Consequently, while it is possible to define a new
function without defining a new type, the reverse is
not true. We therefore describe how to add new functions
- to <ProductName>Postgres</ProductName> before describing how to add new
- types.
- <ProductName>Postgres</ProductName> <Acronym>SQL</Acronym> provides two types of functions: query
- language functions (functions written in <Acronym>SQL</Acronym> and
- programming language functions (functions written in a
- compiled programming language such as <Acronym>C</Acronym>.) Either kind
+ to <ProductName>Postgres</ProductName> before describing
+ how to add new types.
+ <ProductName>Postgres</ProductName> <Acronym>SQL</Acronym>
+ provides two types of functions: query language functions
+ (functions written in <Acronym>SQL</Acronym> and programming
+ language functions (functions written in a compiled
+ programming language such as <Acronym>C</Acronym>.) Either kind
of function can take a base type, a composite type or
some combination as arguments (parameters). In addition,
both kinds of functions can return a base type or
- a composite type. It's easier to define <Acronym>SQL</Acronym> functions,
- so we'll start with those.
- Examples in this section can also be found in <FileName>funcs.sql</FileName>
- and <FileName>C-code/funcs.c</FileName>.
+ a composite type. It's easier to define <Acronym>SQL</Acronym>
+ functions, so we'll start with those. Examples in this section
+ can also be found in <FileName>funcs.sql</FileName>
+ and <FileName>funcs.c</FileName>.
</Para>
<Sect1>
instead of one.
</Para>
<Para>
- It's almost as easy to define <Acronym>SQL</Acronym> functions that take
- base types as arguments. In the example below, notice
+ It's 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 $1
and $2.
SELECT name, double_salary(EMP) AS dream
FROM EMP
- WHERE EMP.dept = 'toy';
+ WHERE EMP.cubicle ~= '(2,1)'::point;
+
+-----+-------+
|name | dream |
AS 'SELECT \'None\'::text AS name,
1000 AS salary,
25 AS age,
- \'none\'::text AS dept;'
+ \'(2,2)\'::point AS cubicle'
LANGUAGE 'sql';
</ProgramListing>
<ItemizedList>
<ListItem>
<Para>
-The target list order must be exactly the same as
- that in which the attributes appear in the CREATE
- TABLE statement (or when you execute a .* query).
+ The target list order must be exactly the same as
+ that in which the attributes appear in the CREATE
+ TABLE statement (or when you execute a .* query).
</Para>
</ListItem>
<ListItem>
<Para>
-You must typecast the expressions
- (using ::) very carefully or you will see the following error:
+You must typecast the expressions (using ::) very carefully
+or you will see the following error:
<ProgramListing>
- WARN::function declared to return type EMP does not retrieve (EMP.*)
+ WARN::function declared to return type EMP does not retrieve (EMP.*)
</ProgramListing>
</Para>
</ListItem>
</ItemizedList>
</para>
<Para>
- Any collection of commands in the <Acronym>SQL</Acronym> query language
- can be packaged together and defined as a function.
- The commands can include updates (i.e., <Acronym>insert</Acronym>, <Acronym>update</Acronym>
- and <Acronym>delete</Acronym>) as well as <Acronym>select</Acronym> queries. However, the
- final command must be a <Acronym>select</Acronym> that returns whatever is
+ Any collection of commands in the <Acronym>SQL</Acronym> query
+ language can be packaged together and defined as a function.
+ The commands can include updates (i.e., <Acronym>insert</Acronym>,
+ <Acronym>update</Acronym> and <Acronym>delete</Acronym>) as well
+ as <Acronym>select</Acronym> queries. However, the final command
+ must be a <Acronym>select</Acronym> that returns whatever is
specified as the function's returntype.
<ProgramListing>
+--+
|1 |
+--+
+
</ProgramListing>
</Para>
</sect2>
<Title>Programming Language Functions on Base Types</Title>
<Para>
- Internally, <ProductName>Postgres</ProductName> regards a base type as a "blob of
- memory." The user-defined functions that you define
- over a type in turn define the way that <ProductName>Postgres</ProductName> can
- operate on it. That is, <ProductName>Postgres</ProductName> will only store and
- retrieve the data from disk and use your user-defined
- functions to input, process, and output the data.
+ Internally, <ProductName>Postgres</ProductName> regards a
+ base type as a "blob of memory." The user-defined
+ functions that you define over a type in turn define the
+ way that <ProductName>Postgres</ProductName> can operate
+ on it. That is, <ProductName>Postgres</ProductName> will
+ only store and retrieve the data from disk and use your
+ user-defined functions to input, process, and output the data.
