acronym>SQL</acronym> Functions on Composite Types</title>
<para>
- When writing functions with arguments of composite
- types, we must not only specify which
- argument we want (as we did above with <literal>$1</> and <literal>$2</literal>) but
- also the desired attribute (field) of that argument. For example,
- suppose that
+ When writing functions with arguments of composite types, we must not
+ only specify which argument we want but also the desired attribute
+ (field) of that argument. For example, suppose that
<type>emp</type> is a table containing employee data, and therefore
also the name of the composite type of each row of the table. Here
is a function <function>double_salary</function> that computes what someone's
cubicle point
);
+INSERT INTO emp VALUES ('Bill', 4200, 45, '(2,1)');
+
CREATE FUNCTION double_salary(emp) RETURNS numeric AS $$
SELECT $1.salary * 2 AS salary;
$$ LANGUAGE SQL;
<para>
Notice the use of the syntax <literal>$1.salary</literal>
to select one field of the argument row value. Also notice
- how the calling <command>SELECT</> command uses <literal>*</>
- to select
+ how the calling <command>SELECT</command> command
+ uses <replaceable>table_name</replaceable><literal>.*</literal> to select
the entire current row of a table as a composite value. The table
row can alternatively be referenced using just the table name,
like this:
WHERE emp.cubicle ~= point '(2,1)';
</screen>
but this usage is deprecated since it's easy to get confused.
+ (See <xref linkend="rowtypes-usage"> for details about these
+ two notations for the composite value of a table row.)
</para>
<para>
Sometimes it is handy to construct a composite argument value
- on-the-fly. This can be done with the <literal>ROW</> construct.
+ on-the-fly. This can be done with the <literal>ROW</literal> construct.
For example, we could adjust the data being passed to the function:
<screen>
SELECT name, double_salary(ROW(name, salary*1.1, age, cubicle)) AS dream
<para>
It is also possible to build a function that returns a composite type.
- This is an example of a function
+ This is an example of a function
that returns a single <type>emp</type> row:
<programlisting>
</listitem>
<listitem>
<para>
- You must typecast the expressions to match the
- definition of the composite type, or you will get errors like this:
+ We must ensure each expression's type matches the corresponding
+ column of the composite type, inserting a cast if necessary.
+ Otherwise we'll get errors like this:
<screen>
<computeroutput>
ERROR: function declared to return emp returns varchar instead of text at column 1
</computeroutput>
</screen>
+ As with the base-type case, the function will not insert any casts
+ automatically.
</para>
</listitem>
</itemizedlist>
- </para>
+ </para>
<para>
A different way to define the same function is:
$$ LANGUAGE SQL;
</programlisting>
- Here we wrote a <command>SELECT</> that returns just a single
+ Here we wrote a <command>SELECT</command> that returns just a single
column of the correct composite type. This isn't really better
in this situation, but it is a handy alternative in some cases
— for example, if we need to compute the result by calling
another function that returns the desired composite value.
- </para>
+ Another example is that if we are trying to write a function that
+ returns a domain over composite, rather than a plain composite type,
+ it is always necessary to write it as returning a single column,
+ since there is no other way to produce a value that is exactly of
+ the domain type.
+ </para>
<para>
- We could call this function directly in either of two ways:
+ We could call this function directly either by using it in
+ a value expression:
<screen>
SELECT new_emp();
new_emp
--------------------------
(None,1000.0,25,"(2,2)")
+</screen>
+
+ or by calling it as a table function:
+<screen>
SELECT * FROM new_emp();
name | salary | age | cubicle
The second way is described more fully in <xref
linkend="xfunc-sql-table-functions">.
- </para>
+ </para>
<para>
When you use a function that returns a composite type,
<screen>
SELECT new_emp().name;
-ERROR: syntax error at or near "." at character 17
+ERROR: syntax error at or near "."
LINE 1: SELECT new_emp().name;
^
</screen>
</para>
<para>
- Another option is to use
- functional notation for extracting an attribute. The simple way
- to explain this is that we can use the
- notations <literal>attribute(table)</> and <literal>table.attribute</>
- interchangeably.
+ Another option is to use functional notation for extracting an attribute:
<screen>
SELECT name(new_emp());
None
</screen>
-<screen>
--- This is the same as:
--- SELECT emp.name AS youngster FROM emp WHERE emp.age < 30;
-
-SELECT name(emp) AS youngster FROM emp WHERE age(emp) < 30;
-
- youngster
------------
- Sam
- Andy
-</screen>
+ As explained in <xref linkend="rowtypes-usage">, the field notation and
+ functional notation are equivalent.
</para>
- <tip>
- <para>
- The equivalence between functional notation and attribute notation
- makes it possible to use functions on composite types to emulate
- <quote>computed fields</>.
- <indexterm>
- <primary>computed field</primary>
- </indexterm>
- <indexterm>
- <primary>field</primary>
- <secondary>computed</secondary>
- </indexterm>
- For example, using the previous definition
- for <literal>double_salary(emp)</>, we can write
-
-<screen>
-SELECT emp.name, emp.double_salary FROM emp;
-</screen>
-
- An application using this wouldn't need to be directly aware that
- <literal>double_salary</> isn't a real column of the table.
- (You can also emulate computed fields with views.)
- </para>
- </tip>
-
<para>
Another way to use a function returning a composite type is to pass the
result to another function that accepts the correct row type as input:
None
(1 row)
</screen>
- </para>
-
- <para>
- Still another way to use a function that returns a composite type is to
- call it as a table function, as described in <xref
- linkend="xfunc-sql-table-functions">.
</para>
</sect2>
<sect2 id="xfunc-output-parameters">
- <title>Functions with Output Parameters</title>
+ <title><acronym>SQL</acronym> Functions with Output Parameters</title>
<indexterm>
<primary>function</primary>
<para>
An alternative way of describing a function's results is to define it
- with <firstterm>output parameters</>, as in this example:
+ with <firstterm>output parameters</firstterm>, as in this example:
<screen>
CREATE FUNCTION add_em (IN x int, IN y int, OUT sum int)
-AS 'SELECT $1 + $2'
+AS 'SELECT x + y'
LANGUAGE SQL;
SELECT add_em(3,7);
(1 row)
</screen>
- This is not essentially different from the version of <literal>add_em</>
+ This is not essentially different from the version of <literal>add_em</literal>
shown in <xref linkend="xfunc-sql-base-functions">. The real value of
output parameters is that they provide a convenient way of defining
functions that return several columns. For example,
<screen>
CREATE FUNCTION sum_n_product (x int, y int, OUT sum int, OUT product int)
-AS 'SELECT $1 + $2, $1 * $2'
+AS 'SELECT x + y, x * y'
LANGUAGE SQL;
SELECT * FROM sum_n_product(11,42);
</screen>
but not having to bother with the separate composite type definition
- is often handy.
+ is often handy. Notice that the names attached to the output parameters
+ are not just decoration, but determine the column names of the anonymous
+ composite type. (If you omit a name for an output parameter, the
+ system will choose a name on its own.)
</para>
<para>
</para>
<para>
- Parameters can be marked as <literal>IN</> (the default),
- <literal>OUT</>, or <literal>INOUT</>. An <literal>INOUT</>
+ Parameters can be marked as <literal>IN</literal> (the default),
+ <literal>OUT</literal>, <literal>INOUT</literal>, or <literal>VARIADIC</literal>.
+ An <literal>INOUT</literal>
parameter serves as both an input parameter (part of the calling
argument list) and an output parameter (part of the result record type).
+ <literal>VARIADIC</literal> parameters are input parameters, but are treated
+ specially as described next.
+ </para>
+ </sect2>
+
+ <sect2 id="xfunc-sql-variadic-functions">
+ <title><acronym>SQL</acronym> Functions with Variable Numbers of Arguments</title>
+
+ <indexterm>
+ <primary>function</primary>
+ <secondary>variadic</secondary>
+ </indexterm>
+
+ <indexterm>
+ <primary>variadic function</primary>
+ </indexterm>
+
+ <para>
+ <acronym>SQL</acronym> functions can be declared to accept
+ variable numbers of arguments, so long as all the <quote>optional</quote>
+ arguments are of the same data type. The optional arguments will be
+ passed to the function as an array. The function is declared by
+ marking the last parameter as <literal>VARIADIC</literal>; this parameter
+ must be declared as being of an array type. For example:
+
+<screen>
+CREATE FUNCTION mleast(VARIADIC arr numeric[]) RETURNS numeric AS $$
+ SELECT min($1[i]) FROM generate_subscripts($1, 1) g(i);
+$$ LANGUAGE SQL;
+
+SELECT mleast(10, -1, 5, 4.4);
+ mleast
+--------
+ -1
+(1 row)
+</screen>
+
+ Effectively, all the actual arguments at or beyond the
+ <literal>VARIADIC</literal> position are gathered up into a one-dimensional
+ array, as if you had written
+
+<screen>
+SELECT mleast(ARRAY[10, -1, 5, 4.4]); -- doesn't work
+</screen>
+
+ You can't actually write that, though — or at least, it will
+ not match this function definition. A parameter marked
+ <literal>VARIADIC</literal> matches one or more occurrences of its element
+ type, not of its own type.
+ </para>
+
+ <para>
+ Sometimes it is useful to be able to pass an already-constructed array
+ to a variadic function; this is particularly handy when one variadic
+ function wants to pass on its array parameter to another one. You can
+ do that by specifying <literal>VARIADIC</literal> in the call:
+
+<screen>
+SELECT mleast(VARIADIC ARRAY[10, -1, 5, 4.4]);
+</screen>
+
+ This prevents expansion of the function's variadic parameter into its
+ element type, thereby allowing the array argument value to match
+ normally. <literal>VARIADIC</literal> can only be attached to the last
+ actual argument of a function call.
+ </para>
+
+ <para>
+ Specifying <literal>VARIADIC</literal> in the call is also the only way to
+ pass an empty array to a variadic function, for example:
+
+<screen>
+SELECT mleast(VARIADIC ARRAY[]::numeric[]);
+</screen>
+
+ Simply writing <literal>SELECT mleast()</literal> does not work because a
+ variadic parameter must match at least one actual argument.
+ (You could define a second function also named <literal>mleast</literal>,
+ with no parameters, if you wanted to allow such calls.)
+ </para>
+
+ <para>
+ The array element parameters generated from a variadic parameter are
+ treated as not having any names of their own. This means it is not
+ possible to call a variadic function using named arguments (<xref
+ linkend="sql-syntax-calling-funcs">), except when you specify
+ <literal>VARIADIC</literal>. For example, this will work:
+
+<screen>
+SELECT mleast(VARIADIC arr => ARRAY[10, -1, 5, 4.4]);
+</screen>
+
+ but not these:
+
+<screen>
+SELECT mleast(arr => 10);
+SELECT mleast(arr => ARRAY[10, -1, 5, 4.4]);
+</screen>
+ </para>
+ </sect2>
+
+ <sect2 id="xfunc-sql-parameter-defaults">
+ <title><acronym>SQL</acronym> Functions with Default Values for Arguments</title>
+
+ <indexterm>
+ <primary>function</primary>
+ <secondary>default values for arguments</secondary>
+ </indexterm>
+
+ <para>
+ Functions can be declared with default values for some or all input
+ arguments. The default values are inserted whenever the function is
+ called with insufficiently many actual arguments. Since arguments
+ can only be omitted from the end of the actual argument list, all
+ parameters after a parameter with a default value have to have
+ default values as well. (Although the use of named argument notation
+ could allow this restriction to be relaxed, it's still enforced so that
+ positional argument notation works sensibly.)
+ </para>
+
+ <para>
+ For example:
+<screen>
+CREATE FUNCTION foo(a int, b int DEFAULT 2, c int DEFAULT 3)
+RETURNS int
+LANGUAGE SQL
+AS $$
+ SELECT $1 + $2 + $3;
+$$;
+
+SELECT foo(10, 20, 30);
+ foo
+-----
+ 60
+(1 row)
+
+SELECT foo(10, 20);
+ foo
+-----
+ 33
+(1 row)
+
+SELECT foo(10);
+ foo
+-----
+ 15
+(1 row)
+
+SELECT foo(); -- fails since there is no default for the first argument
+ERROR: function foo() does not exist
+</screen>
+ The <literal>=</literal> sign can also be used in place of the
+ key word <literal>DEFAULT</literal>.
</para>
</sect2>
<title><acronym>SQL</acronym> Functions as Table Sources</title>
<para>
- All SQL functions may be used in the <literal>FROM</> clause of a query,
+ All SQL functions can be used in the <literal>FROM</literal> clause of a query,
but it is particularly useful for functions returning composite types.
If the function is defined to return a base type, the table function
produces a one-column table. If the function is defined to return
fooid | foosubid | fooname | upper
-------+----------+---------+-------
1 | 1 | Joe | JOE
-(2 rows)
+(1 row)
</screen>
As the example shows, we can work with the columns of the function's
<para>
Note that we only got one row out of the function. This is because
- we did not use <literal>SETOF</>. That is described in the next section.
+ we did not use <literal>SETOF</literal>. That is described in the next section.
</para>
</sect2>
- <sect2>
+ <sect2 id="xfunc-sql-functions-returning-set">
<title><acronym>SQL</acronym> Functions Returning Sets</title>
+ <indexterm>
+ <primary>function</primary>
+ <secondary>with SETOF</secondary>
+ </indexterm>
+
<para>
When an SQL function is declared as returning <literal>SETOF
- <replaceable>sometype</></literal>, the function's final
- <command>SELECT</> query is executed to completion, and each row it
+ <replaceable>sometype</replaceable></literal>, the function's final
+ query is executed to completion, and each row it
outputs is returned as an element of the result set.
