-<!-- $PostgreSQL: pgsql/doc/src/sgml/array.sgml,v 1.56 2007/02/01 00:28:16 momjian Exp $ -->
+<!-- doc/src/sgml/array.sgml -->
<sect1 id="arrays">
<title>Arrays</title>
<para>
<productname>PostgreSQL</productname> allows columns of a table to be
defined as variable-length multidimensional arrays. Arrays of any
- built-in or user-defined base type can be created. (Arrays of
- composite types or domains are not yet supported, however.)
+ built-in or user-defined base type, enum type, or composite type
+ can be created.
+ Arrays of domains are not yet supported.
</para>
- <sect2>
+ <sect2 id="arrays-declaration">
<title>Declaration of Array Types</title>
+ <indexterm>
+ <primary>array</primary>
+ <secondary>declaration</secondary>
+ </indexterm>
+
<para>
To illustrate the use of array types, we create this table:
<programlisting>
);
</programlisting>
- However, the current implementation does not enforce the array size
- limits — the behavior is the same as for arrays of unspecified
+ However, the current implementation ignores any supplied array size
+ limits, i.e., the behavior is the same as for arrays of unspecified
length.
</para>
<para>
- Actually, the current implementation does not enforce the declared
+ The current implementation does not enforce the declared
number of dimensions either. Arrays of a particular element type are
all considered to be of the same type, regardless of size or number
- of dimensions. So, declaring number of dimensions or sizes in
- <command>CREATE TABLE</command> is simply documentation, it does not
+ of dimensions. So, declaring the array size or number of dimensions in
+ <command>CREATE TABLE</command> is simply documentation; it does not
affect run-time behavior.
</para>
<para>
- An alternative syntax, which conforms to the SQL standard, can
- be used for one-dimensional arrays.
+ An alternative syntax, which conforms to the SQL standard by using
+ the keyword <literal>ARRAY</>, can be used for one-dimensional arrays.
<structfield>pay_by_quarter</structfield> could have been defined
as:
<programlisting>
pay_by_quarter integer ARRAY[4],
</programlisting>
- This syntax requires an integer constant to denote the array size.
+ Or, if no array size is to be specified:
+<programlisting>
+ pay_by_quarter integer ARRAY,
+</programlisting>
As before, however, <productname>PostgreSQL</> does not enforce the
- size restriction.
+ size restriction in any case.
</para>
</sect2>
- <sect2>
+ <sect2 id="arrays-input">
<title>Array Value Input</title>
<indexterm>
where <replaceable>delim</replaceable> is the delimiter character
for the type, as recorded in its <literal>pg_type</literal> entry.
Among the standard data types provided in the
- <productname>PostgreSQL</productname> distribution, type
- <literal>box</> uses a semicolon (<literal>;</>) but all the others
- use comma (<literal>,</>). Each <replaceable>val</replaceable> is
+ <productname>PostgreSQL</productname> distribution, all use a comma
+ (<literal>,</>), except for type <type>box</> which uses a semicolon
+ (<literal>;</>). Each <replaceable>val</replaceable> is
either a constant of the array element type, or a subarray. An example
of an array constant is:
<programlisting>
</programlisting>
</para>
+ <para>
+ Multidimensional arrays must have matching extents for each
+ dimension. A mismatch causes an error, for example:
+
+<programlisting>
+INSERT INTO sal_emp
+ VALUES ('Bill',
+ '{10000, 10000, 10000, 10000}',
+ '{{"meeting", "lunch"}, {"meeting"}}');
+ERROR: multidimensional arrays must have array expressions with matching dimensions
+</programlisting>
+ </para>
+
<para>
The <literal>ARRAY</> constructor syntax can also be used:
<programlisting>
constructor syntax is discussed in more detail in
<xref linkend="sql-syntax-array-constructors">.
