CREATE FUNCTION — define a new function
CREATE [ OR REPLACE ] FUNCTION
    name ( [ [ argmode ] [ argname ] argtype [ { DEFAULT | = } default_expr ] [, ...] ] )
    [ RETURNS rettype
      | RETURNS TABLE ( column_name column_type [, ...] ) ]
  { LANGUAGE lang_name
    | TRANSFORM { FOR TYPE type_name } [, ... ]
    | WINDOW
    | IMMUTABLE | STABLE | VOLATILE | [ NOT ] LEAKPROOF
    | CALLED ON NULL INPUT | RETURNS NULL ON NULL INPUT | STRICT
    | [ EXTERNAL ] SECURITY INVOKER | [ EXTERNAL ] SECURITY DEFINER
    | PARALLEL { UNSAFE | RESTRICTED | SAFE }
    | COST execution_cost
    | ROWS result_rows
    | SUPPORT support_function
    | SET configuration_parameter { TO value | = value | FROM CURRENT }
    | AS 'definition'
    | AS 'obj_file', 'link_symbol'
  } ...
   CREATE FUNCTION defines a new function.
   CREATE OR REPLACE FUNCTION will either create a
   new function, or replace an existing definition.
   To be able to define a function, the user must have the
   USAGE privilege on the language.
  
If a schema name is included, then the function is created in the specified schema. Otherwise it is created in the current schema. The name of the new function must not match any existing function or procedure with the same input argument types in the same schema. However, functions and procedures of different argument types can share a name (this is called overloading).
   To replace the current definition of an existing function, use
   CREATE OR REPLACE FUNCTION.  It is not possible
   to change the name or argument types of a function this way (if you
   tried, you would actually be creating a new, distinct function).
   Also, CREATE OR REPLACE FUNCTION will not let
   you change the return type of an existing function.  To do that,
   you must drop and recreate the function.  (When using OUT
   parameters, that means you cannot change the types of any
   OUT parameters except by dropping the function.)
  
   When CREATE OR REPLACE FUNCTION is used to replace an
   existing function, the ownership and permissions of the function
   do not change.  All other function properties are assigned the
   values specified or implied in the command.  You must own the function
   to replace it (this includes being a member of the owning role).
  
   If you drop and then recreate a function, the new function is not
   the same entity as the old; you will have to drop existing rules, views,
   triggers, etc. that refer to the old function.  Use
   CREATE OR REPLACE FUNCTION to change a function
   definition without breaking objects that refer to the function.
   Also, ALTER FUNCTION can be used to change most of the
   auxiliary properties of an existing function.
  
The user that creates the function becomes the owner of the function.
   To be able to create a function, you must have USAGE
   privilege on the argument types and the return type.
  
nameThe name (optionally schema-qualified) of the function to create.
argmode
       The mode of an argument: IN, OUT,
       INOUT, or VARIADIC.
       If omitted, the default is IN.
       Only OUT arguments can follow a VARIADIC one.
       Also, OUT and INOUT arguments cannot be used
       together with the RETURNS TABLE notation.
      
argnameThe name of an argument. Some languages (including SQL and PL/pgSQL) let you use the name in the function body. For other languages the name of an input argument is just extra documentation, so far as the function itself is concerned; but you can use input argument names when calling a function to improve readability (see Section 4.3). In any case, the name of an output argument is significant, because it defines the column name in the result row type. (If you omit the name for an output argument, the system will choose a default column name.)
argtypeThe data type(s) of the function's arguments (optionally schema-qualified), if any. The argument types can be base, composite, or domain types, or can reference the type of a table column.
       Depending on the implementation language it might also be allowed
       to specify “pseudo-types” such as cstring.
       Pseudo-types indicate that the actual argument type is either
       incompletely specified, or outside the set of ordinary SQL data types.
      
       The type of a column is referenced by writing
       table_name.column_name%TYPE
default_expr
       An expression to be used as default value if the parameter is
       not specified.  The expression has to be coercible to the
       argument type of the parameter.
       Only input (including INOUT) parameters can have a default
        value.  All input parameters following a
       parameter with a default value must have default values as well.
      
rettype
       The return data type (optionally schema-qualified). The return type
       can be a base, composite, or domain type,
       or can reference the type of a table column.
       Depending on the implementation language it might also be allowed
       to specify “pseudo-types” such as cstring.
       If the function is not supposed to return a value, specify
       void as the return type.
      
       When there are OUT or INOUT parameters,
       the RETURNS clause can be omitted.  If present, it
       must agree with the result type implied by the output parameters:
       RECORD if there are multiple output parameters, or
       the same type as the single output parameter.
      
       The SETOF
       modifier indicates that the function will return a set of
       items, rather than a single item.
      