</para>
<para>
- This feature is normally used when calling the function in the <literal>FROM</>
+ This feature is normally used when calling the function in the <literal>FROM</literal>
clause. In this case each row returned by the function becomes
a row of the table seen by the query. For example, assume that
- table <literal>foo</> has the same contents as above, and we say:
+ table <literal>foo</literal> has the same contents as above, and we say:
<programlisting>
CREATE FUNCTION getfoo(int) RETURNS SETOF foo AS $$
</para>
<para>
- Currently, functions returning sets may also be called in the select list
- of a query. For each row that the query
- generates by itself, the function returning set is invoked, and an output
- row is generated for each element of the function's result set. Note,
- however, that this capability is deprecated and may be removed in future
- releases. The following is an example function returning a set from the
- select list:
+ It is also possible to return multiple rows with the columns defined by
+ output parameters, like this:
-<screen>
-CREATE FUNCTION listchildren(text) RETURNS SETOF text AS $$
- SELECT name FROM nodes WHERE parent = $1
+<programlisting>
+CREATE TABLE tab (y int, z int);
+INSERT INTO tab VALUES (1, 2), (3, 4), (5, 6), (7, 8);
+
+CREATE FUNCTION sum_n_product_with_tab (x int, OUT sum int, OUT product int)
+RETURNS SETOF record
+AS $$
+ SELECT $1 + tab.y, $1 * tab.y FROM tab;
$$ LANGUAGE SQL;
+SELECT * FROM sum_n_product_with_tab(10);
+ sum | product
+-----+---------
+ 11 | 10
+ 13 | 30
+ 15 | 50
+ 17 | 70
+(4 rows)
+</programlisting>
+
+ The key point here is that you must write <literal>RETURNS SETOF record</literal>
+ to indicate that the function returns multiple rows instead of just one.
+ If there is only one output parameter, write that parameter's type
+ instead of <type>record</type>.
+ </para>
+
+ <para>
+ It is frequently useful to construct a query's result by invoking a
+ set-returning function multiple times, with the parameters for each
+ invocation coming from successive rows of a table or subquery. The
+ preferred way to do this is to use the <literal>LATERAL</literal> key word,
+ which is described in <xref linkend="queries-lateral">.
+ Here is an example using a set-returning function to enumerate
+ elements of a tree structure:
+
+<screen>
SELECT * FROM nodes;
name | parent
-----------+--------
SubChild2 | Child1
(6 rows)
+CREATE FUNCTION listchildren(text) RETURNS SETOF text AS $$
+ SELECT name FROM nodes WHERE parent = $1
+$$ LANGUAGE SQL STABLE;
+
+SELECT * FROM listchildren('Top');
+ listchildren
+--------------
+ Child1
+ Child2
+ Child3
+(3 rows)
+
+SELECT name, child FROM nodes, LATERAL listchildren(name) AS child;
+ name | child
+--------+-----------
+ Top | Child1
+ Top | Child2
+ Top | Child3
+ Child1 | SubChild1
+ Child1 | SubChild2
+(5 rows)
+</screen>
+
+ This example does not do anything that we couldn't have done with a
+ simple join, but in more complex calculations the option to put
+ some of the work into a function can be quite convenient.
+ </para>
+
+ <para>
+ Functions returning sets can also be called in the select list
+ of a query. For each row that the query
+ generates by itself, the set-returning function is invoked, and an output
+ row is generated for each element of the function's result set.
+ The previous example could also be done with queries like
+ these:
+
+<screen>
SELECT listchildren('Top');
listchildren
--------------
</screen>
In the last <command>SELECT</command>,
- notice that no output row appears for <literal>Child2</>, <literal>Child3</>, etc.
+ notice that no output row appears for <literal>Child2</literal>, <literal>Child3</literal>, etc.
This happens because <function>listchildren</function> returns an empty set
- for those arguments, so no result rows are generated.
+ for those arguments, so no result rows are generated. This is the same
+ behavior as we got from an inner join to the function result when using
+ the <literal>LATERAL</literal> syntax.
+ </para>
+
+ <para>
+ <productname>PostgreSQL</productname>'s behavior for a set-returning function in a
+ query's select list is almost exactly the same as if the set-returning
+ function had been written in a <literal>LATERAL FROM</literal>-clause item
+ instead. For example,
+<programlisting>
+SELECT x, generate_series(1,5) AS g FROM tab;
+</programlisting>
+ is almost equivalent to
+<programlisting>
+SELECT x, g FROM tab, LATERAL generate_series(1,5) AS g;
+</programlisting>
+ It would be exactly the same, except that in this specific example,
+ the planner could choose to put <structname>g</structname> on the outside of the
+ nestloop join, since <structname>g</structname> has no actual lateral dependency
+ on <structname>tab</structname>. That would result in a different output row
+ order. Set-returning functions in the select list are always evaluated
+ as though they are on the inside of a nestloop join with the rest of
+ the <literal>FROM</literal> clause, so that the function(s) are run to
+ completion before the next row from the <literal>FROM</literal> clause is
+ considered.
+ </para>
+
+ <para>
+ If there is more than one set-returning function in the query's select
+ list, the behavior is similar to what you get from putting the functions
+ into a single <literal>LATERAL ROWS FROM( ... )</literal> <literal>FROM</literal>-clause
+ item. For each row from the underlying query, there is an output row
+ using the first result from each function, then an output row using the
+ second result, and so on. If some of the set-returning functions
+ produce fewer outputs than others, null values are substituted for the
+ missing data, so that the total number of rows emitted for one
+ underlying row is the same as for the set-returning function that
+ produced the most outputs. Thus the set-returning functions
+ run <quote>in lockstep</quote> until they are all exhausted, and then
+ execution continues with the next underlying row.
+ </para>
+
+ <para>
+ Set-returning functions can be nested in a select list, although that is
+ not allowed in <literal>FROM</literal>-clause items. In such cases, each level
+ of nesting is treated separately, as though it were
+ a separate <literal>LATERAL ROWS FROM( ... )</literal> item. For example, in
+<programlisting>
+SELECT srf1(srf2(x), srf3(y)), srf4(srf5(z)) FROM tab;
+</programlisting>
+ the set-returning functions <function>srf2</function>, <function>srf3</function>,
+ and <function>srf5</function> would be run in lockstep for each row
+ of <structname>tab</structname>, and then <function>srf1</function> and <function>srf4</function>
+ would be applied in lockstep to each row produced by the lower
+ functions.
+ </para>
+
+ <para>
+ Set-returning functions cannot be used within conditional-evaluation
+ constructs, such as <literal>CASE</literal> or <literal>COALESCE</literal>. For
+ example, consider
+<programlisting>
+SELECT x, CASE WHEN x > 0 THEN generate_series(1, 5) ELSE 0 END FROM tab;
+</programlisting>
+ It might seem that this should produce five repetitions of input rows
+ that have <literal>x > 0</literal>, and a single repetition of those that do
+ not; but actually, because <function>generate_series(1, 5)</function> would be
+ run in an implicit <literal>LATERAL FROM</literal> item before
+ the <literal>CASE</literal> expression is ever evaluated, it would produce five
+ repetitions of every input row. To reduce confusion, such cases produce
+ a parse-time error instead.
+ </para>
+
+ <note>
+ <para>
+ If a function's last command is <command>INSERT</command>, <command>UPDATE</command>,
+ or <command>DELETE</command> with <literal>RETURNING</literal>, that command will
+ always be executed to completion, even if the function is not declared
+ with <literal>SETOF</literal> or the calling query does not fetch all the
+ result rows. Any extra rows produced by the <literal>RETURNING</literal>
+ clause are silently dropped, but the commanded table modifications
+ still happen (and are all completed before returning from the function).
+ </para>
+ </note>
+
+ <note>
+ <para>
+ Before <productname>PostgreSQL</productname> 10, putting more than one
+ set-returning function in the same select list did not behave very
+ sensibly unless they always produced equal numbers of rows. Otherwise,
+ what you got was a number of output rows equal to the least common
+ multiple of the numbers of rows produced by the set-returning
+ functions. Also, nested set-returning functions did not work as
+ described above; instead, a set-returning function could have at most
+ one set-returning argument, and each nest of set-returning functions
+ was run independently. Also, conditional execution (set-returning
+ functions inside <literal>CASE</literal> etc) was previously allowed,
+ complicating things even more.
+ Use of the <literal>LATERAL</literal> syntax is recommended when writing
+ queries that need to work in older <productname>PostgreSQL</productname> versions,
+ because that will give consistent results across different versions.
+ If you have a query that is relying on conditional execution of a
+ set-returning function, you may be able to fix it by moving the
+ conditional test into a custom set-returning function. For example,
+<programlisting>
+SELECT x, CASE WHEN y > 0 THEN generate_series(1, z) ELSE 5 END FROM tab;
+</programlisting>
+ could become
+<programlisting>
+CREATE FUNCTION case_generate_series(cond bool, start int, fin int, els int)
+ RETURNS SETOF int AS $$
+BEGIN
+ IF cond THEN
+ RETURN QUERY SELECT generate_series(start, fin);
+ ELSE
+ RETURN QUERY SELECT els;
+ END IF;
+END$$ LANGUAGE plpgsql;
+
+SELECT x, case_generate_series(y > 0, 1, z, 5) FROM tab;
+</programlisting>
+ This formulation will work the same in all versions
+ of <productname>PostgreSQL</productname>.
+ </para>
+ </note>
+ </sect2>
+
+ <sect2 id="xfunc-sql-functions-returning-table">
+ <title><acronym>SQL</acronym> Functions Returning <literal>TABLE</literal></title>
+
+ <indexterm>
+ <primary>function</primary>
+ <secondary>RETURNS TABLE</secondary>
+ </indexterm>
+
+ <para>
+ There is another way to declare a function as returning a set,
+ which is to use the syntax
+ <literal>RETURNS TABLE(<replaceable>columns</replaceable>)</literal>.
+ This is equivalent to using one or more <literal>OUT</literal> parameters plus
+ marking the function as returning <literal>SETOF record</literal> (or
+ <literal>SETOF</literal> a single output parameter's type, as appropriate).
+ This notation is specified in recent versions of the SQL standard, and
+ thus may be more portable than using <literal>SETOF</literal>.
+ </para>
+
+ <para>
+ For example, the preceding sum-and-product example could also be
+ done this way:
+
+<programlisting>
+CREATE FUNCTION sum_n_product_with_tab (x int)
+RETURNS TABLE(sum int, product int) AS $$
+ SELECT $1 + tab.y, $1 * tab.y FROM tab;
+$$ LANGUAGE SQL;
+</programlisting>
+
+ It is not allowed to use explicit <literal>OUT</literal> or <literal>INOUT</literal>
+ parameters with the <literal>RETURNS TABLE</literal> notation — you must
+ put all the output columns in the <literal>TABLE</literal> list.
</para>
</sect2>
<title>Polymorphic <acronym>SQL</acronym> Functions</title>
<para>
- <acronym>SQL</acronym> functions may be declared to accept and
- return the polymorphic types <type>anyelement</type> and
- <type>anyarray</type>. See <xref
+ <acronym>SQL</acronym> functions can be declared to accept and
+ return the polymorphic types <type>anyelement</type>,
+ <type>anyarray</type>, <type>anynonarray</type>,
+ <type>anyenum</type>, and <type>anyrange</type>. See <xref
linkend="extend-types-polymorphic"> for a more detailed
explanation of polymorphic functions. Here is a polymorphic
function <function>make_array</function> that builds up an array
Without the typecast, you will get errors like this:
<screen>
<computeroutput>
-ERROR: could not determine "anyarray"/"anyelement" type because input has type "unknown"
+ERROR: could not determine polymorphic type because input has type "unknown"
</computeroutput>
</screen>
</para>
SELECT 1;
$$ LANGUAGE SQL;
ERROR: cannot determine result data type
-DETAIL: A function returning "anyarray" or "anyelement" must have at least one argument of either type.
+DETAIL: A function returning a polymorphic type must have at least one polymorphic argument.
</screen>
</para>
For example:
<screen>
CREATE FUNCTION dup (f1 anyelement, OUT f2 anyelement, OUT f3 anyarray)
-AS 'select $1, array[$1,$1]' LANGUAGE sql;
+AS 'select $1, array[$1,$1]' LANGUAGE SQL;
SELECT * FROM dup(22);
f2 | f3
(1 row)
</screen>
</para>
+
+ <para>
+ Polymorphism can also be used with variadic functions.