</para>
-
- <para>
- Multidimensional arrays must have matching extents for each
- dimension. A mismatch causes an error report, for example:
-
-<programlisting>
-INSERT INTO sal_emp
- VALUES ('Bill',
- '{10000, 10000, 10000, 10000}',
- '{{"meeting", "lunch"}, {"meeting"}}');
-ERROR: multidimensional arrays must have array expressions with matching dimensions
-</programlisting>
- </para>
</sect2>
- <sect2>
+ <sect2 id="arrays-accessing">
<title>Accessing Arrays</title>
+ <indexterm>
+ <primary>array</primary>
+ <secondary>accessing</secondary>
+ </indexterm>
+
<para>
Now, we can run some queries on the table.
- First, we show how to access a single element of an array at a time.
+ First, we show how to access a single element of an array.
This query retrieves the names of the employees whose pay changed in
the second quarter:
-
+
<programlisting>
SELECT name FROM sal_emp WHERE pay_by_quarter[1] <> pay_by_quarter[2];
</programlisting>
The array subscript numbers are written within square brackets.
- By default <productname>PostgreSQL</productname> uses the
+ By default <productname>PostgreSQL</productname> uses a
one-based numbering convention for arrays, that is,
an array of <replaceable>n</> elements starts with <literal>array[1]</literal> and
ends with <literal>array[<replaceable>n</>]</literal>.
<para>
This query retrieves the third quarter pay of all employees:
-
+
<programlisting>
SELECT pay_by_quarter[3] FROM sal_emp;
<literal><replaceable>lower-bound</replaceable>:<replaceable>upper-bound</replaceable></literal>
for one or more array dimensions. For example, this query retrieves the first
item on Bill's schedule for the first two days of the week:
-
+
<programlisting>
SELECT schedule[1:2][1:1] FROM sal_emp WHERE name = 'Bill';
(1 row)
</programlisting>
- We could also have written:
-
-<programlisting>
-SELECT schedule[1:2][1] FROM sal_emp WHERE name = 'Bill';
-</programlisting>
+ If any dimension is written as a slice, i.e., contains a colon, then all
+ dimensions are treated as slices. Any dimension that has only a single
+ number (no colon) is treated as being from 1
+ to the number specified. For example, <literal>[2]</> is treated as
+ <literal>[1:2]</>, as in this example:
- with the same result. An array subscripting operation is always taken to
- represent an array slice if any of the subscripts are written in the form
- <literal><replaceable>lower</replaceable>:<replaceable>upper</replaceable></literal>.
- A lower bound of 1 is assumed for any subscript where only one value
- is specified, as in this example:
<programlisting>
SELECT schedule[1:2][2] FROM sal_emp WHERE name = 'Bill';
-------------------------------------------
{{meeting,lunch},{training,presentation}}
(1 row)
+</programlisting>
+
+ To avoid confusion with the non-slice case, it's best to use slice syntax
+ for all dimensions, e.g., <literal>[1:2][1:1]</>, not <literal>[2][1:1]</>.
+ </para>
+
+ <para>
+ It is possible to omit the <replaceable>lower-bound</replaceable> and/or
+ <replaceable>upper-bound</replaceable> of a slice specifier; the missing
+ bound is replaced by the lower or upper limit of the array's subscripts.
+ For example:
+
+<programlisting>
+SELECT schedule[:2][2:] FROM sal_emp WHERE name = 'Bill';
+
+ schedule
+------------------------
+ {{lunch},{presentation}}
+(1 row)
+
+SELECT schedule[:][1:1] FROM sal_emp WHERE name = 'Bill';
+
+ schedule
+------------------------
+ {{meeting},{training}}
+(1 row)
</programlisting>
</para>
<para>
An array slice expression likewise yields null if the array itself or
- any of the subscript expressions are null. However, in other corner
+ any of the subscript expressions are null. However, in other
cases such as selecting an array slice that
is completely outside the current array bounds, a slice expression
- yields an empty (zero-dimensional) array instead of null.
+ yields an empty (zero-dimensional) array instead of null. (This
+ does not match non-slice behavior and is done for historical reasons.)
If the requested slice partially overlaps the array bounds, then it
- is silently reduced to just the overlapping region.
+ is silently reduced to just the overlapping region instead of
+ returning null.
