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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>38.5. Query Language (SQL) Functions</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="xproc.html" title="38.4. User-Defined Procedures" /><link rel="next" href="xfunc-overload.html" title="38.6. Function Overloading" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">38.5. Query Language (<acronym class="acronym">SQL</acronym>) Functions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="xproc.html" title="38.4. User-Defined Procedures">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="extend.html" title="Chapter 38. Extending SQL">Up</a></td><th width="60%" align="center">Chapter 38. Extending <acronym class="acronym">SQL</acronym></th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="xfunc-overload.html" title="38.6. Function Overloading">Next</a></td></tr></table><hr /></div><div class="sect1" id="XFUNC-SQL"><div class="titlepage"><div><div><h2 class="title" style="clear: both">38.5. Query Language (<acronym class="acronym">SQL</acronym>) Functions <a href="#XFUNC-SQL" class="id_link">#</a></h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="xfunc-sql.html#XFUNC-SQL-FUNCTION-ARGUMENTS">38.5.1. Arguments for <acronym class="acronym">SQL</acronym> Functions</a></span></dt><dt><span class="sect2"><a href="xfunc-sql.html#XFUNC-SQL-BASE-FUNCTIONS">38.5.2. <acronym class="acronym">SQL</acronym> Functions on Base Types</a></span></dt><dt><span class="sect2"><a href="xfunc-sql.html#XFUNC-SQL-COMPOSITE-FUNCTIONS">38.5.3. <acronym class="acronym">SQL</acronym> Functions on Composite Types</a></span></dt><dt><span class="sect2"><a href="xfunc-sql.html#XFUNC-OUTPUT-PARAMETERS">38.5.4. <acronym class="acronym">SQL</acronym> Functions with Output Parameters</a></span></dt><dt><span class="sect2"><a href="xfunc-sql.html#XFUNC-OUTPUT-PARAMETERS-PROC">38.5.5. <acronym class="acronym">SQL</acronym> Procedures with Output Parameters</a></span></dt><dt><span class="sect2"><a href="xfunc-sql.html#XFUNC-SQL-VARIADIC-FUNCTIONS">38.5.6. <acronym class="acronym">SQL</acronym> Functions with Variable Numbers of Arguments</a></span></dt><dt><span class="sect2"><a href="xfunc-sql.html#XFUNC-SQL-PARAMETER-DEFAULTS">38.5.7. <acronym class="acronym">SQL</acronym> Functions with Default Values for Arguments</a></span></dt><dt><span class="sect2"><a href="xfunc-sql.html#XFUNC-SQL-TABLE-FUNCTIONS">38.5.8. <acronym class="acronym">SQL</acronym> Functions as Table Sources</a></span></dt><dt><span class="sect2"><a href="xfunc-sql.html#XFUNC-SQL-FUNCTIONS-RETURNING-SET">38.5.9. <acronym class="acronym">SQL</acronym> Functions Returning Sets</a></span></dt><dt><span class="sect2"><a href="xfunc-sql.html#XFUNC-SQL-FUNCTIONS-RETURNING-TABLE">38.5.10. <acronym class="acronym">SQL</acronym> Functions Returning <code class="literal">TABLE</code></a></span></dt><dt><span class="sect2"><a href="xfunc-sql.html#XFUNC-SQL-POLYMORPHIC-FUNCTIONS">38.5.11. Polymorphic <acronym class="acronym">SQL</acronym> Functions</a></span></dt><dt><span class="sect2"><a href="xfunc-sql.html#XFUNC-SQL-COLLATIONS">38.5.12. <acronym class="acronym">SQL</acronym> Functions with Collations</a></span></dt></dl></div><a id="id-1.8.3.8.2" class="indexterm"></a><p>
SQL functions execute an arbitrary list of SQL statements, returning
the result of the last query in the list.
In the simple (non-set)
case, the first row of the last query's result will be returned.
(Bear in mind that <span class="quote">“<span class="quote">the first row</span>”</span> of a multirow
result is not well-defined unless you use <code class="literal">ORDER BY</code>.)
If the last query happens
to return no rows at all, the null value will be returned.
</p><p>
Alternatively, an SQL function can be declared to return a set (that is,
multiple rows) by specifying the function's return type as <code class="literal">SETOF
<em class="replaceable"><code>sometype</code></em></code>, or equivalently by declaring it as
<code class="literal">RETURNS TABLE(<em class="replaceable"><code>columns</code></em>)</code>. In this case
all rows of the last query's result are returned. Further details appear
below.
</p><p>
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
<code class="type">void</code>, the last statement must be a <code class="command">SELECT</code>,
or an <code class="command">INSERT</code>, <code class="command">UPDATE</code>, or <code class="command">DELETE</code>
that has a <code class="literal">RETURNING</code> clause.
</p><p>
Any collection of commands in the <acronym class="acronym">SQL</acronym>
language can be packaged together and defined as a function.
Besides <code class="command">SELECT</code> queries, the commands can include data
modification queries (<code class="command">INSERT</code>,
<code class="command">UPDATE</code>, <code class="command">DELETE</code>, and
<code class="command">MERGE</code>), as well as
other SQL commands. (You cannot use transaction control commands, e.g.,
<code class="command">COMMIT</code>, <code class="command">SAVEPOINT</code>, and some utility
commands, e.g., <code class="literal">VACUUM</code>, in <acronym class="acronym">SQL</acronym> functions.)