Base types can have one of three internal formats:
<ItemizedList>
<ListItem><Para>pass by value, fixed-length</Para>
<Para>
By-value types can only be 1, 2 or 4 bytes in length
(even if your computer supports by-value types of other
- sizes). <ProductName>Postgres</ProductName> itself only passes integer types by
- value. You should be careful to define your types such
- that they will be the same size (in bytes) on all
- architectures. For example, the <Acronym>long</Acronym> type is dangerous
- because it is 4 bytes on some machines and 8 bytes on
- others, whereas <Acronym>int</Acronym> type is 4 bytes on most <Acronym>UNIX</Acronym>
- machines (though not on most personal computers). A
- reasonable implementation of the <Acronym>int4</Acronym> type on <Acronym>UNIX</Acronym>
+ sizes). <ProductName>Postgres</ProductName> itself
+ only passes integer types by value. You should be careful
+ to define your types such that they will be the same
+ size (in bytes) on all architectures. For example, the
+ <Acronym>long</Acronym> type is dangerous because it
+ is 4 bytes on some machines and 8 bytes on others, whereas
+ <Acronym>int</Acronym> type is 4 bytes on most
+ <Acronym>UNIX</Acronym> machines (though not on most
+ personal computers). A reasonable implementation of
+ the <Acronym>int4</Acronym> type on <Acronym>UNIX</Acronym>
machines might be:
<ProgramListing>
<ProgramListing>
/* 16-byte structure, passed by reference */
- typedef struct {
- char data[16];
- } char16;
+ typedef struct
+ {
+ double x, y;
+ } Point;
</ProgramListing>
</Para>
<Para>
Obviously, the data field is not long enough to hold
all possible strings -- it's impossible to declare such
- a structure in <Acronym>C</Acronym>. When manipulating variable-length
- types, we must be careful to allocate the correct
- amount of memory and initialize the length field. For
- example, if we wanted to store 40 bytes in a text
+ a structure in <Acronym>C</Acronym>. When manipulating
+ variable-length types, we must be careful to allocate
+ the correct amount of memory and initialize the length field.
+ For example, if we wanted to store 40 bytes in a text
structure, we might use a code fragment like this:
<ProgramListing>
#include "postgres.h"
<ProgramListing>
#include <string.h>
#include "postgres.h"
+
+ /* By Value */
+
int
add_one(int arg)
{
return(arg + 1);
}
- text *
- concat_text(text *arg1, text *arg2)
+
+ /* By Reference, Fixed Length */
+
+ Point *
+ makepoint(Point *pointx, Point *pointy )
{
- int32 new_text_size = VARSIZE(arg1) + VARSIZE(arg2) - VARHDRSZ;
- text *new_text = (text *) palloc(new_text_size);
-
- memset((void *) new_text, 0, new_text_size);
- VARSIZE(new_text) = new_text_size;
- strncpy(VARDATA(new_text), VARDATA(arg1), VARSIZE(arg1)-VARHDRSZ);
- strncat(VARDATA(new_text), VARDATA(arg2), VARSIZE(arg2)-VARHDRSZ);
- return (new_text);
+ 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(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);
+
+ memset((void *) new_text, 0, new_text_size);
+ VARSIZE(new_text) = new_text_size;
+ strncpy(VARDATA(new_text), VARDATA(arg1), VARSIZE(arg1)-VARHDRSZ);
+ strncat(VARDATA(new_text), VARDATA(arg2), VARSIZE(arg2)-VARHDRSZ);
+ return (new_text);
+ }
</ProgramListing>
</Para>
<ProgramListing>
CREATE FUNCTION add_one(int4) RETURNS int4
- AS 'PGROOT/tutorial/obj/funcs.so' LANGUAGE 'c';
+ AS 'PGROOT/tutorial/funcs.so' LANGUAGE 'c';
+ CREATE FUNCTION makepoint(point, point) RETURNS point
+ AS 'PGROOT/tutorial/funcs.so' LANGUAGE 'c';
+
CREATE FUNCTION concat_text(text, text) RETURNS text
- AS 'PGROOT/tutorial/obj/funcs.so' LANGUAGE 'c';
-
+ AS 'PGROOT/tutorial/funcs.so' LANGUAGE 'c';
+
CREATE FUNCTION copytext(text) RETURNS text
- AS 'PGROOT/tutorial/obj/funcs.so' LANGUAGE 'c';
+ AS 'PGROOT/tutorial/funcs.so' LANGUAGE 'c';
</ProgramListing>
</Para>
null fields. In addition, composite types that are
part of an inheritance hierarchy may have different
fields than other members of the same inheritance hierarchy.
- Therefore, <ProductName>Postgres</ProductName> provides a procedural
- interface for accessing fields of composite types from
- C.
- As <ProductName>Postgres</ProductName> processes a set of instances, each instance
-
will be passed into your function as an opaque structure of type <Acronym>TUPLE</Acronym>.
+ Therefore, <ProductName>Postgres</ProductName> provides
+ a procedural interface for accessing fields of composite types
+ from C. As <ProductName>Postgres</ProductName> processes
+ a set of instances, each instance will be passed into your
+ function as an opaque structure of type <Acronym>TUPLE</Acronym>.
