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37.5.1. Arguments forSQL Functions

37.5.2.SQL Functions on Base Types

37.5.3.SQL Functions on Composite Types

37.5.4.SQL Functions with Output Parameters

37.5.5.SQL Procedures with Output Parameters

37.5.6.SQL Functions with Variable Numbers of Arguments

37.5.7.SQL Functions with Default Values for Arguments

37.5.8.SQL Functions as Table Sources

37.5.9.SQL Functions Returning Sets

37.5.10.SQL Functions ReturningTABLE

37.5.11. PolymorphicSQL Functions

37.5.12.SQL Functions with Collations

Note

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.,CREATE TABLE), the effects of such commands will not be visible during parse analysis of later commands in the function. Thus, for example,CREATE TABLE foo (...); INSERT INTO foo VALUES(...); will not work as desired if packaged up into a single SQL function, sincefoo won't exist yet when theINSERT command is parsed. It's recommended to usePL/pgSQL instead of an SQL function in this type of situation.

Arguments of an SQL function can be referenced in the function body using either names or numbers. Examples of both methods appear below.

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 isfunction_name.argument_name. (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.)

In the older numeric approach, arguments are referenced using the syntax$n:$1 refers to the first input argument,$2 to the second, and so on. This will work whether or not the particular argument was declared with a name.

If an argument is of a composite type, then the dot notation, e.g.,argname.fieldname or$1.fieldname, 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.

SQL function arguments can only be used as data values, not as identifiers. Thus for example this is reasonable:

INSERT INTO mytable VALUES ($1);

but this will not work:

INSERT INTO $1 VALUES (42);

The simplest possibleSQL function has no arguments and simply returns a base type, such asinteger:

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

Notice that we defined a column alias within the function body for the result of the function (with the nameresult), but this column alias is not visible outside the function. Hence, the result is labeledone instead ofresult.

It is almost as easy to defineSQL functions that take base types as arguments:

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

Alternatively, we could dispense with names for the arguments and use numbers:

CREATE FUNCTION add_em(integer, integer) RETURNS integer AS $$    SELECT $1 + $2;$$ LANGUAGE SQL;SELECT add_em(1, 2) AS answer; answer--------      3

Here is a more useful function, which might be used to debit a bank account:

CREATE FUNCTION tf1 (accountno integer, debit numeric) RETURNS numeric AS $$    UPDATE bank        SET balance = balance - debit        WHERE accountno = tf1.accountno;    SELECT 1;$$ LANGUAGE SQL;

A user could execute this function to debit account 17 by $100.00 as follows:

SELECT tf1(17, 100.0);

In this example, we chose the nameaccountno for the first argument, but this is the same as the name of a column in thebank table. Within theUPDATE command,accountno refers to the columnbank.accountno, sotf1.accountno must be used to refer to the argument. We could of course avoid this by using a different name for the argument.

In practice one would probably like a more useful result from the function than a constant 1, so a more likely definition is:

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;

which adjusts the balance and returns the new balance. The same thing could be done in one command usingRETURNING:

CREATE FUNCTION tf1 (accountno integer, debit numeric) RETURNS numeric AS $$    UPDATE bank        SET balance = balance - debit        WHERE accountno = tf1.accountno    RETURNING balance;$$ LANGUAGE SQL;

If the finalSELECT orRETURNING clause in anSQL function does not return exactly the function's declared result type,Postgres Pro 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 previousadd_em function to return typefloat8 instead. It's sufficient to write

CREATE FUNCTION add_em(integer, integer) RETURNS float8 AS $$    SELECT $1 + $2;$$ LANGUAGE SQL;

since theinteger sum can be implicitly cast tofloat8. (SeeChapter 10 orCREATE CAST for more about casts.)

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 thatemp is a table containing employee data, and therefore also the name of the composite type of each row of the table. Here is a functiondouble_salary that computes what someone's salary would be if it were doubled:

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

Notice the use of the syntax$1.salary to select one field of the argument row value. Also notice how the callingSELECT command usestable_name.* 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:

SELECT name, double_salary(emp) AS dream    FROM emp    WHERE emp.cubicle ~= point '(2,1)';

but this usage is deprecated since it's easy to get confused. (SeeSection 8.16.5 for details about these two notations for the composite value of a table row.)

Sometimes it is handy to construct a composite argument value on-the-fly. This can be done with theROW construct. For example, we could adjust the data being passed to the function:

SELECT name, double_salary(ROW(name, salary*1.1, age, cubicle)) AS dream    FROM emp;

It is also possible to build a function that returns a composite type. This is an example of a function that returns a singleemp row:

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;

In this example we have specified each of the attributes with a constant value, but any computation could have been substituted for these constants.

Note two important things about defining the function:

A different way to define the same function is:

CREATE FUNCTION new_emp() RETURNS emp AS $$    SELECT ROW('None', 1000.0, 25, '(2,2)')::emp;$$ LANGUAGE SQL;

Here we wrote aSELECT 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.

