Convert HTML docs to more streamlined TXT

[ai-pg] / full-docs / txt / xfunc-sql.txt

36.5. Query Language (SQL) Functions #

   36.5.1. Arguments for SQL Functions
   36.5.2. SQL Functions on Base Types
   36.5.3. SQL Functions on Composite Types
   36.5.4. SQL Functions with Output Parameters
   36.5.5. SQL Procedures with Output Parameters
   36.5.6. SQL Functions with Variable Numbers of Arguments
   36.5.7. SQL Functions with Default Values for Arguments
   36.5.8. SQL Functions as Table Sources
   36.5.9. SQL Functions Returning Sets
   36.5.10. SQL Functions Returning TABLE
   36.5.11. Polymorphic SQL Functions
   36.5.12. SQL Functions with Collations

   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 “the first row” of a multirow result is not well-defined
   unless you use ORDER BY.) If the last query happens to return no rows
   at all, the null value will be returned.

   Alternatively, an SQL function can be declared to return a set (that
   is, multiple rows) by specifying the function's return type as SETOF
   sometype, or equivalently by declaring it as RETURNS TABLE(columns). In
   this case all rows of the last query's result are returned. Further
   details appear below.

   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 void, the last statement must be a
   SELECT, or an INSERT, UPDATE, DELETE, or MERGE that has a RETURNING
   clause.

   Any collection of commands in the SQL language can be packaged together
   and defined as a function. Besides SELECT queries, the commands can
   include data modification queries (INSERT, UPDATE, DELETE, and MERGE),
   as well as other SQL commands. (You cannot use transaction control
   commands, e.g., COMMIT, SAVEPOINT, and some utility commands, e.g.,
   VACUUM, in SQL functions.) However, the final command must be a SELECT
   or have a RETURNING 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 void. For example, this function removes
   rows with negative salaries from the emp table:
CREATE FUNCTION clean_emp() RETURNS void AS '
    DELETE FROM emp
        WHERE salary < 0;
' LANGUAGE SQL;

SELECT clean_emp();

 clean_emp
-----------

(1 row)

   You can also write this as a procedure, thus avoiding the issue of the
   return type. For example:
CREATE PROCEDURE clean_emp() AS '
    DELETE FROM emp
        WHERE salary < 0;
' LANGUAGE SQL;

CALL clean_emp();

   In simple cases like this, the difference between a function returning
   void 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 void is a PostgreSQL extension.

   The syntax of the CREATE FUNCTION command requires the function body to
   be written as a string constant. It is usually most convenient to use
   dollar quoting (see Section 4.1.2.4) for the string constant. If you
   choose to use regular single-quoted string constant syntax, you must
   double single quote marks (') and backslashes (\) (assuming escape
   string syntax) in the body of the function (see Section 4.1.2.1).

36.5.1. Arguments for SQL Functions #

   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 is
   function_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);

Note

   The ability to use names to reference SQL function arguments was added
   in PostgreSQL 9.2. Functions to be used in older servers must use the
   $n notation.

36.5.2. SQL Functions on Base Types #

   The simplest possible SQL function has no arguments and simply returns
   a base type, such as integer:
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 name result), but this column alias is
   not visible outside the function. Hence, the result is labeled one
   instead of result.

   It is almost as easy to define SQL 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 name accountno for the first argument,
   but this is the same as the name of a column in the bank table. Within
   the UPDATE command, accountno refers to the column bank.accountno, so
   tf1.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 using RETURNING:
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 final SELECT or RETURNING clause in an SQL function does not
   return exactly the function's declared result type, PostgreSQL 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 add_em
   function to return type float8 instead. It's sufficient to write
CREATE FUNCTION add_em(integer, integer) RETURNS float8 AS $$
    SELECT $1 + $2;
$$ LANGUAGE SQL;

   since the integer sum can be implicitly cast to float8. (See Chapter 10
   or CREATE CAST for more about casts.)

36.5.3. SQL Functions on Composite Types #

   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 emp 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 double_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 calling SELECT command uses
   table_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. (See
   Section 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 the ROW 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 single emp 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:
     * 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.)
     * 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:

ERROR:  return type mismatch in function declared to return emp
DETAIL:  Final statement returns text instead of point at column 4.


       As with the base-type case, the system will not insert explicit
       casts automatically, only implicit or assignment casts.

   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 a SELECT 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 in Section 36.5.8.

   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 in Section 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)

36.5.4. SQL Functions with Output Parameters #

   An alternative way of describing a function's results is to define it
   with output 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 of add_em shown in
   Section 36.5.2. 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_prod
AS '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 because PostgreSQL
   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 as IN (the default), OUT, INOUT, or VARIADIC.
   An INOUT 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.