However, the final command
must be a <code class="command">SELECT</code> or have a <code class="literal">RETURNING</code>
clause that returns whatever is
specified as the function's return type. Alternatively, if you
want to define an SQL function that performs actions but has no
useful value to return, you can define it as returning <code class="type">void</code>.
For example, this function removes rows with negative salaries from
the <code class="literal">emp</code> table:
</p><pre class="screen">
CREATE FUNCTION clean_emp() RETURNS void AS '
DELETE FROM emp
WHERE salary < 0;
' LANGUAGE SQL;
SELECT clean_emp();
clean_emp
-----------
(1 row)
</pre><p>
</p><p>
You can also write this as a procedure, thus avoiding the issue of the
return type. For example:
</p><pre class="screen">
CREATE PROCEDURE clean_emp() AS '
DELETE FROM emp
WHERE salary < 0;
' LANGUAGE SQL;
CALL clean_emp();
</pre><p>
In simple cases like this, the difference between a function returning
<code class="type">void</code> and a procedure is mostly stylistic. However,
procedures offer additional functionality such as transaction control
that is not available in functions. Also, procedures are SQL standard
whereas returning <code class="type">void</code> is a PostgreSQL extension.
</p><div class="note"><h3 class="title">Note</h3><p>
The entire body of an SQL function is parsed before any of it is
executed. While an SQL function can contain commands that alter
the system catalogs (e.g., <code class="command">CREATE TABLE</code>), the effects
of such commands will not be visible during parse analysis of
later commands in the function. Thus, for example,
<code class="literal">CREATE TABLE foo (...); INSERT INTO foo VALUES(...);</code>
will not work as desired if packaged up into a single SQL function,
since <code class="structname">foo</code> won't exist yet when the <code class="command">INSERT</code>
command is parsed. It's recommended to use <span class="application">PL/pgSQL</span>
instead of an SQL function in this type of situation.
</p></div><p>
The syntax of the <code class="command">CREATE FUNCTION</code> command requires
the function body to be written as a string constant. It is usually
most convenient to use dollar quoting (see <a class="xref" href="sql-syntax-lexical.html#SQL-SYNTAX-DOLLAR-QUOTING" title="4.1.2.4. Dollar-Quoted String Constants">Section 4.1.2.4</a>) for the string constant.
If you choose to use regular single-quoted string constant syntax,
you must double single quote marks (<code class="literal">'</code>) and backslashes
(<code class="literal">\</code>) (assuming escape string syntax) in the body of
the function (see <a class="xref" href="sql-syntax-lexical.html#SQL-SYNTAX-STRINGS" title="4.1.2.1. String Constants">Section 4.1.2.1</a>).
</p><div class="sect2" id="XFUNC-SQL-FUNCTION-ARGUMENTS"><div class="titlepage"><div><div><h3 class="title">38.5.1. Arguments for <acronym class="acronym">SQL</acronym> Functions <a href="#XFUNC-SQL-FUNCTION-ARGUMENTS" class="id_link">#</a></h3></div></div></div><a id="id-1.8.3.8.10.2" class="indexterm"></a><p>
Arguments of an SQL function can be referenced in the function
body using either names or numbers. Examples of both methods appear
below.
</p><p>
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
<code class="literal"><em class="replaceable"><code>function_name</code></em>.<em class="replaceable"><code>argument_name</code></em></code>.
(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.)
</p><p>
In the older numeric approach, arguments are referenced using the syntax
<code class="literal">$<em class="replaceable"><code>n</code></em></code>: <code class="literal">$1</code> refers to the first input
argument, <code class="literal">$2</code> to the second, and so on. This will work
whether or not the particular argument was declared with a name.
</p><p>
If an argument is of a composite type, then the dot notation,
e.g., <code class="literal"><em class="replaceable"><code>argname</code></em>.<em class="replaceable"><code>fieldname</code></em></code> or
<code class="literal">$1.<em class="replaceable"><code>fieldname</code></em></code>, 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.
</p><p>
SQL function arguments can only be used as data values,
not as identifiers. Thus for example this is reasonable:
</p><pre class="programlisting">
INSERT INTO mytable VALUES ($1);
</pre><p>
but this will not work:
</p><pre class="programlisting">
INSERT INTO $1 VALUES (42);
</pre><p>
</p><div class="note"><h3 class="title">Note</h3><p>
The ability to use names to reference SQL function arguments was added
in <span class="productname">PostgreSQL</span> 9.2. Functions to be used in
older servers must use the <code class="literal">$<em class="replaceable"><code>n</code></em></code> notation.