+ For example:
+<screen>
+CREATE FUNCTION anyleast (VARIADIC anyarray) RETURNS anyelement AS $$
+ SELECT min($1[i]) FROM generate_subscripts($1, 1) g(i);
+$$ LANGUAGE SQL;
+
+SELECT anyleast(10, -1, 5, 4);
+ anyleast
+----------
+ -1
+(1 row)
+
+SELECT anyleast('abc'::text, 'def');
+ anyleast
+----------
+ abc
+(1 row)
+
+CREATE FUNCTION concat_values(text, VARIADIC anyarray) RETURNS text AS $$
+ SELECT array_to_string($2, $1);
+$$ LANGUAGE SQL;
+
+SELECT concat_values('|', 1, 4, 2);
+ concat_values
+---------------
+ 1|4|2
+(1 row)
+</screen>
+ </para>
+ </sect2>
+
+ <sect2>
+ <title><acronym>SQL</acronym> Functions with Collations</title>
+
+ <indexterm>
+ <primary>collation</primary>
+ <secondary>in SQL functions</secondary>
+ </indexterm>
+
+ <para>
+ When a SQL function has one or more parameters of collatable data types,
+ a collation is identified for each function call depending on the
+ collations assigned to the actual arguments, as described in <xref
+ linkend="collation">. If a collation is successfully identified
+ (i.e., there are no conflicts of implicit collations among the arguments)
+ then all the collatable parameters are treated as having that collation
+ implicitly. This will affect the behavior of collation-sensitive
+ operations within the function. For example, using the
+ <function>anyleast</function> function described above, the result of
+<programlisting>
+SELECT anyleast('abc'::text, 'ABC');
+</programlisting>
+ will depend on the database's default collation. In <literal>C</literal> locale
+ the result will be <literal>ABC</literal>, but in many other locales it will
+ be <literal>abc</literal>. The collation to use can be forced by adding
+ a <literal>COLLATE</literal> clause to any of the arguments, for example
+<programlisting>
+SELECT anyleast('abc'::text, 'ABC' COLLATE "C");
+</programlisting>
+ Alternatively, if you wish a function to operate with a particular
+ collation regardless of what it is called with, insert
+ <literal>COLLATE</literal> clauses as needed in the function definition.
+ This version of <function>anyleast</function> would always use <literal>en_US</literal>
+ locale to compare strings:
+<programlisting>
+CREATE FUNCTION anyleast (VARIADIC anyarray) RETURNS anyelement AS $$
+ SELECT min($1[i] COLLATE "en_US") FROM generate_subscripts($1, 1) g(i);
+$$ LANGUAGE SQL;
+</programlisting>
+ But note that this will throw an error if applied to a non-collatable
+ data type.
+ </para>
+
+ <para>
+ If no common collation can be identified among the actual arguments,
+ then a SQL function treats its parameters as having their data types'
+ default collation (which is usually the database's default collation,
+ but could be different for parameters of domain types).
+ </para>
+
+ <para>
+ The behavior of collatable parameters can be thought of as a limited
+ form of polymorphism, applicable only to textual data types.
+ </para>
</sect2>
</sect1>
</indexterm>
<para>
- More than one function may be defined with the same SQL name, so long
+ More than one function can be defined with the same SQL name, so long
as the arguments they take are different. In other words,
function names can be <firstterm>overloaded</firstterm>. When a
query is executed, the server will determine which function to
<para>
When creating a family of overloaded functions, one should be
careful not to create ambiguities. For instance, given the
- functions
+ functions:
<programlisting>
CREATE FUNCTION test(int, real) RETURNS ...
CREATE FUNCTION test(smallint, double precision) RETURNS ...
<para>
A function that takes a single argument of a composite type should
generally not have the same name as any attribute (field) of that type.
- Recall that <literal>attribute(table)</literal> is considered equivalent
- to <literal>table.attribute</literal>. In the case that there is an
+ Recall that <literal><replaceable>attribute</replaceable>(<replaceable>table</replaceable>)</literal>
+ is considered equivalent
+ to <literal><replaceable>table</replaceable>.<replaceable>attribute</replaceable></literal>.
+ In the case that there is an
ambiguity between a function on a composite type and an attribute of
the composite type, the attribute will always be used. It is possible
to override that choice by schema-qualifying the function name
- (that is, <literal>schema.func(table)</literal>) but it's better to
+ (that is, <literal><replaceable>schema</replaceable>.<replaceable>func</replaceable>(<replaceable>table</replaceable>)
+ </literal>) but it's better to
avoid the problem by not choosing conflicting names.
</para>
+ <para>
+ Another possible conflict is between variadic and non-variadic functions.
+ For instance, it is possible to create both <literal>foo(numeric)</literal> and
+ <literal>foo(VARIADIC numeric[])</literal>. In this case it is unclear which one
+ should be matched to a call providing a single numeric argument, such as
+ <literal>foo(10.1)</literal>. The rule is that the function appearing
+ earlier in the search path is used, or if the two functions are in the
+ same schema, the non-variadic one is preferred.
+ </para>
+
<para>
When overloading C-language functions, there is an additional
constraint: The C name of each function in the family of
rule is violated, the behavior is not portable. You might get a
run-time linker error, or one of the functions will get called
(usually the internal one). The alternative form of the
- <literal>AS</> clause for the SQL <command>CREATE
+ <literal>AS</literal> clause for the SQL <command>CREATE
FUNCTION</command> command decouples the SQL function name from
- the function name in the C source code. For instance,
+ the function name in the C source code. For instance:
<programlisting>
CREATE FUNCTION test(int) RETURNS int
- AS '<replaceable>filename</>', 'test_1arg'
+ AS '<replaceable>filename</replaceable>', 'test_1arg'
LANGUAGE C;
CREATE FUNCTION test(int, int) RETURNS int
- AS '<replaceable>filename</>', 'test_2arg'
+ AS '<replaceable>filename</replaceable>', 'test_2arg'
LANGUAGE C;
</programlisting>
The names of the C functions here reflect one of many possible conventions.
</indexterm>
<para>
- Every function has a <firstterm>volatility</> classification, with
- the possibilities being <literal>VOLATILE</>, <literal>STABLE</>, or
- <literal>IMMUTABLE</>. <literal>VOLATILE</> is the default if the
- <command>CREATE FUNCTION</command> command does not specify a category.
- The volatility category is a promise to the optimizer about the behavior
- of the function:
+ Every function has a <firstterm>volatility</firstterm> classification, with
+ the possibilities being <literal>VOLATILE</literal>, <literal>STABLE</literal>, or
+ <literal>IMMUTABLE</literal>. <literal>VOLATILE</literal> is the default if the
+ <xref linkend="sql-createfunction">
+ command does not specify a category. The volatility category is a
+ promise to the optimizer about the behavior of the function:
<itemizedlist>
<listitem>
<para>
- A <literal>VOLATILE</> function can do anything, including modifying
+ A <literal>VOLATILE</literal> function can do anything, including modifying
the database. It can return different results on successive calls with
the same arguments. The optimizer makes no assumptions about the
behavior of such functions. A query using a volatile function will
</listitem>
<listitem>
<para>
- A <literal>STABLE</> function cannot modify the database and is
+ A <literal>STABLE</literal> function cannot modify the database and is
guaranteed to return the same results given the same arguments
- for all calls within a single surrounding query. This category
- allows the optimizer to optimize away multiple calls of the function
- within a single query. In particular, it is safe to use an expression
- containing such a function in an index scan condition. (Since an
- index scan will evaluate the comparison value only once, not once at
- each row, it is not valid to use a <literal>VOLATILE</> function in
- an index scan condition.)
+ for all rows within a single statement. This category allows the
+ optimizer to optimize multiple calls of the function to a single
+ call. In particular, it is safe to use an expression containing
+ such a function in an index scan condition. (Since an index scan
+ will evaluate the comparison value only once, not once at each
+ row, it is not valid to use a <literal>VOLATILE</literal> function in an
+ index scan condition.)
</para>
</listitem>
<listitem>
<para>
- An <literal>IMMUTABLE</> function cannot modify the database and is
+ An <literal>IMMUTABLE</literal> function cannot modify the database and is
guaranteed to return the same results given the same arguments forever.
This category allows the optimizer to pre-evaluate the function when
a query calls it with constant arguments. For example, a query like
- <literal>SELECT ... WHERE x = 2 + 2</> can be simplified on sight to
- <literal>SELECT ... WHERE x = 4</>, because the function underlying
- the integer addition operator is marked <literal>IMMUTABLE</>.
+ <literal>SELECT ... WHERE x = 2 + 2</literal> can be simplified on sight to
+ <literal>SELECT ... WHERE x = 4</literal>, because the function underlying
+ the integer addition operator is marked <literal>IMMUTABLE</literal>.
</para>
</listitem>
</itemizedlist>
</para>
<para>
- Any function with side-effects <emphasis>must</> be labeled
- <literal>VOLATILE</>, so that calls to it cannot be optimized away.
+ Any function with side-effects <emphasis>must</emphasis> be labeled
+ <literal>VOLATILE</literal>, so that calls to it cannot be optimized away.
Even a function with no side-effects needs to be labeled
- <literal>VOLATILE</> if its value can change within a single query;
- some examples are <literal>random()</>, <literal>currval()</>,
- <literal>timeofday()</>.
+ <literal>VOLATILE</literal> if its value can change within a single query;
+ some examples are <literal>random()</literal>, <literal>currval()</literal>,
+ <literal>timeofday()</literal>.
+ </para>
+
+ <para>
+ Another important example is that the <function>current_timestamp</function>
+ family of functions qualify as <literal>STABLE</literal>, since their values do
+ not change within a transaction.
</para>
<para>
- There is relatively little difference between <literal>STABLE</> and
- <literal>IMMUTABLE</> categories when considering simple interactive
+ There is relatively little difference between <literal>STABLE</literal> and
+ <literal>IMMUTABLE</literal> categories when considering simple interactive
queries that are planned and immediately executed: it doesn't matter
a lot whether a function is executed once during planning or once during
query execution startup. But there is a big difference if the plan is
- saved and reused later. Labeling a function <literal>IMMUTABLE</> when
- it really isn't may allow it to be prematurely folded to a constant during
+ saved and reused later. Labeling a function <literal>IMMUTABLE</literal> when
+ it really isn't might allow it to be prematurely folded to a constant during
planning, resulting in a stale value being re-used during subsequent uses
of the plan. This is a hazard when using prepared statements or when
using function languages that cache plans (such as
- <application>PL/pgSQL</>).
+ <application>PL/pgSQL</application>).
</para>
<para>
- Because of the snapshotting behavior of MVCC (see <xref linkend="mvcc">)
- a function containing only <command>SELECT</> commands can safely be
- marked <literal>STABLE</>, even if it selects from tables that might be
+ For functions written in SQL or in any of the standard procedural
+ languages, there is a second important property determined by the
+ volatility category, namely the visibility of any data changes that have
+ been made by the SQL command that is calling the function. A
+ <literal>VOLATILE</literal> function will see such changes, a <literal>STABLE</literal>
+ or <literal>IMMUTABLE</literal> function will not. This behavior is implemented
+ using the snapshotting behavior of MVCC (see <xref linkend="mvcc">):
+ <literal>STABLE</literal> and <literal>IMMUTABLE</literal> functions use a snapshot
+ established as of the start of the calling query, whereas
+ <literal>VOLATILE</literal> functions obtain a fresh snapshot at the start of
+ each query they execute.
+ </para>
+
+ <note>
+ <para>
+ Functions written in C can manage snapshots however they want, but it's
+ usually a good idea to make C functions work this way too.
+ </para>
+ </note>
+
+ <para>
+ Because of this snapshotting behavior,
+ a function containing only <command>SELECT</command> commands can safely be
+ marked <literal>STABLE</literal>, even if it selects from tables that might be
undergoing modifications by concurrent queries.
- <productname>PostgreSQL</productname> will execute a <literal>STABLE</>
- function using the snapshot established for the calling query, and so it
- will see a fixed view of the database throughout that query.
- Also note
- that the <function>current_timestamp</> family of functions qualify
- as stable, since their values do not change within a transaction.
+ <productname>PostgreSQL</productname> will execute all commands of a
+ <literal>STABLE</literal> function using the snapshot established for the
+ calling query, and so it will see a fixed view of the database throughout
+ that query.
</para>
<para>
- The same snapshotting behavior is used for <command>SELECT</> commands
- within <literal>IMMUTABLE</> functions. It is generally unwise to select
- from database tables within an <literal>IMMUTABLE</> function at all,
+ The same snapshotting behavior is used for <command>SELECT</command> commands
+ within <literal>IMMUTABLE</literal> functions. It is generally unwise to select
+ from database tables within an <literal>IMMUTABLE</literal> function at all,
since the immutability will be broken if the table contents ever change.
However, <productname>PostgreSQL</productname> does not enforce that you
do not do that.
</para>
<para>
- A common error is to label a function <literal>IMMUTABLE</> when its
+ A common error is to label a function <literal>IMMUTABLE</literal> when its
results depend on a configuration parameter. For example, a function
that manipulates timestamps might well have results that depend on the
<xref linkend="guc-timezone"> setting. For safety, such functions should
- be labeled <literal>STABLE</> instead.
+ be labeled <literal>STABLE</literal> instead.
</para>
<note>
<para>
- Before <productname>PostgreSQL</productname> release 8.0, the requirement
- that <literal>STABLE</> and <literal>IMMUTABLE</> functions cannot modify
- the database was not enforced by the system. Release 8.0 enforces it
- by requiring SQL functions and procedural language functions of these
- categories to contain no SQL commands other than <command>SELECT</>.
+ <productname>PostgreSQL</productname> requires that <literal>STABLE</literal>
+ and <literal>IMMUTABLE</literal> functions contain no SQL commands other
+ than <command>SELECT</command> to prevent data modification.
(This is not a completely bulletproof test, since such functions could
- still call <literal>VOLATILE</> functions that modify the database.
- If you do that, you will find that the <literal>STABLE</> or
- <literal>IMMUTABLE</> function does not notice the database changes
- applied by the called function.)
+ still call <literal>VOLATILE</literal> functions that modify the database.
+ If you do that, you will find that the <literal>STABLE</literal> or
+ <literal>IMMUTABLE</literal> function does not notice the database changes
+ applied by the called function, since they are hidden from its snapshot.)