Suppose we want to write a function to answer the query
<ProgramListing>
* SELECT name, c_overpaid(EMP, 1500) AS overpaid
<ProgramListing>
#include "postgres.h"
- #include "libpq-fe.h" /* for TUPLE */
+ #include "executor/executor.h" /* for GetAttributeByName() */
+
bool
- c_overpaid(TUPLE t,/* the current instance of EMP */
+ c_overpaid(TupleTableSlot *t, /* the current instance of EMP */
int4 limit)
{
bool isnull = false;
</Para>
<Para>
- <Acronym>GetAttributeByName</Acronym> is the <ProductName>Postgres</ProductName> system function that
+ <Acronym>GetAttributeByName</Acronym> is the
+ <ProductName>Postgres</ProductName> system function that
returns attributes out of the current instance. It has
three arguments: the argument of type TUPLE passed into
the function, the name of the desired attribute, and a
return parameter that describes whether the attribute
- is null. <Acronym>GetAttributeByName</Acronym> will align data properly
- so you can cast its return value to the desired type.
- For example, if you have an attribute name which is of
- the type name, the <Acronym>GetAttributeByName</Acronym> call would look
- like:
+ is null. <Acronym>GetAttributeByName</Acronym> will
+ align data properly so you can cast its return value to
+ the desired type. For example, if you have an attribute
+ name which is of the type name, the <Acronym>GetAttributeByName</Acronym>
+ call would look like:
<ProgramListing>
char *str;
...
</Para>
<Para>
- The following query lets <ProductName>Postgres</ProductName> know about the
- c_overpaid function:
+ The following query lets <ProductName>Postgres</ProductName>
+ know about the c_overpaid function:
<ProgramListing>
* CREATE FUNCTION c_overpaid(EMP, int4) RETURNS bool
AS 'PGROOT/tutorial/obj/funcs.so' LANGUAGE 'c';
We now turn to the more difficult task of writing
programming language functions. Be warned: this section
of the manual will not make you a programmer. You must
- have a good understanding of <Acronym>C</Acronym> (including the use of
- pointers and the malloc memory manager) before trying
- to write <Acronym>C</Acronym> functions for use with <ProductName>Postgres</ProductName>.
- While it may be possible to load functions written in
- languages other than <Acronym>C</Acronym> into <ProductName>Postgres</ProductName>, this is often
- difficult (when it is possible at all) because other
- languages, such as <Acronym>FORTRAN</Acronym> and <Acronym>Pascal</Acronym> often do not follow
- the same "calling convention" as <Acronym>C</Acronym>. That is, other
+ have a good understanding of <Acronym>C</Acronym>
+ (including the use of pointers and the malloc memory manager)
+ before trying to write <Acronym>C</Acronym> functions for
+ use with <ProductName>Postgres</ProductName>. While it may
+ be possible to load functions written in languages other
+ than <Acronym>C</Acronym> into <ProductName>Postgres</ProductName>,
+ this is often difficult (when it is possible at all)
+ because other languages, such as <Acronym>FORTRAN</Acronym>
+ and <Acronym>Pascal</Acronym> often do not follow the same
+ "calling convention" as <Acronym>C</Acronym>. That is, other
languages do not pass argument and return values
between functions in the same way. For this reason, we
will assume that your programming language functions
are written in <Acronym>C</Acronym>.
- The basic rules for building <Acronym>C</Acronym> functions are as follows:
+ The basic rules for building <Acronym>C</Acronym> functions
+ are as follows:
<ItemizedList>
<ListItem>
<Para>
-Most of the header (include) files for <ProductName>Postgres</ProductName>
+ Most of the header (include) files for
+ <ProductName>Postgres</ProductName>
should already be installed in
<FileName>PGROOT/include</FileName> (see Figure 2).
You should always include
</para>
</ListItem>
<ListItem>
-<Para> When allocating memory, use the <ProductName>Postgres</ProductName>
+<Para> When allocating memory, use the
+ <ProductName>Postgres</ProductName>
routines palloc and pfree instead of the
- corresponding <Acronym>C</Acronym> library routines malloc and free.
+ corresponding <Acronym>C</Acronym> library routines
+ malloc and free.
The memory allocated by palloc will be freed
automatically at the end of each transaction,
preventing memory leaks.
</Para>
</ListItem>
<ListItem>
-<Para> Most of the internal <ProductName>Postgres</ProductName> types are declared
- in postgres.h, so it's a good idea to always
- include that file as well. Including postgres.h
- will also include elog.h and palloc.h for you.
+<Para> Most of the internal <ProductName>Postgres</ProductName>
+ types are declared in postgres.h, so it's a good
+ idea to always include that file as well. Including
+ postgres.h will also include elog.h and palloc.h for you.
</Para>
</ListItem>
<ListItem>
-<Para> Compiling and loading your object code so that
- it can be dynamically loaded into <ProductName>Postgres</ProductName>
+<Para> Compiling and loading your object code so that
+ it can be dynamically loaded into
+ <ProductName>Postgres</ProductName>
always requires special flags. See Appendix A
for a detailed explanation of how to do it for
your particular operating system.
projects / postgresql / commitdiff