TABLE
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
, or equivalently by declaring it as
sometype
RETURNS TABLE(
. In this case
all rows of the last query's result are returned. Further details appear
below.
columns
)
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
, or DELETE
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
, and DELETE
), 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 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,
since foo
won't exist yet when the INSERT
command is parsed. It's recommended to use PL/pgSQL
instead of an SQL function in this type of situation.
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).
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
.
(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.)
function_name
.argument_name
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.,
or
argname
.fieldname
$1.
, 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.
fieldname
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 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 $
notation.
n
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 a 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.)
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 38.5.8.
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)
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 38.5.2. 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.
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 43.6.3 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 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]);
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
.
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.
When an SQL function is declared as returning SETOF
, the function's final
query is executed to completion, and each row it
outputs is returned as an element of the result set.
sometype
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.
If a function's last command is INSERT
, UPDATE
,
or DELETE
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).
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.
TABLE
There is another way to declare a function as returning a set,
which is to use the syntax
RETURNS TABLE(
.
This is equivalent to using one or more columns
)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.
SQL functions can be declared to accept and
return the polymorphic types described in Section 38.2.5. 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 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 in Section 24.2. 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.