</para>
</note>
</sect1>
<productname>PostgreSQL</productname> allows user-defined functions
to be written in other languages besides SQL and C. These other
languages are generically called <firstterm>procedural
- languages</firstterm> (<acronym>PL</>s).
+ languages</firstterm> (<acronym>PL</acronym>s).
Procedural languages aren't built into the
<productname>PostgreSQL</productname> server; they are offered
by loadable modules.
<sect1 id="xfunc-internal">
<title>Internal Functions</title>
- <indexterm zone="xfunc-internal"><primary>function</><secondary>internal</></>
+ <indexterm zone="xfunc-internal"><primary>function</primary><secondary>internal</secondary></indexterm>
<para>
Internal functions are functions written in C that have been statically
The <quote>body</quote> of the function definition
specifies the C-language name of the function, which need not be the
same as the name being declared for SQL use.
- (For reasons of backwards compatibility, an empty body
+ (For reasons of backward compatibility, an empty body
is accepted as meaning that the C-language function name is the
same as the SQL name.)
</para>
<para>
Normally, all internal functions present in the
- server are declared during the initialization of the database cluster (<command>initdb</command>),
+ server are declared during the initialization of the database cluster
+ (see <xref linkend="creating-cluster">),
but a user could use <command>CREATE FUNCTION</command>
to create additional alias names for an internal function.
Internal functions are declared in <command>CREATE FUNCTION</command>
be made compatible with C, such as C++). Such functions are
compiled into dynamically loadable objects (also called shared
libraries) and are loaded by the server on demand. The dynamic
- loading feature is what distinguishes <quote>C language</> functions
- from <quote>internal</> functions — the actual coding conventions
+ loading feature is what distinguishes <quote>C language</quote> functions
+ from <quote>internal</quote> functions — the actual coding conventions
are essentially the same for both. (Hence, the standard internal
function library is a rich source of coding examples for user-defined
C functions.)
</para>
<para>
- Two different calling conventions are currently used for C functions.
- The newer <quote>version 1</quote> calling convention is indicated by writing
- a <literal>PG_FUNCTION_INFO_V1()</literal> macro call for the function,
- as illustrated below. Lack of such a macro indicates an old-style
- (<quote>version 0</quote>) function. The language name specified in <command>CREATE FUNCTION</command>
- is <literal>C</literal> in either case. Old-style functions are now deprecated
- because of portability problems and lack of functionality, but they
- are still supported for compatibility reasons.
+ Currently only one calling convention is used for C functions
+ (<quote>version 1</quote>). Support for that calling convention is
+ indicated by writing a <literal>PG_FUNCTION_INFO_V1()</literal> macro
+ call for the function, as illustrated below.
</para>
<sect2 id="xfunc-c-dynload">
<listitem>
<para>
If the name starts with the string <literal>$libdir</literal>,
- that part is replaced by the <productname>PostgreSQL</> package
+ that part is replaced by the <productname>PostgreSQL</productname> package
library directory
- name, which is determined at build time.<indexterm><primary>$libdir</></>
+ name, which is determined at build time.<indexterm><primary>$libdir</primary></indexterm>
</para>
</listitem>
<para>
If the name does not contain a directory part, the file is
searched for in the path specified by the configuration variable
- <xref linkend="guc-dynamic-library-path">.<indexterm><primary>dynamic_library_path</></>
+ <xref linkend="guc-dynamic-library-path">.<indexterm><primary>dynamic_library_path</primary></indexterm>
</para>
</listitem>
that fails as well, the load will fail.
</para>
+ <para>
+ It is recommended to locate shared libraries either relative to
+ <literal>$libdir</literal> or through the dynamic library path.
+ This simplifies version upgrades if the new installation is at a
+ different location. The actual directory that
+ <literal>$libdir</literal> stands for can be found out with the
+ command <literal>pg_config --pkglibdir</literal>.
+ </para>
+
<para>
The user ID the <productname>PostgreSQL</productname> server runs
as must be able to traverse the path to the file you intend to
<productname>PostgreSQL</productname> will not compile a C function
automatically. The object file must be compiled before it is referenced
in a <command>CREATE
- FUNCTION</> command. See <xref linkend="dfunc"> for additional
+ FUNCTION</command> command. See <xref linkend="dfunc"> for additional
information.
</para>
</note>
+ <indexterm zone="xfunc-c-dynload">
+ <primary>magic block</primary>
+ </indexterm>
+
+ <para>
+ To ensure that a dynamically loaded object file is not loaded into an
+ incompatible server, <productname>PostgreSQL</productname> checks that the
+ file contains a <quote>magic block</quote> with the appropriate contents.
+ This allows the server to detect obvious incompatibilities, such as code
+ compiled for a different major version of
+ <productname>PostgreSQL</productname>. To include a magic block,
+ write this in one (and only one) of the module source files, after having
+ included the header <filename>fmgr.h</filename>:
+
+<programlisting>
+PG_MODULE_MAGIC;
+</programlisting>
+ </para>
+
<para>
After it is used for the first time, a dynamically loaded object
file is retained in memory. Future calls in the same session to
the function(s) in that file will only incur the small overhead of
a symbol table lookup. If you need to force a reload of an object
- file, for example after recompiling it, use the <command>LOAD</>
- command or begin a fresh session.
+ file, for example after recompiling it, begin a fresh session.
</para>
- <para>
- It is recommended to locate shared libraries either relative to
- <literal>$libdir</literal> or through the dynamic library path.
- This simplifies version upgrades if the new installation is at a
- different location. The actual directory that
- <literal>$libdir</literal> stands for can be found out with the
- command <literal>pg_config --pkglibdir</literal>.
- </para>
+ <indexterm zone="xfunc-c-dynload">
+ <primary>_PG_init</primary>
+ </indexterm>
+ <indexterm zone="xfunc-c-dynload">
+ <primary>_PG_fini</primary>
+ </indexterm>
+ <indexterm zone="xfunc-c-dynload">
+ <primary>library initialization function</primary>
+ </indexterm>
+ <indexterm zone="xfunc-c-dynload">
+ <primary>library finalization function</primary>
+ </indexterm>
<para>
- Before <productname>PostgreSQL</productname> release 7.2, only
- exact absolute paths to object files could be specified in
- <command>CREATE FUNCTION</>. This approach is now deprecated
- since it makes the function definition unnecessarily unportable.
- It's best to specify just the shared library name with no path nor
- extension, and let the search mechanism provide that information
- instead.
+ Optionally, a dynamically loaded file can contain initialization and
+ finalization functions. If the file includes a function named
+ <function>_PG_init</function>, that function will be called immediately after
+ loading the file. The function receives no parameters and should
+ return void. If the file includes a function named
+ <function>_PG_fini</function>, that function will be called immediately before
+ unloading the file. Likewise, the function receives no parameters and
+ should return void. Note that <function>_PG_fini</function> will only be called
+ during an unload of the file, not during process termination.
+ (Presently, unloads are disabled and will never occur, but this may
+ change in the future.)
</para>
+
</sect2>
<sect2 id="xfunc-c-basetype">
<indexterm zone="xfunc-c-basetype">
<primary>data type</primary>
- <secondary>internal organisation</secondary>
+ <secondary>internal organization</secondary>
</indexterm>
<para>
<para>
By-value types can only be 1, 2, or 4 bytes in length
(also 8 bytes, if <literal>sizeof(Datum)</literal> is 8 on your machine).
- You should be careful
- to define your types such that they will be the same
- size (in bytes) on all architectures. For example, the
- <literal>long</literal> type is dangerous because it
- is 4 bytes on some machines and 8 bytes on others, whereas
- <type>int</type> type is 4 bytes on most
- Unix machines. A reasonable implementation of
- the <type>int4</type> type on Unix
- machines might be:
-
+ You should be careful to define your types such that they will be the
+ same size (in bytes) on all architectures. For example, the
+ <literal>long</literal> type is dangerous because it is 4 bytes on some
+ machines and 8 bytes on others, whereas <type>int</type> type is 4 bytes
+ on most Unix machines. A reasonable implementation of the
+ <type>int4</type> type on Unix machines might be:
+
<programlisting>
/* 4-byte integer, passed by value */
typedef int int4;
</programlisting>
+
+ (The actual PostgreSQL C code calls this type <type>int32</type>, because
+ it is a convention in C that <type>int<replaceable>XX</replaceable></type>
+ means <replaceable>XX</replaceable> <emphasis>bits</emphasis>. Note
+ therefore also that the C type <type>int8</type> is 1 byte in size. The
+ SQL type <type>int8</type> is called <type>int64</type> in C. See also
+ <xref linkend="xfunc-c-type-table">.)
</para>
<para>
- On the other hand, fixed-length types of any size may
+ On the other hand, fixed-length types of any size can
be passed by-reference. For example, here is a sample
implementation of a <productname>PostgreSQL</productname> type:
-
+
<programlisting>
/* 16-byte structure, passed by reference */
typedef struct
<para>
Finally, all variable-length types must also be passed
by reference. All variable-length types must begin
- with a length field of exactly 4 bytes, and all data to
- be stored within that type must be located in the memory
+ with an opaque length field of exactly 4 bytes, which will be set
+ by <symbol>SET_VARSIZE</symbol>; never set this field directly! All data to
+ be stored within that type must be located in the memory
immediately following that length field. The
length field contains the total length of the structure,
that is, it includes the size of the length field
itself.
</para>
+ <para>
+ Another important point is to avoid leaving any uninitialized bits
+ within data type values; for example, take care to zero out any
+ alignment padding bytes that might be present in structs. Without
+ this, logically-equivalent constants of your data type might be
+ seen as unequal by the planner, leading to inefficient (though not
+ incorrect) plans.
+ </para>
+
<warning>
<para>
- <emphasis>Never</> modify the contents of a pass-by-reference input
+ <emphasis>Never</emphasis> modify the contents of a pass-by-reference input
value. If you do so you are likely to corrupt on-disk data, since
- the pointer you are given may well point directly into a disk buffer.
+ the pointer you are given might point directly into a disk buffer.
The sole exception to this rule is explained in
<xref linkend="xaggr">.
</para>
<programlisting>
typedef struct {
- int4 length;
- char data[1];
+ int32 length;
+ char data[FLEXIBLE_ARRAY_MEMBER];
} text;
</programlisting>
- Obviously, the data field declared here is not long enough to hold
- all possible strings. Since it's impossible to declare a variable-size
- structure in <acronym>C</acronym>, we rely on the knowledge that the
- <acronym>C</acronym> compiler won't range-check array subscripts. We
- just allocate the necessary amount of space and then access the array as
- if it were declared the right length. (This is a common trick, which
- you can read about in many textbooks about C.)
+ The <literal>[FLEXIBLE_ARRAY_MEMBER]</literal> notation means that the actual
+ length of the data part is not specified by this declaration.
</para>
<para>
- When manipulating
- variable-length types, we must be careful to allocate
+ When manipulating
+ variable-length types, we must be careful to allocate
the correct amount of memory and set the length field correctly.
- For example, if we wanted to store 40 bytes in a <structname>text</>
+ For example, if we wanted to store 40 bytes in a <structname>text</structname>
structure, we might use a code fragment like this:
-<programlisting>
+<programlisting><![CDATA[
#include "postgres.h"
...
char buffer[40]; /* our source data */
...
text *destination = (text *) palloc(VARHDRSZ + 40);
-destination->length = VARHDRSZ + 40;
-memcpy(destination->data, buffer, 40);
+SET_VARSIZE(destination, VARHDRSZ + 40);
+memcpy(destination->data, buffer, 40);
...
+]]>
</programlisting>
- <literal>VARHDRSZ</> is the same as <literal>sizeof(int4)</>, but
- it's considered good style to use the macro <literal>VARHDRSZ</>
+ <literal>VARHDRSZ</literal> is the same as <literal>sizeof(int32)</literal>, but
+ it's considered good style to use the macro <literal>VARHDRSZ</literal>
to refer to the size of the overhead for a variable-length type.
+ Also, the length field <emphasis>must</emphasis> be set using the
+ <literal>SET_VARSIZE</literal> macro, not by simple assignment.
</para>
<para>
<xref linkend="xfunc-c-type-table"> specifies which C type
corresponds to which SQL type when writing a C-language function
- that uses a built-in type of <productname>PostgreSQL</>.
+ that uses a built-in type of <productname>PostgreSQL</productname>.
The <quote>Defined In</quote> column gives the header file that
needs to be included to get the type definition. (The actual
- definition may be in a different file that is included by the
+ definition might be in a different file that is included by the
listed file. It is recommended that users stick to the defined
interface.) Note that you should always include
<filename>postgres.h</filename> first in any source file, because
</para>
<table tocentry="1" id="xfunc-c-type-table">
- <title>Equivalent C Types for Built-In SQL Types</title>
+ <title>Equivalent C Types for Built-in SQL Types</title>
<tgroup cols="3">
<thead>
<row>
<entry><type>AbsoluteTime</type></entry>
<entry><filename>utils/nabstime.h</filename></entry>
</row>
+ <row>
+ <entry><type>bigint</type> (<type>int8</type>)</entry>
+ <entry><type>int64</type></entry>
+ <entry><filename>postgres.h</filename></entry>
+ </row>
<row>
<entry><type>boolean</type></entry>
<entry><type>bool</type></entry>
</row>
<row>
<entry><type>smallint</type> (<type>int2</type>)</entry>
- <entry><type>int2</type> or <type>int16</type></entry>
+ <entry><type>int16</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
</row>
<row>
<entry><type>integer</type> (<type>int4</type>)</entry>
- <entry><type>int4</type> or <type>int32</type></entry>
+ <entry><type>int32</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<row>
<entry><type>interval</type></entry>
<entry><type>Interval*</type></entry>
- <entry><filename>utils/timestamp.h</filename></entry>
+ <entry><filename>datatype/timestamp.h</filename></entry>
</row>
<row>
<entry><type>lseg</type></entry>
<row>
<entry><type>timestamp</type></entry>
<entry><type>Timestamp*</type></entry>
- <entry><filename>utils/timestamp.h</filename></entry>
+ <entry><filename>datatype/timestamp.h</filename></entry>
</row>
<row>
<entry><type>tinterval</type></entry>
</sect2>
<sect2>
- <title>Calling Conventions Version 0 for C-Language Functions</title>
-
- <para>
- We present the <quote>old style</quote> calling convention first — although
- this approach is now deprecated, it's easier to get a handle on
- initially. In the version-0 method, the arguments and result
- of the C function are just declared in normal C style, but being
- careful to use the C representation of each SQL data type as shown
- above.