36.5.5. SQL Procedures with Output Parameters #

   Output parameters are also supported in procedures, but they work a bit
   differently from functions. In CALL 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 the OUT parameter must be
   included. It's customary to write NULL:
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 from PL/pgSQL, instead of writing NULL you
   must write a variable that will receive the procedure's output. See
   Section 41.6.3 for details.

36.5.6. SQL Functions with Variable Numbers of Arguments #

   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 as VARIADIC; 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 the VARIADIC
   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 marked VARIADIC 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; see
   Section 10.3. You can do this by specifying VARIADIC 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.

   Specifying VARIADIC 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 writing SELECT mleast() does not work because a variadic
   parameter must match at least one actual argument. (You could define a
   second function also named mleast, 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 specify VARIADIC. 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]);

36.5.7. SQL Functions with Default Values for Arguments #

   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
   Section 10.3.

   For example:
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

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

36.5.8. SQL Functions as Table Sources #

   All SQL functions can be used in the FROM 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 use SETOF. That is described in the next section.

36.5.9. SQL Functions Returning Sets #

   When an SQL function is declared as returning SETOF sometype, 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 the FROM
   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 foo 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 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)

   The key point here is that you must write RETURNS 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 of record.

   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 LATERAL key word, which is
   described in Section 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 last SELECT, notice that no output row appears for Child2,
   Child3, etc. This happens because listchildren 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 LATERAL syntax.

   PostgreSQL'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 LATERAL 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 put g on the outside of the nested-loop join,
   since g has no actual lateral dependency on tab. 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 FROM clause, so that the function(s) are run
   to completion before the next row from the FROM 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 single LATERAL 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 in FROM-clause items. In such cases, each level of
   nesting is treated separately, as though it were a separate LATERAL
   ROWS FROM( ... ) item. For example, in
SELECT srf1(srf2(x), srf3(y)), srf4(srf5(z)) FROM tab;

   the set-returning functions srf2, srf3, and srf5 would be run in
   lockstep for each row of tab, and then srf1 and srf4 would be applied
   in lockstep to each row produced by the lower functions.

   Set-returning functions cannot be used within conditional-evaluation
   constructs, such as CASE or COALESCE. 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 have x > 0, and a single repetition of those that do not; but
   actually, because generate_series(1, 5) would be run in an implicit
   LATERAL FROM item before the CASE expression is ever evaluated, it
   would produce five repetitions of every input row. To reduce confusion,
   such cases produce a parse-time error instead.

Note

   If a function's last command is INSERT, UPDATE, DELETE, or MERGE with
   RETURNING, that command will always be executed to completion, even if
   the function is not declared with SETOF or the calling query does not
   fetch all the result rows. Any extra rows produced by the RETURNING
   clause are silently dropped, but the commanded table modifications
   still happen (and are all completed before returning from the
   function).

Note

   Before PostgreSQL 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 CASE etc.) was previously
   allowed, complicating things even more. Use of the LATERAL syntax is
   recommended when writing queries that need to work in older PostgreSQL
   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,
SELECT x, CASE WHEN y > 0 THEN generate_series(1, z) ELSE 5 END FROM tab;

   could become
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;

   This formulation will work the same in all versions of PostgreSQL.

36.5.10. SQL Functions Returning TABLE #

   There is another way to declare a function as returning a set, which is
   to use the syntax RETURNS TABLE(columns). This is equivalent to using
   one or more OUT parameters plus marking the function as returning SETOF
   record (or SETOF 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 SETOF.

   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 explicit OUT or INOUT parameters with the
   RETURNS TABLE notation — you must put all the output columns in the
   TABLE list.

36.5.11. Polymorphic SQL Functions #

   SQL functions can be declared to accept and return the polymorphic
   types described in Section 36.2.5. Here is a polymorphic function
   make_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 type text. This is required if the argument is just a string
   literal, since otherwise it would be treated as type unknown, and array
   of unknown 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

   With make_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 type
   text 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 type
DETAIL:  A result of type anyelement requires at least one input of type anyelem
ent, 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)

36.5.12. SQL Functions with Collations #

   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
   Section 23.2. 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 the anyleast
   function described above, the result of
SELECT anyleast('abc'::text, 'ABC');

   will depend on the database's default collation. In C locale the result
   will be ABC, but in many other locales it will be abc. The collation to
   use can be forced by adding a COLLATE 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, insert COLLATE clauses
   as needed in the function definition. This version of anyleast would
   always use en_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.