</p></div></div><div class="sect2" id="XFUNC-SQL-BASE-FUNCTIONS"><div class="titlepage"><div><div><h3 class="title">38.5.2. <acronym class="acronym">SQL</acronym> Functions on Base Types <a href="#XFUNC-SQL-BASE-FUNCTIONS" class="id_link">#</a></h3></div></div></div><p>
The simplest possible <acronym class="acronym">SQL</acronym> function has no arguments and
simply returns a base type, such as <code class="type">integer</code>:
</p><pre class="screen">
CREATE FUNCTION one() RETURNS integer AS $$
SELECT 1 AS result;
$$ LANGUAGE SQL;
-- Alternative syntax for string literal:
CREATE FUNCTION one() RETURNS integer AS '
SELECT 1 AS result;
' LANGUAGE SQL;
SELECT one();
one
-----
1
</pre><p>
</p><p>
Notice that we defined a column alias within the function body for the result of the function
(with the name <code class="literal">result</code>), but this column alias is not visible
outside the function. Hence, the result is labeled <code class="literal">one</code>
instead of <code class="literal">result</code>.
</p><p>
It is almost as easy to define <acronym class="acronym">SQL</acronym> functions
that take base types as arguments:
</p><pre class="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
</pre><p>
</p><p>
Alternatively, we could dispense with names for the arguments and
use numbers:
</p><pre class="screen">
CREATE FUNCTION add_em(integer, integer) RETURNS integer AS $$
SELECT $1 + $2;
$$ LANGUAGE SQL;
SELECT add_em(1, 2) AS answer;
answer
--------
3
</pre><p>
</p><p>
Here is a more useful function, which might be used to debit a
bank account:
</p><pre class="programlisting">
CREATE FUNCTION tf1 (accountno integer, debit numeric) RETURNS numeric AS $$
UPDATE bank
SET balance = balance - debit
WHERE accountno = tf1.accountno;
SELECT 1;
$$ LANGUAGE SQL;
</pre><p>
A user could execute this function to debit account 17 by $100.00 as
follows:
</p><pre class="programlisting">
SELECT tf1(17, 100.0);
</pre><p>
</p><p>
In this example, we chose the name <code class="literal">accountno</code> for the first
argument, but this is the same as the name of a column in the
<code class="literal">bank</code> table. Within the <code class="command">UPDATE</code> command,
<code class="literal">accountno</code> refers to the column <code class="literal">bank.accountno</code>,
so <code class="literal">tf1.accountno</code> must be used to refer to the argument.
We could of course avoid this by using a different name for the argument.
</p><p>
In practice one would probably like a more useful result from the
function than a constant 1, so a more likely definition
is:
</p><pre class="programlisting">
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;
</pre><p>
which adjusts the balance and returns the new balance.
The same thing could be done in one command using <code class="literal">RETURNING</code>:
</p><pre class="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;
</pre><p>
</p><p>
If the final <code class="literal">SELECT</code> or <code class="literal">RETURNING</code>
clause in an <acronym class="acronym">SQL</acronym> function does not return exactly
the function's declared result
type, <span class="productname">PostgreSQL</span> will automatically cast
the value to the required type, if that is possible with an implicit
or assignment cast. Otherwise, you must write an explicit cast.
For example, suppose we wanted the
previous <code class="function">add_em</code> function to return
type <code class="type">float8</code> instead. It's sufficient to write
</p><pre class="programlisting">
CREATE FUNCTION add_em(integer, integer) RETURNS float8 AS $$
SELECT $1 + $2;
$$ LANGUAGE SQL;
</pre><p>
since the <code class="type">integer</code> sum can be implicitly cast
to <code class="type">float8</code>.
(See <a class="xref" href="typeconv.html" title="Chapter 10. Type Conversion">Chapter 10</a> or <a class="xref" href="sql-createcast.html" title="CREATE CAST"><span class="refentrytitle">CREATE CAST</span></a>
for more about casts.)
</p></div><div class="sect2" id="XFUNC-SQL-COMPOSITE-FUNCTIONS"><div class="titlepage"><div><div><h3 class="title">38.5.3. <acronym class="acronym">SQL</acronym> Functions on Composite Types <a href="#XFUNC-SQL-COMPOSITE-FUNCTIONS" class="id_link">#</a></h3></div></div></div><p>
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
<code class="type">emp</code> 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 <code class="function">double_salary</code> that computes what someone's
salary would be if it were doubled:
</p><pre class="screen">
CREATE TABLE emp (
name text,
salary numeric,
age integer,
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;
SELECT name, double_salary(emp.*) AS dream
FROM emp
WHERE emp.cubicle ~= point '(2,1)';
name | dream
------+-------
Bill | 8400
</pre><p>
</p><p>
Notice the use of the syntax <code class="literal">$1.salary</code>
to select one field of the argument row value. Also notice
how the calling <code class="command">SELECT</code> command
uses <em class="replaceable"><code>table_name</code></em><code class="literal">.*</code> 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:
</p><pre class="screen">
SELECT name, double_salary(emp) AS dream
FROM emp
WHERE emp.cubicle ~= point '(2,1)';
</pre><p>
but this usage is deprecated since it's easy to get confused.
(See <a class="xref" href="rowtypes.html#ROWTYPES-USAGE" title="8.16.5. Using Composite Types in Queries">Section 8.16.5</a> for details about these
two notations for the composite value of a table row.)
</p><p>
Sometimes it is handy to construct a composite argument value
on-the-fly. This can be done with the <code class="literal">ROW</code> construct.
For example, we could adjust the data being passed to the function:
</p><pre class="screen">
SELECT name, double_salary(ROW(name, salary*1.1, age, cubicle)) AS dream
FROM emp;
</pre><p>
</p><p>
It is also possible to build a function that returns a composite type.