- </para>
-
- <para>
- Here are some examples:
-
-<programlisting>
-#include "postgres.h"
-#include <string.h>
-
-/* by value */
-
-int
-add_one(int arg)
-{
- return arg + 1;
-}
-
-/* by reference, fixed length */
-
-float8 *
-add_one_float8(float8 *arg)
-{
- float8 *result = (float8 *) palloc(sizeof(float8));
-
- *result = *arg + 1.0;
-
- return result;
-}
-
-Point *
-makepoint(Point *pointx, Point *pointy)
-{
- Point *new_point = (Point *) palloc(sizeof(Point));
-
- new_point->x = pointx->x;
- new_point->y = pointy->y;
-
- return new_point;
-}
-
-/* by reference, variable length */
-
-text *
-copytext(text *t)
-{
- /*
- * VARSIZE is the total size of the struct in bytes.
- */
- text *new_t = (text *) palloc(VARSIZE(t));
- VARATT_SIZEP(new_t) = VARSIZE(t);
- /*
- * VARDATA is a pointer to the data region of the struct.
- */
- memcpy((void *) VARDATA(new_t), /* destination */
- (void *) VARDATA(t), /* source */
- VARSIZE(t)-VARHDRSZ); /* how many bytes */
- return new_t;
-}
-
-text *
-concat_text(text *arg1, text *arg2)
-{
- int32 new_text_size = VARSIZE(arg1) + VARSIZE(arg2) - VARHDRSZ;
- text *new_text = (text *) palloc(new_text_size);
-
- VARATT_SIZEP(new_text) = new_text_size;
- memcpy(VARDATA(new_text), VARDATA(arg1), VARSIZE(arg1)-VARHDRSZ);
- memcpy(VARDATA(new_text) + (VARSIZE(arg1)-VARHDRSZ),
- VARDATA(arg2), VARSIZE(arg2)-VARHDRSZ);
- return new_text;
-}
-</programlisting>
- </para>
-
- <para>
- Supposing that the above code has been prepared in file
- <filename>funcs.c</filename> and compiled into a shared object,
- we could define the functions to <productname>PostgreSQL</productname>
- with commands like this:
-
-<programlisting>
-CREATE FUNCTION add_one(integer) RETURNS integer
- AS '<replaceable>DIRECTORY</replaceable>/funcs', 'add_one'
- LANGUAGE C STRICT;
-
--- note overloading of SQL function name "add_one"
-CREATE FUNCTION add_one(double precision) RETURNS double precision
- AS '<replaceable>DIRECTORY</replaceable>/funcs', 'add_one_float8'
- LANGUAGE C STRICT;
-
-CREATE FUNCTION makepoint(point, point) RETURNS point
- AS '<replaceable>DIRECTORY</replaceable>/funcs', 'makepoint'
- LANGUAGE C STRICT;
-
-CREATE FUNCTION copytext(text) RETURNS text
- AS '<replaceable>DIRECTORY</replaceable>/funcs', 'copytext'
- LANGUAGE C STRICT;
-
-CREATE FUNCTION concat_text(text, text) RETURNS text
- AS '<replaceable>DIRECTORY</replaceable>/funcs', 'concat_text',
- LANGUAGE C STRICT;
-</programlisting>
- </para>
-
- <para>
- Here, <replaceable>DIRECTORY</replaceable> stands for the
- directory of the shared library file (for instance the
- <productname>PostgreSQL</productname> tutorial directory, which
- contains the code for the examples used in this section).
- (Better style would be to use just <literal>'funcs'</> in the
- <literal>AS</> clause, after having added
- <replaceable>DIRECTORY</replaceable> to the search path. In any
- case, we may omit the system-specific extension for a shared
- library, commonly <literal>.so</literal> or
- <literal>.sl</literal>.)
- </para>
-
- <para>
- Notice that we have specified the functions as <quote>strict</quote>,
- meaning that
- the system should automatically assume a null result if any input
- value is null. By doing this, we avoid having to check for null inputs
- in the function code. Without this, we'd have to check for null values
- explicitly, by checking for a null pointer for each
- pass-by-reference argument. (For pass-by-value arguments, we don't
- even have a way to check!)
- </para>
-
- <para>
- Although this calling convention is simple to use,
- it is not very portable; on some architectures there are problems
- with passing data types that are smaller than <type>int</type> this way. Also, there is
- no simple way to return a null result, nor to cope with null arguments
- in any way other than making the function strict. The version-1
- convention, presented next, overcomes these objections.
- </para>
- </sect2>
-
- <sect2>
- <title>Calling Conventions Version 1 for C-Language Functions</title>
+ <title>Version 1 Calling Conventions</title>
<para>
The version-1 calling convention relies on macros to suppress most
of the complexity of passing arguments and results. The C declaration
- of a version-1 function is always
+ of a version-1 function is always:
<programlisting>
Datum funcname(PG_FUNCTION_ARGS)
</programlisting>
- In addition, the macro call
+ In addition, the macro call:
<programlisting>
PG_FUNCTION_INFO_V1(funcname);
</programlisting>
- must appear in the same source file. (Conventionally. it's
+ must appear in the same source file. (Conventionally, it's
written just before the function itself.) This macro call is not
- needed for <literal>internal</>-language functions, since
- <productname>PostgreSQL</> assumes that all internal functions
+ needed for <literal>internal</literal>-language functions, since
+ <productname>PostgreSQL</productname> assumes that all internal functions
use the version-1 convention. It is, however, required for
dynamically-loaded functions.
</para>
<para>
In a version-1 function, each actual argument is fetched using a
<function>PG_GETARG_<replaceable>xxx</replaceable>()</function>
- macro that corresponds to the argument's data type, and the
- result is returned using a
+ macro that corresponds to the argument's data type. In non-strict
+ functions there needs to be a previous check about argument null-ness
+ using <function>PG_ARGNULL_<replaceable>xxx</replaceable>()</function>.
+ The result is returned using a
<function>PG_RETURN_<replaceable>xxx</replaceable>()</function>
macro for the return type.
<function>PG_GETARG_<replaceable>xxx</replaceable>()</function>
</para>
<para>
- Here we show the same functions as above, coded in version-1 style:
+ Here are some examples using the version-1 calling convention:
+ </para>
-<programlisting>
+<programlisting><![CDATA[
#include "postgres.h"
-#include <string.h>
+#include <string.h>
#include "fmgr.h"
+#include "utils/geo_decls.h"
+
+PG_MODULE_MAGIC;
/* by value */
PG_FUNCTION_INFO_V1(add_one);
-
+
Datum
add_one(PG_FUNCTION_ARGS)
{
Point *pointy = PG_GETARG_POINT_P(1);
Point *new_point = (Point *) palloc(sizeof(Point));
- new_point->x = pointx->x;
- new_point->y = pointy->y;
-
+ new_point->x = pointx->x;
+ new_point->y = pointy->y;
+
PG_RETURN_POINT_P(new_point);
}
Datum
copytext(PG_FUNCTION_ARGS)
{
- text *t = PG_GETARG_TEXT_P(0);
+ text *t = PG_GETARG_TEXT_PP(0);
+
/*
- * VARSIZE is the total size of the struct in bytes.
+ * VARSIZE_ANY_EXHDR is the size of the struct in bytes, minus the
+ * VARHDRSZ or VARHDRSZ_SHORT of its header. Construct the copy with a
+ * full-length header.
*/
- text *new_t = (text *) palloc(VARSIZE(t));
- VARATT_SIZEP(new_t) = VARSIZE(t);
+ text *new_t = (text *) palloc(VARSIZE_ANY_EXHDR(t) + VARHDRSZ);
+ SET_VARSIZE(new_t, VARSIZE_ANY_EXHDR(t) + VARHDRSZ);
+
/*
- * VARDATA is a pointer to the data region of the struct.
+ * VARDATA is a pointer to the data region of the new struct. The source
+ * could be a short datum, so retrieve its data through VARDATA_ANY.
*/
memcpy((void *) VARDATA(new_t), /* destination */
- (void *) VARDATA(t), /* source */
- VARSIZE(t)-VARHDRSZ); /* how many bytes */
+ (void *) VARDATA_ANY(t), /* source */
+ VARSIZE_ANY_EXHDR(t)); /* how many bytes */
PG_RETURN_TEXT_P(new_t);
}
Datum
concat_text(PG_FUNCTION_ARGS)
{
- text *arg1 = PG_GETARG_TEXT_P(0);
- text *arg2 = PG_GETARG_TEXT_P(1);
- int32 new_text_size = VARSIZE(arg1) + VARSIZE(arg2) - VARHDRSZ;
+ text *arg1 = PG_GETARG_TEXT_PP(0);
+ text *arg2 = PG_GETARG_TEXT_PP(1);
+ int32 arg1_size = VARSIZE_ANY_EXHDR(arg1);
+ int32 arg2_size = VARSIZE_ANY_EXHDR(arg2);
+ int32 new_text_size = arg1_size + arg2_size + VARHDRSZ;
text *new_text = (text *) palloc(new_text_size);
- VARATT_SIZEP(new_text) = new_text_size;
- memcpy(VARDATA(new_text), VARDATA(arg1), VARSIZE(arg1)-VARHDRSZ);
- memcpy(VARDATA(new_text) + (VARSIZE(arg1)-VARHDRSZ),
- VARDATA(arg2), VARSIZE(arg2)-VARHDRSZ);
+ SET_VARSIZE(new_text, new_text_size);
+ memcpy(VARDATA(new_text), VARDATA_ANY(arg1), arg1_size);
+ memcpy(VARDATA(new_text) + arg1_size, VARDATA_ANY(arg2), arg2_size);
PG_RETURN_TEXT_P(new_text);
}
+]]>
+</programlisting>
+
+ <para>
+ Supposing that the above code has been prepared in file
+ <filename>funcs.c</filename> and compiled into a shared object,
+ we could define the functions to <productname>PostgreSQL</productname>
+ with commands like this:
+ </para>
+
+<programlisting>
+CREATE FUNCTION add_one(integer) RETURNS integer
+ AS '<replaceable>DIRECTORY</replaceable>/funcs', 'add_one'
+ LANGUAGE C STRICT;
+
+-- note overloading of SQL function name "add_one"
+CREATE FUNCTION add_one(double precision) RETURNS double precision
+ AS '<replaceable>DIRECTORY</replaceable>/funcs', 'add_one_float8'
+ LANGUAGE C STRICT;
+
+CREATE FUNCTION makepoint(point, point) RETURNS point
+ AS '<replaceable>DIRECTORY</replaceable>/funcs', 'makepoint'
+ LANGUAGE C STRICT;
+
+CREATE FUNCTION copytext(text) RETURNS text
+ AS '<replaceable>DIRECTORY</replaceable>/funcs', 'copytext'
+ LANGUAGE C STRICT;
+
+CREATE FUNCTION concat_text(text, text) RETURNS text
+ AS '<replaceable>DIRECTORY</replaceable>/funcs', 'concat_text'
+ LANGUAGE C STRICT;
</programlisting>
+
+ <para>
+ Here, <replaceable>DIRECTORY</replaceable> stands for the
+ directory of the shared library file (for instance the
+ <productname>PostgreSQL</productname> tutorial directory, which
+ contains the code for the examples used in this section).
+ (Better style would be to use just <literal>'funcs'</literal> in the
+ <literal>AS</literal> clause, after having added
+ <replaceable>DIRECTORY</replaceable> to the search path. In any
+ case, we can omit the system-specific extension for a shared
+ library, commonly <literal>.so</literal>.)
</para>
<para>
- The <command>CREATE FUNCTION</command> commands are the same as
- for the version-0 equivalents.
+ Notice that we have specified the functions as <quote>strict</quote>,
+ meaning that
+ the system should automatically assume a null result if any input
+ value is null. By doing this, we avoid having to check for null inputs
+ in the function code. Without this, we'd have to check for null values
+ explicitly, using <function>PG_ARGISNULL()</function>.
</para>
<para>
- At first glance, the version-1 coding conventions may appear to
- be just pointless obscurantism. They do, however, offer a number
- of improvements, because the macros can hide unnecessary detail.
- An example is that in coding <function>add_one_float8</>, we no longer need to
- be aware that <type>float8</type> is a pass-by-reference type. Another
- example is that the <literal>GETARG</> macros for variable-length types allow
- for more efficient fetching of <quote>toasted</quote> (compressed or
+ At first glance, the version-1 coding conventions might appear to be just
+ pointless obscurantism, over using plain <literal>C</literal> calling
+ conventions. They do however allow to deal with <literal>NULL</literal>able
+ arguments/return values, and <quote>toasted</quote> (compressed or
out-of-line) values.