</para>
<para>
array_dims
------------
- [1:2][1:1]
+ [1:2][1:2]
(1 row)
</programlisting>
<function>array_dims</function> produces a <type>text</type> result,
- which is convenient for people to read but perhaps not so convenient
+ which is convenient for people to read but perhaps inconvenient
for programs. Dimensions can also be retrieved with
<function>array_upper</function> and <function>array_lower</function>,
which return the upper and lower bound of a
-------------
2
(1 row)
+</programlisting>
+
+ <function>array_length</function> will return the length of a specified
+ array dimension:
+
+<programlisting>
+SELECT array_length(schedule, 1) FROM sal_emp WHERE name = 'Carol';
+
+ array_length
+--------------
+ 2
+(1 row)
+</programlisting>
+
+ <function>cardinality</function> returns the total number of elements in an
+ array across all dimensions. It is effectively the number of rows a call to
+ <function>unnest</function> would yield:
+
+<programlisting>
+SELECT cardinality(schedule) FROM sal_emp WHERE name = 'Carol';
+
+ cardinality
+-------------
+ 4
+(1 row)
</programlisting>
</para>
</sect2>
- <sect2>
+ <sect2 id="arrays-modifying">
<title>Modifying Arrays</title>
+ <indexterm>
+ <primary>array</primary>
+ <secondary>modifying</secondary>
+ </indexterm>
+
<para>
An array value can be replaced completely:
<programlisting>
UPDATE sal_emp SET pay_by_quarter[4] = 15000
WHERE name = 'Bill';
-</programListing>
+</programlisting>
or updated in a slice:
WHERE name = 'Carol';
</programlisting>
+ The slice syntaxes with omitted <replaceable>lower-bound</replaceable> and/or
+ <replaceable>upper-bound</replaceable> can be used too, but only when
+ updating an array value that is not NULL or zero-dimensional (otherwise,
+ there is no existing subscript limit to substitute).
</para>
<para>
- A stored array value can be enlarged by assigning to element(s) not already
+ A stored array value can be enlarged by assigning to elements not already
present. Any positions between those previously present and the newly
- assigned element(s) will be filled with nulls. For example, if array
+ assigned elements will be filled with nulls. For example, if array
<literal>myarray</> currently has 4 elements, it will have six
- elements after an update that assigns to <literal>myarray[6]</>,
- and <literal>myarray[5]</> will contain a null.
+ elements after an update that assigns to <literal>myarray[6]</>;
+ <literal>myarray[5]</> will contain null.
Currently, enlargement in this fashion is only allowed for one-dimensional
arrays, not multidimensional arrays.
</para>
<para>
Subscripted assignment allows creation of arrays that do not use one-based
subscripts. For example one might assign to <literal>myarray[-2:7]</> to
- create an array with subscript values running from -2 to 7.
+ create an array with subscript values from -2 to 7.
</para>
<para>
- New array values can also be constructed by using the concatenation operator,
+ New array values can also be constructed using the concatenation operator,
<literal>||</literal>:
<programlisting>
SELECT ARRAY[1,2] || ARRAY[3,4];
</para>
<para>
- The concatenation operator allows a single element to be pushed on to the
+ The concatenation operator allows a single element to be pushed onto the
beginning or end of a one-dimensional array. It also accepts two
<replaceable>N</>-dimensional arrays, or an <replaceable>N</>-dimensional
and an <replaceable>N+1</>-dimensional array.
</para>
<para>
- When a single element is pushed on to either the beginning or end of a
+ When a single element is pushed onto either the beginning or end of a
one-dimensional array, the result is an array with the same lower bound
subscript as the array operand. For example:
<programlisting>
</para>
<para>
- When an <replaceable>N</>-dimensional array is pushed on to the beginning
+ When an <replaceable>N</>-dimensional array is pushed onto the beginning
or end of an <replaceable>N+1</>-dimensional array, the result is
analogous to the element-array case above. Each <replaceable>N</>-dimensional
sub-array is essentially an element of the <replaceable>N+1</>-dimensional
<function>array_prepend</function>, <function>array_append</function>,
or <function>array_cat</function>. The first two only support one-dimensional
arrays, but <function>array_cat</function> supports multidimensional arrays.