This is an example of a function
that returns a single <code class="type">emp</code> row:
</p><pre class="programlisting">
CREATE FUNCTION new_emp() RETURNS emp AS $$
SELECT text 'None' AS name,
1000.0 AS salary,
25 AS age,
point '(2,2)' AS cubicle;
$$ LANGUAGE SQL;
</pre><p>
In this example we have specified each of the attributes
with a constant value, but any computation
could have been substituted for these constants.
</p><p>
Note two important things about defining the function:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
The select list order in the query must be exactly the same as
that in which the columns appear in the composite type.
(Naming the columns, as we did above,
is irrelevant to the system.)
</p></li><li class="listitem"><p>
We must ensure each expression's type can be cast to that of
the corresponding column of the composite type.
Otherwise we'll get errors like this:
</p><pre class="screen">
<code class="computeroutput">
ERROR: return type mismatch in function declared to return emp
DETAIL: Final statement returns text instead of point at column 4.
</code>
</pre><p>
As with the base-type case, the system will not insert explicit
casts automatically, only implicit or assignment casts.
</p></li></ul></div><p>
</p><p>
A different way to define the same function is:
</p><pre class="programlisting">
CREATE FUNCTION new_emp() RETURNS emp AS $$
SELECT ROW('None', 1000.0, 25, '(2,2)')::emp;
$$ LANGUAGE SQL;
</pre><p>
Here we wrote a <code class="command">SELECT</code> 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.
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 way to cause a coercion of the whole row result.
</p><p>
We could call this function directly either by using it in
a value expression:
</p><pre class="screen">
SELECT new_emp();
new_emp
--------------------------
(None,1000.0,25,"(2,2)")
</pre><p>
or by calling it as a table function:
</p><pre class="screen">
SELECT * FROM new_emp();
name | salary | age | cubicle
------+--------+-----+---------
None | 1000.0 | 25 | (2,2)
</pre><p>
The second way is described more fully in <a class="xref" href="xfunc-sql.html#XFUNC-SQL-TABLE-FUNCTIONS" title="38.5.8. SQL Functions as Table Sources">Section 38.5.8</a>.
</p><p>
When you use a function that returns a composite type,
you might want only one field (attribute) from its result.
You can do that with syntax like this:
</p><pre class="screen">
SELECT (new_emp()).name;
name
------
None
</pre><p>
The extra parentheses are needed to keep the parser from getting
confused. If you try to do it without them, you get something like this:
</p><pre class="screen">
SELECT new_emp().name;
ERROR: syntax error at or near "."
LINE 1: SELECT new_emp().name;
^
</pre><p>
</p><p>
Another option is to use functional notation for extracting an attribute:
</p><pre class="screen">
SELECT name(new_emp());
name
------
None
</pre><p>
As explained in <a class="xref" href="rowtypes.html#ROWTYPES-USAGE" title="8.16.5. Using Composite Types in Queries">Section 8.16.5</a>, the field notation and
functional notation are equivalent.
</p><p>
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:
</p><pre class="screen">
CREATE FUNCTION getname(emp) RETURNS text AS $$
SELECT $1.name;
$$ LANGUAGE SQL;
SELECT getname(new_emp());
getname
---------
None
(1 row)
</pre><p>
</p></div><div class="sect2" id="XFUNC-OUTPUT-PARAMETERS"><div class="titlepage"><div><div><h3 class="title">38.5.4. <acronym class="acronym">SQL</acronym> Functions with Output Parameters <a href="#XFUNC-OUTPUT-PARAMETERS" class="id_link">#</a></h3></div></div></div><a id="id-1.8.3.8.13.2" class="indexterm"></a><p>
An alternative way of describing a function's results is to define it
with <em class="firstterm">output parameters</em>, as in this example:
</p><pre class="screen">
CREATE FUNCTION add_em (IN x int, IN y int, OUT sum int)
AS 'SELECT x + y'
LANGUAGE SQL;
SELECT add_em(3,7);
add_em
--------
10
(1 row)
</pre><p>
This is not essentially different from the version of <code class="literal">add_em</code>
shown in <a class="xref" href="xfunc-sql.html#XFUNC-SQL-BASE-FUNCTIONS" title="38.5.2. SQL Functions on Base Types">Section 38.5.2</a>. The real value of
output parameters is that they provide a convenient way of defining
functions that return several columns. For example,
</p><pre class="screen">
CREATE FUNCTION sum_n_product (x int, y int, OUT sum int, OUT product int)
AS 'SELECT x + y, x * y'
LANGUAGE SQL;
SELECT * FROM sum_n_product(11,42);
sum | product
-----+---------
53 | 462
(1 row)
</pre><p>
What has essentially happened here is that we have created an anonymous
composite type for the result of the function. The above example has
the same end result as
</p><pre class="screen">
CREATE TYPE sum_prod AS (sum int, product int);
CREATE FUNCTION sum_n_product (int, int) RETURNS sum_prod
AS 'SELECT $1 + $2, $1 * $2'
LANGUAGE SQL;
</pre><p>
but not having to bother with the separate composite type definition
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.)