</para>
<para>
- One big improvement in version-1 functions is better handling of null
- inputs and results. The macro <function>PG_ARGISNULL(<replaceable>n</>)</function>
+ The macro <function>PG_ARGISNULL(<replaceable>n</replaceable>)</function>
allows a function to test whether each input is null. (Of course, doing
- this is only necessary in functions not declared <quote>strict</>.)
+ this is only necessary in functions not declared <quote>strict</quote>.)
As with the
<function>PG_GETARG_<replaceable>xxx</replaceable>()</function> macros,
the input arguments are counted beginning at zero. Note that one
</para>
<para>
- Other options provided in the new-style interface are two
+ Other options provided by the version-1 interface are two
variants of the
<function>PG_GETARG_<replaceable>xxx</replaceable>()</function>
macros. The first of these,
<literal>ALTER TABLE <replaceable>tablename</replaceable> ALTER
COLUMN <replaceable>colname</replaceable> SET STORAGE
<replaceable>storagetype</replaceable></literal>. <replaceable>storagetype</replaceable> is one of
- <literal>plain</>, <literal>external</>, <literal>extended</literal>,
- or <literal>main</>.)
+ <literal>plain</literal>, <literal>external</literal>, <literal>extended</literal>,
+ or <literal>main</literal>.)
</para>
<para>
to return set results (<xref linkend="xfunc-c-return-set">) and
implement trigger functions (<xref linkend="triggers">) and
procedural-language call handlers (<xref
- linkend="plhandler">). Version-1 code is also more
- portable than version-0, because it does not break restrictions
- on function call protocol in the C standard. For more details
+ linkend="plhandler">). For more details
see <filename>src/backend/utils/fmgr/README</filename> in the
source distribution.
</para>
<para>
Before we turn to the more advanced topics, we should discuss
some coding rules for <productname>PostgreSQL</productname>
- C-language functions. While it may be possible to load functions
+ C-language functions. While it might be possible to load functions
written in languages other than C into
<productname>PostgreSQL</productname>, this is usually difficult
(when it is possible at all) because other languages, such as
<listitem>
<para>
Use <literal>pg_config
- --includedir-server</literal><indexterm><primary>pg_config</><secondary>with user-defined C functions</></>
- to find out where the <productname>PostgreSQL</> server header
+ --includedir-server</literal><indexterm><primary>pg_config</primary><secondary>with user-defined C functions</secondary></indexterm>
+ to find out where the <productname>PostgreSQL</productname> server header
files are installed on your system (or the system that your
- users will be running on). This option is new with
- <productname>PostgreSQL</> 7.2. For
- <productname>PostgreSQL</> 7.1 you should use the option
- <option>--includedir</option>. (<command>pg_config</command>
- will exit with a non-zero status if it encounters an unknown
- option.) For releases prior to 7.1 you will have to guess,
- but since that was before the current calling conventions were
- introduced, it is unlikely that you want to support those
- releases.
+ users will be running on).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Compiling and linking your code so that it can be dynamically
+ loaded into <productname>PostgreSQL</productname> always
+ requires special flags. See <xref linkend="dfunc"> for a
+ detailed explanation of how to do it for your particular
+ operating system.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Remember to define a <quote>magic block</quote> for your shared library,
+ as described in <xref linkend="xfunc-c-dynload">.
</para>
</listitem>
<para>
When allocating memory, use the
<productname>PostgreSQL</productname> functions
- <function>palloc</function><indexterm><primary>palloc</></> and <function>pfree</function><indexterm><primary>pfree</></>
+ <function>palloc</function><indexterm><primary>palloc</primary></indexterm> and <function>pfree</function><indexterm><primary>pfree</primary></indexterm>
instead of the corresponding C library functions
<function>malloc</function> and <function>free</function>.
The memory allocated by <function>palloc</function> will be
<listitem>
<para>
- Always zero the bytes of your structures using
- <function>memset</function>. Without this, it's difficult to
+ Always zero the bytes of your structures using <function>memset</function>
+ (or allocate them with <function>palloc0</function> in the first place).
+ Even if you assign to each field of your structure, there might be
+ alignment padding (holes in the structure) that contain
+ garbage values. Without this, it's difficult to
support hash indexes or hash joins, as you must pick out only
the significant bits of your data structure to compute a hash.
- Even if you initialize all fields of your structure, there may be
- alignment padding (holes in the structure) that may contain
- garbage values.
+ The planner also sometimes relies on comparing constants via
+ bitwise equality, so you can get undesirable planning results if
+ logically-equivalent values aren't bitwise equal.
</para>
</listitem>
(<symbol>PG_FUNCTION_ARGS</symbol>, etc.) are in
<filename>fmgr.h</filename>, so you will need to include at
least these two files. For portability reasons it's best to
- include <filename>postgres.h</filename> <emphasis>first</>,
+ include <filename>postgres.h</filename> <emphasis>first</emphasis>,
before any other system or user header files. Including
<filename>postgres.h</filename> will also include
<filename>elog.h</filename> and <filename>palloc.h</filename>
error messages to this effect.
</para>
</listitem>
-
- <listitem>
- <para>
- Compiling and linking your code so that it can be dynamically
- loaded into <productname>PostgreSQL</productname> always
- requires special flags. See <xref linkend="dfunc"> for a
- detailed explanation of how to do it for your particular
- operating system.
- </para>
- </listitem>
</itemizedlist>
</para>
</sect2>
&dfunc;
- <sect2 id="xfunc-c-pgxs">
- <title>Extension Building Infrastructure</title>
-
- <indexterm zone="xfunc-c-pgxs">
- <primary>pgxs</primary>
- </indexterm>
-
- <para>
- If you are thinking about distributing your
- <productname>PostgreSQL</> extension modules, setting up a
- portable build system for them can be fairly difficult. Therefore
- the <productname>PostgreSQL</> installation provides a build
- infrastructure for extensions, called <acronym>PGXS</acronym>, so
- that simple extension modules can be built simply against an
- already installed server. Note that this infrastructure is not
- intended to be a universal build system framework that can be used
- to build all software interfacing to <productname>PostgreSQL</>;
- it simply automates common build rules for simple server extension
- modules. For more complicated packages, you need to write your
- own build system.
- </para>
-
- <para>
- To use the infrastructure for your extension, you must write a
- simple makefile. In that makefile, you need to set some variables
- and finally include the global <acronym>PGXS</acronym> makefile.
- Here is an example that builds an extension module named
- <literal>isbn_issn</literal> consisting of a shared library, an
- SQL script, and a documentation text file:
-<programlisting>
-MODULES = isbn_issn
-DATA_built = isbn_issn.sql
-DOCS = README.isbn_issn
-
-PGXS := $(shell pg_config --pgxs)
-include $(PGXS)
-</programlisting>
- The last two lines should always be the same. Earlier in the
- file, you assign variables or add custom
- <application>make</application> rules.
- </para>
-
- <para>
- The following variables can be set:
-
- <variablelist>
- <varlistentry>
- <term><varname>MODULES</varname></term>
- <listitem>
- <para>
- list of shared objects to be built from source file with same
- stem (do not include suffix in this list)
- </para>
- </listitem>
- </varlistentry>
-
- <varlistentry>
- <term><varname>DATA</varname></term>
- <listitem>
- <para>
- random files to install into <literal><replaceable>prefix</replaceable>/share/contrib</literal>
- </para>
- </listitem>
- </varlistentry>
-
- <varlistentry>
- <term><varname>DATA_built</varname></term>
- <listitem>
- <para>
- random files to install into
- <literal><replaceable>prefix</replaceable>/share/contrib</literal>,
- which need to be built first
- </para>
- </listitem>
- </varlistentry>
-
- <varlistentry>
- <term><varname>DOCS</varname></term>
- <listitem>
- <para>
- random files to install under
- <literal><replaceable>prefix</replaceable>/doc/contrib</literal>
- </para>
- </listitem>
- </varlistentry>
-
- <varlistentry>
- <term><varname>SCRIPTS</varname></term>
- <listitem>
- <para>
- script files (not binaries) to install into
- <literal><replaceable>prefix</replaceable>/bin</literal>
- </para>
- </listitem>
- </varlistentry>
-
- <varlistentry>
- <term><varname>SCRIPTS_built</varname></term>
- <listitem>
- <para>
- script files (not binaries) to install into
- <literal><replaceable>prefix</replaceable>/bin</literal>,
- which need to be built first
- </para>
- </listitem>
- </varlistentry>
-
- <varlistentry>
- <term><varname>REGRESS</varname></term>
- <listitem>
- <para>
- list of regression test cases (without suffix)
- </para>
- </listitem>
- </varlistentry>
- </variablelist>
-
- or at most one of these two:
-
- <variablelist>
- <varlistentry>
- <term><varname>PROGRAM</varname></term>
- <listitem>
- <para>
- a binary program to build (list objects files in <varname>OBJS</varname>)
- </para>
- </listitem>
- </varlistentry>
-
- <varlistentry>
- <term><varname>MODULE_big</varname></term>
- <listitem>
- <para>
- a shared object to build (list object files in <varname>OBJS</varname>)
- </para>
- </listitem>
- </varlistentry>
- </variablelist>
-
- The following can also be set:
-
- <variablelist>
-
- <varlistentry>
- <term><varname>EXTRA_CLEAN</varname></term>
- <listitem>
- <para>
- extra files to remove in <literal>make clean</literal>
- </para>
- </listitem>
- </varlistentry>
-
- <varlistentry>
- <term><varname>PG_CPPFLAGS</varname></term>
- <listitem>
- <para>
- will be added to <varname>CPPFLAGS</varname>
- </para>
- </listitem>
- </varlistentry>
-
- <varlistentry>
- <term><varname>PG_LIBS</varname></term>
- <listitem>
- <para>
- will be added to <varname>PROGRAM</varname> link line
- </para>
- </listitem>
- </varlistentry>
-
- <varlistentry>
- <term><varname>SHLIB_LINK</varname></term>
- <listitem>
- <para>
- will be added to <varname>MODULE_big</varname> link line
- </para>
- </listitem>
- </varlistentry>
- </variablelist>
- </para>
-
- <para>
- Put this makefile as <literal>Makefile</literal> in the directory
- which holds your extension. Then you can do
- <literal>make</literal> to compile, and later <literal>make
- install</literal> to install your module. The extension is
- compiled and installed for the
- <productname>PostgreSQL</productname> installation that
- corresponds to the first <command>pg_config</command> command
- found in your path.
- </para>
- </sect2>
-
-
<sect2>
- <title>Composite-Type Arguments in C-Language Functions</title>
+ <title>Composite-type Arguments</title>
<para>
Composite types do not have a fixed layout like C structures.
- Instances of a composite type may contain null fields. In
+ Instances of a composite type can contain null fields. In
addition, composite types that are part of an inheritance
- hierarchy may have different fields than other members of the
+ hierarchy can have different fields than other members of the
same inheritance hierarchy. Therefore,
<productname>PostgreSQL</productname> provides a function
interface for accessing fields of composite types from C.
</para>
<para>
- Suppose we want to write a function to answer the query
+ Suppose we want to write a function to answer the query:
<programlisting>
SELECT name, c_overpaid(emp, 1500) AS overpaid
WHERE name = 'Bill' OR name = 'Sam';
</programlisting>
- Using call conventions version 0, we can define
- <function>c_overpaid</> as:
-
-<programlisting>
-#include "postgres.h"
-#include "executor/executor.h" /* for GetAttributeByName() */
-
-bool
-c_overpaid(HeapTupleHeader t, /* the current row of emp */
- int32 limit)
-{
- bool isnull;
- int32 salary;
-
- salary = DatumGetInt32(GetAttributeByName(t, "salary", &isnull));
- if (isnull)
- return false;
- return salary > limit;
-}
-</programlisting>
-
- In version-1 coding, the above would look like this:
+ Using the version-1 calling conventions, we can define
+ <function>c_overpaid</function> as:
-<programlisting>
+<programlisting><![CDATA[
#include "postgres.h"
#include "executor/executor.h" /* for GetAttributeByName() */
+PG_MODULE_MAGIC;
+
PG_FUNCTION_INFO_V1(c_overpaid);
Datum
bool isnull;
Datum salary;
- salary = GetAttributeByName(t, "salary", &isnull);
+ salary = GetAttributeByName(t, "salary", &isnull);
if (isnull)
PG_RETURN_BOOL(false);
/* Alternatively, we might prefer to do PG_RETURN_NULL() for null salary. */
- PG_RETURN_BOOL(DatumGetInt32(salary) > limit);
+ PG_RETURN_BOOL(DatumGetInt32(salary) > limit);
}
+]]>
</programlisting>
</para>
<para>
- <function>GetAttributeByName</function> is the
+ <function>GetAttributeByName</function> is the
<productname>PostgreSQL</productname> system function that
returns attributes out of the specified row. It has
three arguments: the argument of type <type>HeapTupleHeader</type> passed
- into
+ into
the function, the name of the desired attribute, and a
return parameter that tells whether the attribute
is null. <function>GetAttributeByName</function> returns a <type>Datum</type>
LANGUAGE C STRICT;
</programlisting>
- Notice we have used <literal>STRICT</> so that we did not have to
+ Notice we have used <literal>STRICT</literal> so that we did not have to
check whether the input arguments were NULL.