We could call this function directly either by using it in a value expression:

SELECT new_emp();         new_emp-------------------------- (None,1000.0,25,"(2,2)")

or by calling it as a table function:

SELECT * FROM new_emp(); name | salary | age | cubicle------+--------+-----+--------- None | 1000.0 |  25 | (2,2)

The second way is described more fully inSection 37.5.8.

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:

SELECT (new_emp()).name; name------ None

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:

SELECT new_emp().name;ERROR:  syntax error at or near "."LINE 1: SELECT new_emp().name;                        ^

Another option is to use functional notation for extracting an attribute:

SELECT name(new_emp()); name------ None

As explained inSection 8.16.5, the field notation and functional notation are equivalent.

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:

CREATE FUNCTION getname(emp) RETURNS text AS $$    SELECT $1.name;$$ LANGUAGE SQL;SELECT getname(new_emp()); getname--------- None(1 row)

An alternative way of describing a function's results is to define it withoutput parameters, as in this example:

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)

This is not essentially different from the version ofadd_em shown inSection 37.5.2. The real value of output parameters is that they provide a convenient way of defining functions that return several columns. For example,

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)

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

CREATE TYPE sum_prod AS (sum int, product int);CREATE FUNCTION sum_n_product (int, int) RETURNS sum_prodAS 'SELECT $1 + $2, $1 * $2'LANGUAGE SQL;

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.)

Notice that output parameters are not included in the calling argument list when invoking such a function from SQL. This is becausePostgres Pro 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

DROP FUNCTION sum_n_product (x int, y int, OUT sum int, OUT product int);DROP FUNCTION sum_n_product (int, int);

Parameters can be marked asIN (the default),OUT,INOUT, orVARIADIC. AnINOUT parameter serves as both an input parameter (part of the calling argument list) and an output parameter (part of the result record type).VARIADIC parameters are input parameters, but are treated specially as described below.

Output parameters are also supported in procedures, but they work a bit differently from functions. InCALL commands, output parameters must be included in the argument list. For example, the bank account debiting routine from earlier could be written like this:

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;

To call this procedure, an argument matching theOUT parameter must be included. It's customary to writeNULL:

CALL tp1(17, 100.0, NULL);

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.

When calling a procedure fromPL/pgSQL, instead of writingNULL you must write a variable that will receive the procedure's output. SeeSection 42.6.3 for details.

SQL functions can be declared to accept variable numbers of arguments, so long as all the“optional” 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 asVARIADIC; this parameter must be declared as being of an array type. For example:

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)

Effectively, all the actual arguments at or beyond theVARIADIC position are gathered up into a one-dimensional array, as if you had written

SELECT mleast(ARRAY[10, -1, 5, 4.4]);    -- doesn't work

You can't actually write that, though — or at least, it will not match this function definition. A parameter markedVARIADIC matches one or more occurrences of its element type, not of its own type.

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; seeSection 10.3. You can do this by specifyingVARIADIC in the call:

SELECT mleast(VARIADIC ARRAY[10, -1, 5, 4.4]);

This prevents expansion of the function's variadic parameter into its element type, thereby allowing the array argument value to match normally.VARIADIC can only be attached to the last actual argument of a function call.

SpecifyingVARIADIC in the call is also the only way to pass an empty array to a variadic function, for example:

SELECT mleast(VARIADIC ARRAY[]::numeric[]);

Simply writingSELECT mleast() does not work because a variadic parameter must match at least one actual argument. (You could define a second function also namedmleast, with no parameters, if you wanted to allow such calls.)

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 (Section 4.3), except when you specifyVARIADIC. For example, this will work:

SELECT mleast(VARIADIC arr => ARRAY[10, -1, 5, 4.4]);

but not these:

SELECT mleast(arr => 10);SELECT mleast(arr => ARRAY[10, -1, 5, 4.4]);

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; seeSection 10.3.

For example:

CREATE FUNCTION foo(a int, b int DEFAULT 2, c int DEFAULT 3)RETURNS intLANGUAGE SQLAS $$    SELECT $1 + $2 + $3;$$;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 argumentERROR:  function foo() does not exist

The= sign can also be used in place of the key wordDEFAULT.

All SQL functions can be used in theFROM 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.

Here is an example:

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)

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.

Note that we only got one row out of the function. This is because we did not useSETOF. That is described in the next section.

When an SQL function is declared as returningSETOFsometype, the function's final query is executed to completion, and each row it outputs is returned as an element of the result set.

This feature is normally used when calling the function in theFROM clause. In this case each row returned by the function becomes a row of the table seen by the query. For example, assume that tablefoo has the same contents as above, and we say:

CREATE FUNCTION getfoo(int) RETURNS SETOF foo AS $$    SELECT * FROM foo WHERE fooid = $1;$$ LANGUAGE SQL;SELECT * FROM getfoo(1) AS t1;

Then we would get:

 fooid | foosubid | fooname-------+----------+---------     1 |        1 | Joe     1 |        2 | Ed(2 rows)

It is also possible to return multiple rows with the columns defined by output parameters, like this:

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 recordAS $$    SELECT $1 + tab.y, $1 * tab.y FROM tab;$$ LANGUAGE SQL;SELECT * FROM sum_n_product_with_tab(10); sum | product-----+---------  11 |      10  13 |      30  15 |      50  17 |      70(4 rows)

The key point here is that you must writeRETURNS SETOF record 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 ofrecord.