</p><p>
Notice that output parameters are not included in the calling argument
list when invoking such a function from SQL. This is because
<span class="productname">PostgreSQL</span> considers only the input
parameters to define the function's calling signature. That means
also that only the input parameters matter when referencing the function
for purposes such as dropping it. We could drop the above function
with either of
</p><pre class="screen">
DROP FUNCTION sum_n_product (x int, y int, OUT sum int, OUT product int);
DROP FUNCTION sum_n_product (int, int);
</pre><p>
</p><p>
Parameters can be marked as <code class="literal">IN</code> (the default),
<code class="literal">OUT</code>, <code class="literal">INOUT</code>, or <code class="literal">VARIADIC</code>.
An <code class="literal">INOUT</code>
parameter serves as both an input parameter (part of the calling
argument list) and an output parameter (part of the result record type).
<code class="literal">VARIADIC</code> parameters are input parameters, but are treated
specially as described below.
</p></div><div class="sect2" id="XFUNC-OUTPUT-PARAMETERS-PROC"><div class="titlepage"><div><div><h3 class="title">38.5.5. <acronym class="acronym">SQL</acronym> Procedures with Output Parameters <a href="#XFUNC-OUTPUT-PARAMETERS-PROC" class="id_link">#</a></h3></div></div></div><a id="id-1.8.3.8.14.2" class="indexterm"></a><p>
Output parameters are also supported in procedures, but they work a bit
differently from functions. In <code class="command">CALL</code> commands,
output parameters must be included in the argument list.
For example, the bank account debiting routine from earlier could be
written like this:
</p><pre class="programlisting">
CREATE PROCEDURE tp1 (accountno integer, debit numeric, OUT new_balance numeric) AS $$
UPDATE bank
SET balance = balance - debit
WHERE accountno = tp1.accountno
RETURNING balance;
$$ LANGUAGE SQL;
</pre><p>
To call this procedure, an argument matching the <code class="literal">OUT</code>
parameter must be included. It's customary to write
<code class="literal">NULL</code>:
</p><pre class="programlisting">
CALL tp1(17, 100.0, NULL);
</pre><p>
If you write something else, it must be an expression that is implicitly
coercible to the declared type of the parameter, just as for input
parameters. Note however that such an expression will not be evaluated.
</p><p>
When calling a procedure from <span class="application">PL/pgSQL</span>,
instead of writing <code class="literal">NULL</code> you must write a variable
that will receive the procedure's output. See <a class="xref" href="plpgsql-control-structures.html#PLPGSQL-STATEMENTS-CALLING-PROCEDURE" title="43.6.3. Calling a Procedure">Section 43.6.3</a> for details.
</p></div><div class="sect2" id="XFUNC-SQL-VARIADIC-FUNCTIONS"><div class="titlepage"><div><div><h3 class="title">38.5.6. <acronym class="acronym">SQL</acronym> Functions with Variable Numbers of Arguments <a href="#XFUNC-SQL-VARIADIC-FUNCTIONS" class="id_link">#</a></h3></div></div></div><a id="id-1.8.3.8.15.2" class="indexterm"></a><a id="id-1.8.3.8.15.3" class="indexterm"></a><p>
<acronym class="acronym">SQL</acronym> functions can be declared to accept
variable numbers of arguments, so long as all the <span class="quote">“<span class="quote">optional</span>”</span>
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 <code class="literal">VARIADIC</code>; this parameter
must be declared as being of an array type. For example:
</p><pre class="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)
</pre><p>
Effectively, all the actual arguments at or beyond the
<code class="literal">VARIADIC</code> position are gathered up into a one-dimensional
array, as if you had written
</p><pre class="screen">
SELECT mleast(ARRAY[10, -1, 5, 4.4]); -- doesn't work
</pre><p>
You can't actually write that, though — or at least, it will
not match this function definition. A parameter marked
<code class="literal">VARIADIC</code> matches one or more occurrences of its element
type, not of its own type.
</p><p>
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. Also,
this is the only secure way to call a variadic function found in a schema
that permits untrusted users to create objects; see
<a class="xref" href="typeconv-func.html" title="10.3. Functions">Section 10.3</a>. You can do this by
specifying <code class="literal">VARIADIC</code> in the call:
</p><pre class="screen">
SELECT mleast(VARIADIC ARRAY[10, -1, 5, 4.4]);
</pre><p>
This prevents expansion of the function's variadic parameter into its
element type, thereby allowing the array argument value to match
normally. <code class="literal">VARIADIC</code> can only be attached to the last
actual argument of a function call.
</p><p>
Specifying <code class="literal">VARIADIC</code> in the call is also the only way to
pass an empty array to a variadic function, for example:
</p><pre class="screen">
SELECT mleast(VARIADIC ARRAY[]::numeric[]);
</pre><p>
Simply writing <code class="literal">SELECT mleast()</code> does not work because a
variadic parameter must match at least one actual argument.
(You could define a second function also named <code class="literal">mleast</code>,
with no parameters, if you wanted to allow such calls.)