</para>
</sect2>
<sect2>
- <title>Returning Rows (Composite Types) from C-Language Functions</title>
+ <title>Returning Rows (Composite Types)</title>
<para>
To return a row or composite-type value from a C-language
<para>
There are two ways you can build a composite data value (henceforth
- a <quote>tuple</>): you can build it from an array of Datum values,
+ a <quote>tuple</quote>): you can build it from an array of Datum values,
or from an array of C strings that can be passed to the input
conversion functions of the tuple's column data types. In either
- case, you first need to obtain or construct a <structname>TupleDesc</>
+ case, you first need to obtain or construct a <structname>TupleDesc</structname>
descriptor for the tuple structure. When working with Datums, you
- pass the <structname>TupleDesc</> to <function>BlessTupleDesc</>,
- and then call <function>heap_form_tuple</> for each row. When working
- with C strings, you pass the <structname>TupleDesc</> to
- <function>TupleDescGetAttInMetadata</>, and then call
- <function>BuildTupleFromCStrings</> for each row. In the case of a
+ pass the <structname>TupleDesc</structname> to <function>BlessTupleDesc</function>,
+ and then call <function>heap_form_tuple</function> for each row. When working
+ with C strings, you pass the <structname>TupleDesc</structname> to
+ <function>TupleDescGetAttInMetadata</function>, and then call
+ <function>BuildTupleFromCStrings</function> for each row. In the case of a
function returning a set of tuples, the setup steps can all be done
once during the first call of the function.
</para>
<para>
Several helper functions are available for setting up the needed
- <structname>TupleDesc</>. The recommended way to do this in most
- functions returning composite values is to call
+ <structname>TupleDesc</structname>. The recommended way to do this in most
+ functions returning composite values is to call:
<programlisting>
TypeFuncClass get_call_result_type(FunctionCallInfo fcinfo,
Oid *resultTypeId,
TupleDesc *resultTupleDesc)
</programlisting>
- passing the same <literal>fcinfo</> struct passed to the calling function
+ passing the same <literal>fcinfo</literal> struct passed to the calling function
itself. (This of course requires that you use the version-1
- calling conventions.) <varname>resultTypeId</> can be specified
- as <literal>NULL</> or as the address of a local variable to receive the
- function's result type OID. <varname>resultTupleDesc</> should be the
- address of a local <structname>TupleDesc</> variable. Check that the
- result is <literal>TYPEFUNC_COMPOSITE</>; if so,
- <varname>resultTupleDesc</> has been filled with the needed
- <structname>TupleDesc</>. (If it is not, you can report an error along
+ calling conventions.) <varname>resultTypeId</varname> can be specified
+ as <literal>NULL</literal> or as the address of a local variable to receive the
+ function's result type OID. <varname>resultTupleDesc</varname> should be the
+ address of a local <structname>TupleDesc</structname> variable. Check that the
+ result is <literal>TYPEFUNC_COMPOSITE</literal>; if so,
+ <varname>resultTupleDesc</varname> has been filled with the needed
+ <structname>TupleDesc</structname>. (If it is not, you can report an error along
the lines of <quote>function returning record called in context that
cannot accept type record</quote>.)
</para>
<tip>
<para>
- <function>get_call_result_type</> can resolve the actual type of a
+ <function>get_call_result_type</function> can resolve the actual type of a
polymorphic function result; so it is useful in functions that return
scalar polymorphic results, not only functions that return composites.
- The <varname>resultTypeId</> output is primarily useful for functions
+ The <varname>resultTypeId</varname> output is primarily useful for functions
returning polymorphic scalars.
</para>
</tip>
<note>
<para>
- <function>get_call_result_type</> has a sibling
- <function>get_expr_result_type</>, which can be used to resolve the
+ <function>get_call_result_type</function> has a sibling
+ <function>get_expr_result_type</function>, which can be used to resolve the
expected output type for a function call represented by an expression
tree. This can be used when trying to determine the result type from
outside the function itself. There is also
- <function>get_func_result_type</>, which can be used when only the
+ <function>get_func_result_type</function>, which can be used when only the
function's OID is available. However these functions are not able
- to deal with functions declared to return <structname>record</>, and
- <function>get_func_result_type</> cannot resolve polymorphic types,
- so you should preferentially use <function>get_call_result_type</>.
+ to deal with functions declared to return <structname>record</structname>, and
+ <function>get_func_result_type</function> cannot resolve polymorphic types,
+ so you should preferentially use <function>get_call_result_type</function>.
</para>
</note>
<para>
Older, now-deprecated functions for obtaining
- <structname>TupleDesc</>s are
+ <structname>TupleDesc</structname>s are:
<programlisting>
TupleDesc RelationNameGetTupleDesc(const char *relname)
</programlisting>
- to get a <structname>TupleDesc</> for the row type of a named relation,
- and
+ to get a <structname>TupleDesc</structname> for the row type of a named relation,
+ and:
<programlisting>
TupleDesc TypeGetTupleDesc(Oid typeoid, List *colaliases)
</programlisting>
- to get a <structname>TupleDesc</> based on a type OID. This can
- be used to get a <structname>TupleDesc</> for a base or
+ to get a <structname>TupleDesc</structname> based on a type OID. This can
+ be used to get a <structname>TupleDesc</structname> for a base or
composite type. It will not work for a function that returns
- <structname>record</>, however, and it cannot resolve polymorphic
+ <structname>record</structname>, however, and it cannot resolve polymorphic
types.
</para>
<para>
- Once you have a <structname>TupleDesc</>, call
+ Once you have a <structname>TupleDesc</structname>, call:
<programlisting>
TupleDesc BlessTupleDesc(TupleDesc tupdesc)
</programlisting>
- if you plan to work with Datums, or
+ if you plan to work with Datums, or:
<programlisting>
AttInMetadata *TupleDescGetAttInMetadata(TupleDesc tupdesc)
</programlisting>
if you plan to work with C strings. If you are writing a function
returning set, you can save the results of these functions in the
- <structname>FuncCallContext</> structure — use the
- <structfield>tuple_desc</> or <structfield>attinmeta</> field
+ <structname>FuncCallContext</structname> structure — use the
+ <structfield>tuple_desc</structfield> or <structfield>attinmeta</structfield> field
respectively.
</para>
<para>
- When working with Datums, use
+ When working with Datums, use:
<programlisting>
HeapTuple heap_form_tuple(TupleDesc tupdesc, Datum *values, bool *isnull)
</programlisting>
- to build a <structname>HeapTuple</> given user data in Datum form.
+ to build a <structname>HeapTuple</structname> given user data in Datum form.
</para>
<para>
- When working with C strings, use
+ When working with C strings, use:
<programlisting>
HeapTuple BuildTupleFromCStrings(AttInMetadata *attinmeta, char **values)
</programlisting>
- to build a <structname>HeapTuple</> given user data
- in C string form. <literal>values</literal> is an array of C strings,
+ to build a <structname>HeapTuple</structname> given user data
+ in C string form. <parameter>values</parameter> is an array of C strings,
one for each attribute of the return row. Each C string should be in
the form expected by the input function of the attribute data
type. In order to return a null value for one of the attributes,
- the corresponding pointer in the <parameter>values</> array
- should be set to <symbol>NULL</>. This function will need to
+ the corresponding pointer in the <parameter>values</parameter> array
+ should be set to <symbol>NULL</symbol>. This function will need to
be called again for each row you return.
</para>
<para>
Once you have built a tuple to return from your function, it
- must be converted into a <type>Datum</>. Use
+ must be converted into a <type>Datum</type>. Use:
<programlisting>
HeapTupleGetDatum(HeapTuple tuple)
</programlisting>
- to convert a <structname>HeapTuple</> into a valid Datum. This
- <type>Datum</> can be returned directly if you intend to return
+ to convert a <structname>HeapTuple</structname> into a valid Datum. This
+ <type>Datum</type> can be returned directly if you intend to return
just a single row, or it can be used as the current return value
in a set-returning function.
</para>
</sect2>
<sect2 id="xfunc-c-return-set">
- <title>Returning Sets from C-Language Functions</title>
+ <title>Returning Sets</title>
<para>
There is also a special API that provides support for returning
</para>
<para>
- A set-returning function (<acronym>SRF</>) is called
- once for each item it returns. The <acronym>SRF</> must
+ A set-returning function (<acronym>SRF</acronym>) is called
+ once for each item it returns. The <acronym>SRF</acronym> must
therefore save enough state to remember what it was doing and
return the next item on each call.
- The structure <structname>FuncCallContext</> is provided to help
- control this process. Within a function, <literal>fcinfo->flinfo->fn_extra</>
- is used to hold a pointer to <structname>FuncCallContext</>
+ The structure <structname>FuncCallContext</structname> is provided to help
+ control this process. Within a function, <literal>fcinfo->flinfo->fn_extra</literal>
+ is used to hold a pointer to <structname>FuncCallContext</structname>
across calls.
<programlisting>
-typedef struct
+typedef struct FuncCallContext
{
/*
* Number of times we've been called before
- *
+ *
* call_cntr is initialized to 0 for you by SRF_FIRSTCALL_INIT(), and
* incremented for you every time SRF_RETURN_NEXT() is called.
*/
- uint32 call_cntr;
+ uint64 call_cntr;
/*
* OPTIONAL maximum number of calls
* If not set, you must provide alternative means to know when the
* function is done.
*/
- uint32 max_calls;
+ uint64 max_calls;
/*
* OPTIONAL pointer to result slot
- *
- * This is obsolete and only present for backwards compatibility, viz,
+ *
+ * This is obsolete and only present for backward compatibility, viz,
* user-defined SRFs that use the deprecated TupleDescGetSlot().
*/
TupleTableSlot *slot;
/*
* OPTIONAL pointer to miscellaneous user-provided context information
- *
+ *
* user_fctx is for use as a pointer to your own data to retain
* arbitrary context information between calls of your function.
*/
/*
* OPTIONAL pointer to struct containing attribute type input metadata
- *
+ *
* attinmeta is for use when returning tuples (i.e., composite data types)
* and is not used when returning base data types. It is only needed
* if you intend to use BuildTupleFromCStrings() to create the return
/*
* OPTIONAL pointer to struct containing tuple description
*
- * tuple_desc is for use when returning tuples (i.e. composite data types)
+ * tuple_desc is for use when returning tuples (i.e., composite data types)
* and is only needed if you are going to build the tuples with
* heap_form_tuple() rather than with BuildTupleFromCStrings(). Note that
* the TupleDesc pointer stored here should usually have been run through
</para>
<para>
- An <acronym>SRF</> uses several functions and macros that
- automatically manipulate the <structname>FuncCallContext</>
- structure (and expect to find it via <literal>fn_extra</>). Use
+ An <acronym>SRF</acronym> uses several functions and macros that
+ automatically manipulate the <structname>FuncCallContext</structname>
+ structure (and expect to find it via <literal>fn_extra</literal>). Use:
<programlisting>
SRF_IS_FIRSTCALL()
</programlisting>
to determine if your function is being called for the first or a
- subsequent time. On the first call (only) use
+ subsequent time. On the first call (only) use:
<programlisting>
SRF_FIRSTCALL_INIT()
</programlisting>
- to initialize the <structname>FuncCallContext</>. On every function call,
- including the first, use
+ to initialize the <structname>FuncCallContext</structname>. On every function call,
+ including the first, use:
<programlisting>
SRF_PERCALL_SETUP()
</programlisting>
- to properly set up for using the <structname>FuncCallContext</>
+ to properly set up for using the <structname>FuncCallContext</structname>
and clearing any previously returned data left over from the
previous pass.
</para>
<para>
- If your function has data to return, use
+ If your function has data to return, use:
<programlisting>
SRF_RETURN_NEXT(funcctx, result)
</programlisting>
- to return it to the caller. (<literal>result</> must be of type
- <type>Datum</>, either a single value or a tuple prepared as
+ to return it to the caller. (<literal>result</literal> must be of type
+ <type>Datum</type>, either a single value or a tuple prepared as
described above.) Finally, when your function is finished
- returning data, use
+ returning data, use:
<programlisting>
SRF_RETURN_DONE(funcctx)
</programlisting>
- to clean up and end the <acronym>SRF</>.
+ to clean up and end the <acronym>SRF</acronym>.
</para>
<para>
- The memory context that is current when the <acronym>SRF</> is called is
+ The memory context that is current when the <acronym>SRF</acronym> is called is
a transient context that will be cleared between calls. This means
- that you do not need to call <function>pfree</> on everything
- you allocated using <function>palloc</>; it will go away anyway. However, if you want to allocate
+ that you do not need to call <function>pfree</function> on everything
+ you allocated using <function>palloc</function>; it will go away anyway. However, if you want to allocate
any data structures to live across calls, you need to put them somewhere
else. The memory context referenced by
- <structfield>multi_call_memory_ctx</> is a suitable location for any
- data that needs to survive until the <acronym>SRF</> is finished running. In most
+ <structfield>multi_call_memory_ctx</structfield> is a suitable location for any
+ data that needs to survive until the <acronym>SRF</acronym> is finished running. In most
cases, this means that you should switch into
- <structfield>multi_call_memory_ctx</> while doing the first-call setup.
+ <structfield>multi_call_memory_ctx</structfield> while doing the first-call setup.
</para>
+ <warning>
+ <para>
+ While the actual arguments to the function remain unchanged between
+ calls, if you detoast the argument values (which is normally done
+ transparently by the
+ <function>PG_GETARG_<replaceable>xxx</replaceable></function> macro)
+ in the transient context then the detoasted copies will be freed on
+ each cycle. Accordingly, if you keep references to such values in
+ your <structfield>user_fctx</structfield>, you must either copy them into the
+ <structfield>multi_call_memory_ctx</structfield> after detoasting, or ensure
+ that you detoast the values only in that context.