-
- Note that the concatenation operator discussed above is preferred over
- direct use of these functions. In fact, the functions exist primarily for use
- in implementing the concatenation operator. However, they might be directly
- useful in the creation of user-defined aggregates. Some examples:
+ Some examples:
<programlisting>
SELECT array_prepend(1, ARRAY[2,3]);
{{5,6},{1,2},{3,4}}
</programlisting>
</para>
+
+ <para>
+ In simple cases, the concatenation operator discussed above is preferred
+ over direct use of these functions. However, because the concatenation
+ operator is overloaded to serve all three cases, there are situations where
+ use of one of the functions is helpful to avoid ambiguity. For example
+ consider:
+
+<programlisting>
+SELECT ARRAY[1, 2] || '{3, 4}'; -- the untyped literal is taken as an array
+ ?column?
+-----------
+ {1,2,3,4}
+
+SELECT ARRAY[1, 2] || '7'; -- so is this one
+ERROR: malformed array literal: "7"
+
+SELECT ARRAY[1, 2] || NULL; -- so is an undecorated NULL
+ ?column?
+----------
+ {1,2}
+(1 row)
+
+SELECT array_append(ARRAY[1, 2], NULL); -- this might have been meant
+ array_append
+--------------
+ {1,2,NULL}
+</programlisting>
+
+ In the examples above, the parser sees an integer array on one side of the
+ concatenation operator, and a constant of undetermined type on the other.
+ The heuristic it uses to resolve the constant's type is to assume it's of
+ the same type as the operator's other input — in this case,
+ integer array. So the concatenation operator is presumed to
+ represent <function>array_cat</>, not <function>array_append</>. When
+ that's the wrong choice, it could be fixed by casting the constant to the
+ array's element type; but explicit use of <function>array_append</> might
+ be a preferable solution.
+ </para>
</sect2>
- <sect2>
+ <sect2 id="arrays-searching">
<title>Searching in Arrays</title>
+ <indexterm>
+ <primary>array</primary>
+ <secondary>searching</secondary>
+ </indexterm>
+
<para>
- To search for a value in an array, you must check each value of the
- array. This can be done by hand, if you know the size of the array.
+ To search for a value in an array, each value must be checked.
+ This can be done manually, if you know the size of the array.
For example:
<programlisting>
</programlisting>
However, this quickly becomes tedious for large arrays, and is not
- helpful if the size of the array is uncertain. An alternative method is
+ helpful if the size of the array is unknown. An alternative method is
described in <xref linkend="functions-comparisons">. The above
query could be replaced by:
SELECT * FROM sal_emp WHERE 10000 = ANY (pay_by_quarter);
</programlisting>
- In addition, you could find rows where the array had all values
+ In addition, you can find rows where the array has all values
equal to 10000 with:
<programlisting>
</para>
+ <para>
+ Alternatively, the <function>generate_subscripts</> function can be used.
+ For example:
+
+<programlisting>
+SELECT * FROM
+ (SELECT pay_by_quarter,
+ generate_subscripts(pay_by_quarter, 1) AS s
+ FROM sal_emp) AS foo
+ WHERE pay_by_quarter[s] = 10000;
+</programlisting>
+
+ This function is described in <xref linkend="functions-srf-subscripts">.
+ </para>
+
+ <para>
+ You can also search an array using the <literal>&&</> operator,
+ which checks whether the left operand overlaps with the right operand.
+ For instance:
+
+<programlisting>
+SELECT * FROM sal_emp WHERE pay_by_quarter && ARRAY[10000];
+</programlisting>
+
+ This and other array operators are further described in
+ <xref linkend="functions-array">. It can be accelerated by an appropriate
+ index, as described in <xref linkend="indexes-types">.