</p><p>
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 (<a class="xref" href="sql-syntax-calling-funcs.html" title="4.3. Calling Functions">Section 4.3</a>), except when you specify
<code class="literal">VARIADIC</code>. For example, this will work:
</p><pre class="screen">
SELECT mleast(VARIADIC arr => ARRAY[10, -1, 5, 4.4]);
</pre><p>
but not these:
</p><pre class="screen">
SELECT mleast(arr => 10);
SELECT mleast(arr => ARRAY[10, -1, 5, 4.4]);
</pre><p>
</p></div><div class="sect2" id="XFUNC-SQL-PARAMETER-DEFAULTS"><div class="titlepage"><div><div><h3 class="title">38.5.7. <acronym class="acronym">SQL</acronym> Functions with Default Values for Arguments <a href="#XFUNC-SQL-PARAMETER-DEFAULTS" class="id_link">#</a></h3></div></div></div><a id="id-1.8.3.8.16.2" class="indexterm"></a><p>
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.) Whether or not you use it,
this capability creates a need for precautions when calling functions in
databases where some users mistrust other users; see
<a class="xref" href="typeconv-func.html" title="10.3. Functions">Section 10.3</a>.
</p><p>
For example:
</p><pre class="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
</pre><p>
The <code class="literal">=</code> sign can also be used in place of the
key word <code class="literal">DEFAULT</code>.
</p></div><div class="sect2" id="XFUNC-SQL-TABLE-FUNCTIONS"><div class="titlepage"><div><div><h3 class="title">38.5.8. <acronym class="acronym">SQL</acronym> Functions as Table Sources <a href="#XFUNC-SQL-TABLE-FUNCTIONS" class="id_link">#</a></h3></div></div></div><p>
All SQL functions can be used in the <code class="literal">FROM</code> 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
a composite type, the table function produces a column for each attribute
of the composite type.
</p><p>
Here is an example:
</p><pre class="screen">
CREATE TABLE foo (fooid int, foosubid int, fooname text);
INSERT INTO foo VALUES (1, 1, 'Joe');
INSERT INTO foo VALUES (1, 2, 'Ed');
INSERT INTO foo VALUES (2, 1, 'Mary');
CREATE FUNCTION getfoo(int) RETURNS foo AS $$
SELECT * FROM foo WHERE fooid = $1;
$$ LANGUAGE SQL;
SELECT *, upper(fooname) FROM getfoo(1) AS t1;
fooid | foosubid | fooname | upper
-------+----------+---------+-------
1 | 1 | Joe | JOE
(1 row)
</pre><p>
As the example shows, we can work with the columns of the function's
result just the same as if they were columns of a regular table.
</p><p>
Note that we only got one row out of the function. This is because
we did not use <code class="literal">SETOF</code>. That is described in the next section.
</p></div><div class="sect2" id="XFUNC-SQL-FUNCTIONS-RETURNING-SET"><div class="titlepage"><div><div><h3 class="title">38.5.9. <acronym class="acronym">SQL</acronym> Functions Returning Sets <a href="#XFUNC-SQL-FUNCTIONS-RETURNING-SET" class="id_link">#</a></h3></div></div></div><a id="id-1.8.3.8.18.2" class="indexterm"></a><p>
When an SQL function is declared as returning <code class="literal">SETOF
<em class="replaceable"><code>sometype</code></em></code>, the function's final
query is executed to completion, and each row it
outputs is returned as an element of the result set.
</p><p>
This feature is normally used when calling the function in the <code class="literal">FROM</code>
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 <code class="literal">foo</code> has the same contents as above, and we say:
</p><pre class="programlisting">
CREATE FUNCTION getfoo(int) RETURNS SETOF foo AS $$
SELECT * FROM foo WHERE fooid = $1;
$$ LANGUAGE SQL;
SELECT * FROM getfoo(1) AS t1;
</pre><p>
Then we would get:
</p><pre class="screen">
fooid | foosubid | fooname
-------+----------+---------
1 | 1 | Joe
1 | 2 | Ed
(2 rows)
</pre><p>
</p><p>
It is also possible to return multiple rows with the columns defined by
output parameters, like this:
</p><pre class="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)
</pre><p>
The key point here is that you must write <code class="literal">RETURNS SETOF record</code>
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 <code class="type">record</code>.
</p><p>
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 <code class="literal">LATERAL</code> key word,
which is described in <a class="xref" href="queries-table-expressions.html#QUERIES-LATERAL" title="7.2.1.5. LATERAL Subqueries">Section 7.2.1.5</a>.
Here is an example using a set-returning function to enumerate
elements of a tree structure:
</p><pre class="screen">
SELECT * FROM nodes;
name | parent
-----------+--------
Top |
Child1 | Top
Child2 | Top
Child3 | Top
SubChild1 | Child1
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)
</pre><p>
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.
</p><p>
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:
</p><pre class="screen">
SELECT listchildren('Top');
listchildren
--------------
Child1
Child2
Child3
(3 rows)
SELECT name, listchildren(name) FROM nodes;
name | listchildren
--------+--------------
Top | Child1
Top | Child2
Top | Child3
Child1 | SubChild1
Child1 | SubChild2
(5 rows)
</pre><p>
In the last <code class="command">SELECT</code>,
notice that no output row appears for <code class="literal">Child2</code>, <code class="literal">Child3</code>, etc.
This happens because <code class="function">listchildren</code> returns an empty set
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 <code class="literal">LATERAL</code> syntax.