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 theLATERAL key word, which is described inSection 7.2.1.5. Here is an example using a set-returning function to enumerate elements of a tree structure:

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)

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.

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:

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)

In the lastSELECT, notice that no output row appears forChild2,Child3, etc. This happens becauselistchildren 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 theLATERAL syntax.

Postgres Pro'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 aLATERAL FROM-clause item instead. For example,

SELECT x, generate_series(1,5) AS g FROM tab;

is almost equivalent to

SELECT x, g FROM tab, LATERAL generate_series(1,5) AS g;

It would be exactly the same, except that in this specific example, the planner could choose to putg on the outside of the nested-loop join, sinceg has no actual lateral dependency ontab. 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 theFROM clause, so that the function(s) are run to completion before the next row from theFROM clause is considered.

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 singleLATERAL ROWS FROM( ... )FROM-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“in lockstep” until they are all exhausted, and then execution continues with the next underlying row.

Set-returning functions can be nested in a select list, although that is not allowed inFROM-clause items. In such cases, each level of nesting is treated separately, as though it were a separateLATERAL ROWS FROM( ... ) item. For example, in

SELECT srf1(srf2(x), srf3(y)), srf4(srf5(z)) FROM tab;

the set-returning functionssrf2,srf3, andsrf5 would be run in lockstep for each row oftab, and thensrf1 andsrf4 would be applied in lockstep to each row produced by the lower functions.

Set-returning functions cannot be used within conditional-evaluation constructs, such asCASE orCOALESCE. For example, consider

SELECT x, CASE WHEN x > 0 THEN generate_series(1, 5) ELSE 0 END FROM tab;

It might seem that this should produce five repetitions of input rows that havex > 0, and a single repetition of those that do not; but actually, becausegenerate_series(1, 5) would be run in an implicitLATERAL FROM item before theCASE expression is ever evaluated, it would produce five repetitions of every input row. To reduce confusion, such cases produce a parse-time error instead.

SELECT x, CASE WHEN y > 0 THEN generate_series(1, z) ELSE 5 END FROM tab;

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;

There is another way to declare a function as returning a set, which is to use the syntaxRETURNS TABLE(columns). This is equivalent to using one or moreOUT parameters plus marking the function as returningSETOF record (orSETOF 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 usingSETOF.

For example, the preceding sum-and-product example could also be done this way:

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;

It is not allowed to use explicitOUT orINOUT parameters with theRETURNS TABLE notation — you must put all the output columns in theTABLE list.

SQL functions can be declared to accept and return the polymorphic types described inSection 37.2.5. Here is a polymorphic functionmake_array that builds up an array from two arbitrary data type elements:

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)

Notice the use of the typecast'a'::text to specify that the argument is of typetext. This is required if the argument is just a string literal, since otherwise it would be treated as typeunknown, and array ofunknown is not a valid type. Without the typecast, you will get errors like this:

ERROR:  could not determine polymorphic type because input has type unknown

Withmake_array 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:

SELECT make_array(1, 2.5) AS numericarray;ERROR:  function make_array(integer, numeric) does not exist

An alternative approach is to use the“common” family of polymorphic types, which allows the system to try to identify a suitable common type:

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)

Because the rules for common type resolution default to choosing typetext when all inputs are of unknown types, this also works:

SELECT make_array2('a', 'b') AS textarray; textarray----------- {a,b}(1 row)

It is permitted to have polymorphic arguments with a fixed return type, but the converse is not. For example:

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 typeDETAIL:  A result of type anyelement requires at least one input of type anyelement, anyarray, anynonarray, anyenum, or anyrange.

Polymorphism can be used with functions that have output arguments. For example:

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)

Polymorphism can also be used with variadic functions. For example:

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)

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 inSection 22.2. 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 theanyleast function described above, the result of

SELECT anyleast('abc'::text, 'ABC');

will depend on the database's default collation. InC locale the result will beABC, but in many other locales it will beabc. The collation to use can be forced by adding aCOLLATE clause to any of the arguments, for example

SELECT anyleast('abc'::text, 'ABC' COLLATE "C");

Alternatively, if you wish a function to operate with a particular collation regardless of what it is called with, insertCOLLATE clauses as needed in the function definition. This version ofanyleast would always useen_US locale to compare strings:

CREATE FUNCTION anyleast (VARIADIC anyarray) RETURNS anyelement AS $$    SELECT min($1[i] COLLATE "en_US") FROM generate_subscripts($1, 1) g(i);$$ LANGUAGE SQL;

But note that this will throw an error if applied to a non-collatable data type.

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).

The behavior of collatable parameters can be thought of as a limited form of polymorphism, applicable only to textual data types.