+ </para>
+
+ <para>
+ You can also search for specific values in an array using the <function>array_position</>
+ and <function>array_positions</> functions. The former returns the subscript of
+ the first occurrence of a value in an array; the latter returns an array with the
+ subscripts of all occurrences of the value in the array. For example:
+
+<programlisting>
+SELECT array_position(ARRAY['sun','mon','tue','wed','thu','fri','sat'], 'mon');
+ array_positions
+-----------------
+ 2
+
+SELECT array_positions(ARRAY[1, 4, 3, 1, 3, 4, 2, 1], 1);
+ array_positions
+-----------------
+ {1,4,8}
+</programlisting>
+ </para>
+
<tip>
<para>
Arrays are not sets; searching for specific array elements
can be a sign of database misdesign. Consider
using a separate table with a row for each item that would be an
array element. This will be easier to search, and is likely to
- scale up better to large numbers of elements.
+ scale better for a large number of elements.
</para>
</tip>
</sect2>
- <sect2>
+ <sect2 id="arrays-io">
<title>Array Input and Output Syntax</title>
+ <indexterm>
+ <primary>array</primary>
+ <secondary>I/O</secondary>
+ </indexterm>
+
<para>
The external text representation of an array value consists of items that
are interpreted according to the I/O conversion rules for the array's
around the array value plus delimiter characters between adjacent items.
The delimiter character is usually a comma (<literal>,</>) but can be
something else: it is determined by the <literal>typdelim</> setting
- for the array's element type. (Among the standard data types provided
- in the <productname>PostgreSQL</productname> distribution, type
- <literal>box</> uses a semicolon (<literal>;</>) but all the others
- use comma.) In a multidimensional array, each dimension (row, plane,
+ for the array's element type. Among the standard data types provided
+ in the <productname>PostgreSQL</productname> distribution, all use a comma,
+ except for type <type>box</>, which uses a semicolon (<literal>;</>).
+ In a multidimensional array, each dimension (row, plane,
cube, etc.) gets its own level of curly braces, and delimiters
must be written between adjacent curly-braced entities of the same level.
</para>
<literal>NULL</>. Double quotes and backslashes
embedded in element values will be backslash-escaped. For numeric
data types it is safe to assume that double quotes will never appear, but
- for textual data types one should be prepared to cope with either presence
+ for textual data types one should be prepared to cope with either the presence
or absence of quotes.
</para>
If the value written for an element is <literal>NULL</> (in any case
variant), the element is taken to be NULL. The presence of any quotes
or backslashes disables this and allows the literal string value
- <quote>NULL</> to be entered. Also, for backwards compatibility with
+ <quote>NULL</> to be entered. Also, for backward compatibility with
pre-8.2 versions of <productname>PostgreSQL</>, the <xref
- linkend="guc-array-nulls"> configuration parameter might be turned
+ linkend="guc-array-nulls"> configuration parameter can be turned
<literal>off</> to suppress recognition of <literal>NULL</> as a NULL.
</para>
<para>
- As shown previously, when writing an array value you can write double
+ As shown previously, when writing an array value you can use double
quotes around any individual array element. You <emphasis>must</> do so
if the element value would otherwise confuse the array-value parser.
- For example, elements containing curly braces, commas (or whatever the
- delimiter character is), double quotes, backslashes, or leading or trailing
+ For example, elements containing curly braces, commas (or the data type's
+ delimiter character), double quotes, backslashes, or leading or trailing
whitespace must be double-quoted. Empty strings and strings matching the
word <literal>NULL</> must be quoted, too. To put a double quote or
backslash in a quoted array element value, use escape string syntax
- and precede it with a backslash. Alternatively, you can use
+ and precede it with a backslash. Alternatively, you can avoid quotes and use
backslash-escaping to protect all data characters that would otherwise
be taken as array syntax.
</para>
<para>
- You can write whitespace before a left brace or after a right
- brace. You can also write whitespace before or after any individual item
+ You can add whitespace before a left brace or after a right
+ brace. You can also add whitespace before or after any individual item
string. In all of these cases the whitespace will be ignored. However,
whitespace within double-quoted elements, or surrounded on both sides by
non-whitespace characters of an element, is not ignored.