+ </para>
+ </warning>
+
<para>
A complete pseudo-code example looks like the following:
<programlisting>
{
FuncCallContext *funcctx;
Datum result;
- MemoryContext oldcontext;
<replaceable>further declarations as needed</replaceable>
if (SRF_IS_FIRSTCALL())
{
+ MemoryContext oldcontext;
+
funcctx = SRF_FIRSTCALL_INIT();
oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
/* One-time setup code appears here: */
</para>
<para>
- A complete example of a simple <acronym>SRF</> returning a composite type
+ A complete example of a simple <acronym>SRF</acronym> returning a composite type
looks like:
-<programlisting>
+<programlisting><![CDATA[
PG_FUNCTION_INFO_V1(retcomposite);
Datum
TupleDesc tupdesc;
AttInMetadata *attinmeta;
- /* stuff done only on the first call of the function */
- if (SRF_IS_FIRSTCALL())
- {
+ /* stuff done only on the first call of the function */
+ if (SRF_IS_FIRSTCALL())
+ {
MemoryContext oldcontext;
/* create a function context for cross-call persistence */
funcctx = SRF_FIRSTCALL_INIT();
/* switch to memory context appropriate for multiple function calls */
- oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
+ oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
/* total number of tuples to be returned */
- funcctx->max_calls = PG_GETARG_UINT32(0);
+ funcctx->max_calls = PG_GETARG_UINT32(0);
/* Build a tuple descriptor for our result type */
- if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
+ if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("function returning record called in context "
* C strings
*/
attinmeta = TupleDescGetAttInMetadata(tupdesc);
- funcctx->attinmeta = attinmeta;
+ funcctx->attinmeta = attinmeta;
MemoryContextSwitchTo(oldcontext);
}
/* stuff done on every call of the function */
funcctx = SRF_PERCALL_SETUP();
- call_cntr = funcctx->call_cntr;
- max_calls = funcctx->max_calls;
- attinmeta = funcctx->attinmeta;
-
- if (call_cntr < max_calls) /* do when there is more left to send */
+ call_cntr = funcctx->call_cntr;
+ max_calls = funcctx->max_calls;
+ attinmeta = funcctx->attinmeta;
+
+ if (call_cntr < max_calls) /* do when there is more left to send */
{
char **values;
HeapTuple tuple;
SRF_RETURN_DONE(funcctx);
}
}
+]]>
</programlisting>
One way to declare this function in SQL is:
CREATE OR REPLACE FUNCTION retcomposite(integer, integer)
RETURNS SETOF __retcomposite
- AS '<replaceable>filename</>', 'retcomposite'
+ AS '<replaceable>filename</replaceable>', 'retcomposite'
LANGUAGE C IMMUTABLE STRICT;
</programlisting>
A different way is to use OUT parameters:
CREATE OR REPLACE FUNCTION retcomposite(IN integer, IN integer,
OUT f1 integer, OUT f2 integer, OUT f3 integer)
RETURNS SETOF record
- AS '<replaceable>filename</>', 'retcomposite'
+ AS '<replaceable>filename</replaceable>', 'retcomposite'
LANGUAGE C IMMUTABLE STRICT;
</programlisting>
Notice that in this method the output type of the function is formally
- an anonymous <structname>record</> type.
+ an anonymous <structname>record</structname> type.
</para>
<para>
- The directory <filename>contrib/tablefunc</> in the source
- distribution contains more examples of set-returning functions.
+ The directory <link linkend="tablefunc"><filename>contrib/tablefunc</filename></link>
+ module in the source distribution contains more examples of
+ set-returning functions.
</para>
</sect2>
<title>Polymorphic Arguments and Return Types</title>
<para>
- C-language functions may be declared to accept and
+ C-language functions can be declared to accept and
return the polymorphic types
- <type>anyelement</type> and <type>anyarray</type>.
+ <type>anyelement</type>, <type>anyarray</type>, <type>anynonarray</type>,
+ <type>anyenum</type>, and <type>anyrange</type>.
See <xref linkend="extend-types-polymorphic"> for a more detailed explanation
of polymorphic functions. When function arguments or return types
are defined as polymorphic types, the function author cannot know
in advance what data type it will be called with, or
- need to return. There are two routines provided in <filename>fmgr.h</>
+ need to return. There are two routines provided in <filename>fmgr.h</filename>
to allow a version-1 C function to discover the actual data types
of its arguments and the type it is expected to return. The routines are
- called <literal>get_fn_expr_rettype(FmgrInfo *flinfo)</> and
- <literal>get_fn_expr_argtype(FmgrInfo *flinfo, int argnum)</>.
+ called <literal>get_fn_expr_rettype(FmgrInfo *flinfo)</literal> and
+ <literal>get_fn_expr_argtype(FmgrInfo *flinfo, int argnum)</literal>.
They return the result or argument type OID, or <symbol>InvalidOid</symbol> if the
information is not available.
- The structure <literal>flinfo</> is normally accessed as
- <literal>fcinfo->flinfo</>. The parameter <literal>argnum</>
- is zero based. <function>get_call_result_type</> can also be used
- as an alternative to <function>get_fn_expr_rettype</>.
+ The structure <literal>flinfo</literal> is normally accessed as
+ <literal>fcinfo->flinfo</literal>. The parameter <literal>argnum</literal>
+ is zero based. <function>get_call_result_type</function> can also be used
+ as an alternative to <function>get_fn_expr_rettype</function>.
+ There is also <function>get_fn_expr_variadic</function>, which can be used to
+ find out whether variadic arguments have been merged into an array.
+ This is primarily useful for <literal>VARIADIC "any"</literal> functions,
+ since such merging will always have occurred for variadic functions
+ taking ordinary array types.
</para>
<para>
ArrayType *result;
Oid element_type = get_fn_expr_argtype(fcinfo->flinfo, 0);
Datum element;
+ bool isnull;
int16 typlen;
bool typbyval;
char typalign;
if (!OidIsValid(element_type))
elog(ERROR, "could not determine data type of input");
- /* get the provided element */
- element = PG_GETARG_DATUM(0);
+ /* get the provided element, being careful in case it's NULL */
+ isnull = PG_ARGISNULL(0);
+ if (isnull)
+ element = (Datum) 0;
+ else
+ element = PG_GETARG_DATUM(0);
/* we have one dimension */
ndims = 1;
get_typlenbyvalalign(element_type, &typlen, &typbyval, &typalign);
/* now build the array */
- result = construct_md_array(&element, ndims, dims, lbs,
+ result = construct_md_array(&element, &isnull, ndims, dims, lbs,
element_type, typlen, typbyval, typalign);
PG_RETURN_ARRAYTYPE_P(result);
<programlisting>
CREATE FUNCTION make_array(anyelement) RETURNS anyarray
AS '<replaceable>DIRECTORY</replaceable>/funcs', 'make_array'
- LANGUAGE C STRICT;
+ LANGUAGE C IMMUTABLE;
</programlisting>
+ </para>
- Note the use of <literal>STRICT</literal>; this is essential
- since the code is not bothering to test for a null input.
+ <para>
+ There is a variant of polymorphism that is only available to C-language
+ functions: they can be declared to take parameters of type
+ <literal>"any"</literal>. (Note that this type name must be double-quoted,
+ since it's also a SQL reserved word.) This works like
+ <type>anyelement</type> except that it does not constrain different
+ <literal>"any"</literal> arguments to be the same type, nor do they help
+ determine the function's result type. A C-language function can also
+ declare its final parameter to be <literal>VARIADIC "any"</literal>. This will
+ match one or more actual arguments of any type (not necessarily the same
+ type). These arguments will <emphasis>not</emphasis> be gathered into an array
+ as happens with normal variadic functions; they will just be passed to
+ the function separately. The <function>PG_NARGS()</function> macro and the
+ methods described above must be used to determine the number of actual
+ arguments and their types when using this feature. Also, users of such
+ a function might wish to use the <literal>VARIADIC</literal> keyword in their
+ function call, with the expectation that the function would treat the
+ array elements as separate arguments. The function itself must implement
+ that behavior if wanted, after using <function>get_fn_expr_variadic</function> to
+ detect that the actual argument was marked with <literal>VARIADIC</literal>.
+ </para>
+ </sect2>
+
+ <sect2 id="xfunc-transform-functions">
+ <title>Transform Functions</title>
+
+ <para>
+ Some function calls can be simplified during planning based on
+ properties specific to the function. For example,
+ <literal>int4mul(n, 1)</literal> could be simplified to just <literal>n</literal>.
+ To define such function-specific optimizations, write a
+ <firstterm>transform function</firstterm> and place its OID in the
+ <structfield>protransform</structfield> field of the primary function's
+ <structname>pg_proc</structname> entry. The transform function must have the SQL
+ signature <literal>protransform(internal) RETURNS internal</literal>. The
+ argument, actually <type>FuncExpr *</type>, is a dummy node representing a
+ call to the primary function. If the transform function's study of the
+ expression tree proves that a simplified expression tree can substitute
+ for all possible concrete calls represented thereby, build and return
+ that simplified expression. Otherwise, return a <literal>NULL</literal>
+ pointer (<emphasis>not</emphasis> a SQL null).
+ </para>
+
+ <para>
+ We make no guarantee that <productname>PostgreSQL</productname> will never call the
+ primary function in cases that the transform function could simplify.
+ Ensure rigorous equivalence between the simplified expression and an
+ actual call to the primary function.
+ </para>
+
+ <para>
+ Currently, this facility is not exposed to users at the SQL level
+ because of security concerns, so it is only practical to use for
+ optimizing built-in functions.
+ </para>
+ </sect2>
+
+ <sect2>
+ <title>Shared Memory and LWLocks</title>
+
+ <para>
+ Add-ins can reserve LWLocks and an allocation of shared memory on server
+ startup. The add-in's shared library must be preloaded by specifying
+ it in
+ <xref linkend="guc-shared-preload-libraries"><indexterm><primary>shared_preload_libraries</primary></indexterm>.
+ Shared memory is reserved by calling:
+<programlisting>
+void RequestAddinShmemSpace(int size)
+</programlisting>
+ from your <function>_PG_init</function> function.
+ </para>
+ <para>
+ LWLocks are reserved by calling:
+<programlisting>
+void RequestNamedLWLockTranche(const char *tranche_name, int num_lwlocks)
+</programlisting>
+ from <function>_PG_init</function>. This will ensure that an array of
+ <literal>num_lwlocks</literal> LWLocks is available under the name
+ <literal>tranche_name</literal>. Use <function>GetNamedLWLockTranche</function>
+ to get a pointer to this array.
+ </para>
+ <para>
+ To avoid possible race-conditions, each backend should use the LWLock
+ <function>AddinShmemInitLock</function> when connecting to and initializing
+ its allocation of shared memory, as shown here:
+<programlisting>
+static mystruct *ptr = NULL;
+
+if (!ptr)
+{
+ bool found;
+
+ LWLockAcquire(AddinShmemInitLock, LW_EXCLUSIVE);
+ ptr = ShmemInitStruct("my struct name", size, &found);
+ if (!found)
+ {
+ initialize contents of shmem area;
+ acquire any requested LWLocks using:
+ ptr->locks = GetNamedLWLockTranche("my tranche name");
+ }
+ LWLockRelease(AddinShmemInitLock);
+}
+</programlisting>
+ </para>
+ </sect2>
+
+ <sect2 id="extend-cpp">
+ <title>Using C++ for Extensibility</title>
+
+ <indexterm zone="extend-cpp">
+ <primary>C++</primary>
+ </indexterm>
+
+ <para>
+ Although the <productname>PostgreSQL</productname> backend is written in
+ C, it is possible to write extensions in C++ if these guidelines are
+ followed:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ All functions accessed by the backend must present a C interface
+ to the backend; these C functions can then call C++ functions.
+ For example, <literal>extern C</literal> linkage is required for
+ backend-accessed functions. This is also necessary for any
+ functions that are passed as pointers between the backend and
+ C++ code.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Free memory using the appropriate deallocation method. For example,
+ most backend memory is allocated using <function>palloc()</function>, so use
+ <function>pfree()</function> to free it. Using C++
+ <function>delete</function> in such cases will fail.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Prevent exceptions from propagating into the C code (use a catch-all
+ block at the top level of all <literal>extern C</literal> functions). This
+ is necessary even if the C++ code does not explicitly throw any
+ exceptions, because events like out-of-memory can still throw
+ exceptions. Any exceptions must be caught and appropriate errors
+ passed back to the C interface. If possible, compile C++ with
+ <option>-fno-exceptions</option> to eliminate exceptions entirely; in such
+ cases, you must check for failures in your C++ code, e.g. check for
+ NULL returned by <function>new()</function>.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ If calling backend functions from C++ code, be sure that the
+ C++ call stack contains only plain old data structures
+ (<acronym>POD</acronym>). This is necessary because backend errors
+ generate a distant <function>longjmp()</function> that does not properly
+ unroll a C++ call stack with non-POD objects.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+
+ <para>
+ In summary, it is best to place C++ code behind a wall of
+ <literal>extern C</literal> functions that interface to the backend,
+ and avoid exception, memory, and call stack leakage.
</para>
</sect2>
- </sect1>
-<!-- Keep this comment at the end of the file
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+ </sect1>
projects / postgresql / blobdiff