</p><p>
<span class="productname">PostgreSQL</span>'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 <code class="literal">LATERAL FROM</code>-clause item
instead. For example,
</p><pre class="programlisting">
SELECT x, generate_series(1,5) AS g FROM tab;
</pre><p>
is almost equivalent to
</p><pre class="programlisting">
SELECT x, g FROM tab, LATERAL generate_series(1,5) AS g;
</pre><p>
It would be exactly the same, except that in this specific example,
the planner could choose to put <code class="structname">g</code> on the outside of the
nested-loop join, since <code class="structname">g</code> has no actual lateral dependency
on <code class="structname">tab</code>. 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 nested-loop join with the rest of
the <code class="literal">FROM</code> clause, so that the function(s) are run to
completion before the next row from the <code class="literal">FROM</code> clause is
considered.
</p><p>
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 <code class="literal">LATERAL ROWS FROM( ... )</code> <code class="literal">FROM</code>-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 <span class="quote">“<span class="quote">in lockstep</span>”</span> until they are all exhausted, and then
execution continues with the next underlying row.
</p><p>
Set-returning functions can be nested in a select list, although that is
not allowed in <code class="literal">FROM</code>-clause items. In such cases, each level
of nesting is treated separately, as though it were
a separate <code class="literal">LATERAL ROWS FROM( ... )</code> item. For example, in
</p><pre class="programlisting">
SELECT srf1(srf2(x), srf3(y)), srf4(srf5(z)) FROM tab;
</pre><p>
the set-returning functions <code class="function">srf2</code>, <code class="function">srf3</code>,
and <code class="function">srf5</code> would be run in lockstep for each row
of <code class="structname">tab</code>, and then <code class="function">srf1</code> and <code class="function">srf4</code>
would be applied in lockstep to each row produced by the lower
functions.
</p><p>
Set-returning functions cannot be used within conditional-evaluation
constructs, such as <code class="literal">CASE</code> or <code class="literal">COALESCE</code>. For
example, consider
</p><pre class="programlisting">
SELECT x, CASE WHEN x > 0 THEN generate_series(1, 5) ELSE 0 END FROM tab;
</pre><p>
It might seem that this should produce five repetitions of input rows
that have <code class="literal">x > 0</code>, and a single repetition of those that do
not; but actually, because <code class="function">generate_series(1, 5)</code> would be
run in an implicit <code class="literal">LATERAL FROM</code> item before
the <code class="literal">CASE</code> expression is ever evaluated, it would produce five
repetitions of every input row. To reduce confusion, such cases produce
a parse-time error instead.
</p><div class="note"><h3 class="title">Note</h3><p>
If a function's last command is <code class="command">INSERT</code>, <code class="command">UPDATE</code>,
or <code class="command">DELETE</code> with <code class="literal">RETURNING</code>, that command will
always be executed to completion, even if the function is not declared
with <code class="literal">SETOF</code> or the calling query does not fetch all the
result rows. Any extra rows produced by the <code class="literal">RETURNING</code>
clause are silently dropped, but the commanded table modifications
still happen (and are all completed before returning from the function).
</p></div><div class="note"><h3 class="title">Note</h3><p>
Before <span class="productname">PostgreSQL</span> 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 <code class="literal">CASE</code> etc.) was previously allowed,
complicating things even more.
Use of the <code class="literal">LATERAL</code> syntax is recommended when writing
queries that need to work in older <span class="productname">PostgreSQL</span> 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,
</p><pre class="programlisting">
SELECT x, CASE WHEN y > 0 THEN generate_series(1, z) ELSE 5 END FROM tab;
</pre><p>
could become
</p><pre class="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;
</pre><p>
This formulation will work the same in all versions
of <span class="productname">PostgreSQL</span>.
</p></div></div><div class="sect2" id="XFUNC-SQL-FUNCTIONS-RETURNING-TABLE"><div class="titlepage"><div><div><h3 class="title">38.5.10. <acronym class="acronym">SQL</acronym> Functions Returning <code class="literal">TABLE</code> <a href="#XFUNC-SQL-FUNCTIONS-RETURNING-TABLE" class="id_link">#</a></h3></div></div></div><a id="id-1.8.3.8.19.2" class="indexterm"></a><p>
There is another way to declare a function as returning a set,
which is to use the syntax
<code class="literal">RETURNS TABLE(<em class="replaceable"><code>columns</code></em>)</code>.
This is equivalent to using one or more <code class="literal">OUT</code> parameters plus
marking the function as returning <code class="literal">SETOF record</code> (or
<code class="literal">SETOF</code> 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 <code class="literal">SETOF</code>.
</p><p>
For example, the preceding sum-and-product example could also be
done this way:
</p><pre class="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;
</pre><p>
It is not allowed to use explicit <code class="literal">OUT</code> or <code class="literal">INOUT</code>
parameters with the <code class="literal">RETURNS TABLE</code> notation — you must
put all the output columns in the <code class="literal">TABLE</code> list.
</p></div><div class="sect2" id="XFUNC-SQL-POLYMORPHIC-FUNCTIONS"><div class="titlepage"><div><div><h3 class="title">38.5.11. Polymorphic <acronym class="acronym">SQL</acronym> Functions <a href="#XFUNC-SQL-POLYMORPHIC-FUNCTIONS" class="id_link">#</a></h3></div></div></div><p>
<acronym class="acronym">SQL</acronym> functions can be declared to accept and
return the polymorphic types described in <a class="xref" href="extend-type-system.html#EXTEND-TYPES-POLYMORPHIC" title="38.2.5. Polymorphic Types">Section 38.2.5</a>. Here is a polymorphic
function <code class="function">make_array</code> that builds up an array
from two arbitrary data type elements:
</p><pre class="screen">
CREATE FUNCTION make_array(anyelement, anyelement) RETURNS anyarray AS $$
SELECT ARRAY[$1, $2];
$$ LANGUAGE SQL;
SELECT make_array(1, 2) AS intarray, make_array('a'::text, 'b') AS textarray;
intarray | textarray
----------+-----------
{1,2} | {a,b}
(1 row)
</pre><p>
</p><p>
Notice the use of the typecast <code class="literal">'a'::text</code>
to specify that the argument is of type <code class="type">text</code>. This is
required if the argument is just a string literal, since otherwise
it would be treated as type
<code class="type">unknown</code>, and array of <code class="type">unknown</code> is not a valid
type.
Without the typecast, you will get errors like this:
</p><pre class="screen">
ERROR: could not determine polymorphic type because input has type unknown
</pre><p>
</p><p>
With <code class="function">make_array</code> declared as above, you must
provide two arguments that are of exactly the same data type; the
system will not attempt to resolve any type differences. Thus for
example this does not work:
</p><pre class="screen">
SELECT make_array(1, 2.5) AS numericarray;
ERROR: function make_array(integer, numeric) does not exist
</pre><p>
An alternative approach is to use the <span class="quote">“<span class="quote">common</span>”</span> family of
polymorphic types, which allows the system to try to identify a
suitable common type:
</p><pre class="screen">
CREATE FUNCTION make_array2(anycompatible, anycompatible)
RETURNS anycompatiblearray AS $$
SELECT ARRAY[$1, $2];
$$ LANGUAGE SQL;
SELECT make_array2(1, 2.5) AS numericarray;
numericarray
--------------
{1,2.5}
(1 row)
</pre><p>
Because the rules for common type resolution default to choosing
type <code class="type">text</code> when all inputs are of unknown types, this
also works:
</p><pre class="screen">
SELECT make_array2('a', 'b') AS textarray;
textarray
-----------
{a,b}
(1 row)
</pre><p>
</p><p>
It is permitted to have polymorphic arguments with a fixed
return type, but the converse is not. For example:
</p><pre class="screen">
CREATE FUNCTION is_greater(anyelement, anyelement) RETURNS boolean AS $$
SELECT $1 > $2;
$$ LANGUAGE SQL;
SELECT is_greater(1, 2);
is_greater
------------
f
(1 row)
CREATE FUNCTION invalid_func() RETURNS anyelement AS $$
SELECT 1;
$$ LANGUAGE SQL;
ERROR: cannot determine result data type
DETAIL: A result of type anyelement requires at least one input of type anyelement, anyarray, anynonarray, anyenum, or anyrange.
</pre><p>
</p><p>
Polymorphism can be used with functions that have output arguments.
For example:
</p><pre class="screen">
CREATE FUNCTION dup (f1 anyelement, OUT f2 anyelement, OUT f3 anyarray)
AS 'select $1, array[$1,$1]' LANGUAGE SQL;
SELECT * FROM dup(22);
f2 | f3
----+---------
22 | {22,22}
(1 row)
</pre><p>
</p><p>
Polymorphism can also be used with variadic functions.
For example:
</p><pre class="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)
</pre><p>
</p></div><div class="sect2" id="XFUNC-SQL-COLLATIONS"><div class="titlepage"><div><div><h3 class="title">38.5.12. <acronym class="acronym">SQL</acronym> Functions with Collations <a href="#XFUNC-SQL-COLLATIONS" class="id_link">#</a></h3></div></div></div><a id="id-1.8.3.8.21.2" class="indexterm"></a><p>
When an 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 <a class="xref" href="collation.html" title="24.2. Collation Support">Section 24.2</a>. 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
<code class="function">anyleast</code> function described above, the result of
</p><pre class="programlisting">
SELECT anyleast('abc'::text, 'ABC');
</pre><p>
will depend on the database's default collation. In <code class="literal">C</code> locale
the result will be <code class="literal">ABC</code>, but in many other locales it will
be <code class="literal">abc</code>. The collation to use can be forced by adding
a <code class="literal">COLLATE</code> clause to any of the arguments, for example
</p><pre class="programlisting">
SELECT anyleast('abc'::text, 'ABC' COLLATE "C");
</pre><p>
Alternatively, if you wish a function to operate with a particular
collation regardless of what it is called with, insert
<code class="literal">COLLATE</code> clauses as needed in the function definition.
This version of <code class="function">anyleast</code> would always use <code class="literal">en_US</code>
locale to compare strings:
</p><pre class="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;
</pre><p>
But note that this will throw an error if applied to a non-collatable
data type.
</p><p>
If no common collation can be identified among the actual arguments,
then an 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).
</p><p>
The behavior of collatable parameters can be thought of as a limited
form of polymorphism, applicable only to textual data types.
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