       The type of a column is referenced by writing
       table_name.column_name%TYPE
column_name
       The name of an output column in the RETURNS TABLE
       syntax.  This is effectively another way of declaring a named
       OUT parameter, except that RETURNS TABLE
       also implies RETURNS SETOF.
      
column_type
       The data type of an output column in the RETURNS TABLE
       syntax.
      
lang_name
       The name of the language that the function is implemented in.
       It can be sql, c,
       internal, or the name of a user-defined
       procedural language, e.g. plpgsql.  Enclosing the
       name in single quotes is deprecated and requires matching case.
      
TRANSFORM { FOR TYPE type_name } [, ... ] }Lists which transforms a call to the function should apply. Transforms convert between SQL types and language-specific data types; see CREATE TRANSFORM. Procedural language implementations usually have hardcoded knowledge of the built-in types, so those don't need to be listed here. If a procedural language implementation does not know how to handle a type and no transform is supplied, it will fall back to a default behavior for converting data types, but this depends on the implementation.
WINDOWWINDOW indicates that the function is a
       window function rather than a plain function.
       This is currently only useful for functions written in C.
       The WINDOW attribute cannot be changed when
       replacing an existing function definition.
      
IMMUTABLESTABLEVOLATILE
       These attributes inform the query optimizer about the behavior
       of the function.  At most one choice
       can be specified.  If none of these appear,
       VOLATILE is the default assumption.
      
IMMUTABLE indicates that the function
       cannot modify the database and always
       returns the same result when given the same argument values; that
       is, it does not do database lookups or otherwise use information not
       directly present in its argument list.  If this option is given,
       any call of the function with all-constant arguments can be
       immediately replaced with the function value.
      
STABLE indicates that the function
       cannot modify the database,
       and that within a single table scan it will consistently
       return the same result for the same argument values, but that its
       result could change across SQL statements.  This is the appropriate
       selection for functions whose results depend on database lookups,
       parameter variables (such as the current time zone), etc.  (It is
       inappropriate for AFTER triggers that wish to
       query rows modified by the current command.)  Also note
       that the current_timestamp family of functions qualify
       as stable, since their values do not change within a transaction.
      
VOLATILE indicates that the function value can
       change even within a single table scan, so no optimizations can be
       made.  Relatively few database functions are volatile in this sense;
       some examples are random(), currval(),
       timeofday().  But note that any function that has
       side-effects must be classified volatile, even if its result is quite
       predictable, to prevent calls from being optimized away; an example is
       setval().
      
For additional details see Section 37.7.
LEAKPROOF
       LEAKPROOF indicates that the function has no side
       effects.  It reveals no information about its arguments other than by
       its return value.  For example, a function which throws an error message
       for some argument values but not others, or which includes the argument
       values in any error message, is not leakproof.  This affects how the
       system executes queries against views created with the
       security_barrier option or tables with row level
       security enabled.  The system will enforce conditions from security
       policies and security barrier views before any user-supplied conditions
       from the query itself that contain non-leakproof functions, in order to
       prevent the inadvertent exposure of data.  Functions and operators
       marked as leakproof are assumed to be trustworthy, and may be executed
       before conditions from security policies and security barrier views.
       In addition, functions which do not take arguments or which are not
       passed any arguments from the security barrier view or table do not have
       to be marked as leakproof to be executed before security conditions.  See
       CREATE VIEW and Section 40.5.
       This option can only be set by the superuser.
      
CALLED ON NULL INPUTRETURNS NULL ON NULL INPUTSTRICTCALLED ON NULL INPUT (the default) indicates
       that the function will be called normally when some of its
       arguments are null.  It is then the function author's
       responsibility to check for null values if necessary and respond
       appropriately.
      
RETURNS NULL ON NULL INPUT or
       STRICT indicates that the function always
       returns null whenever any of its arguments are null.  If this
       parameter is specified, the function is not executed when there
       are null arguments; instead a null result is assumed
       automatically.
      
[EXTERNAL] SECURITY INVOKER[EXTERNAL] SECURITY DEFINERSECURITY INVOKER indicates that the function
      is to be executed with the privileges of the user that calls it.
      That is the default.  SECURITY DEFINER
      specifies that the function is to be executed with the
      privileges of the user that owns it.
     
      The key word EXTERNAL is allowed for SQL
      conformance, but it is optional since, unlike in SQL, this feature
      applies to all functions not only external ones.
     
PARALLELPARALLEL UNSAFE indicates that the function
      can't be executed in parallel mode and the presence of such a
      function in an SQL statement forces a serial execution plan.  This is
      the default.  PARALLEL RESTRICTED indicates that
      the function can be executed in parallel mode, but the execution is
      restricted to parallel group leader.  PARALLEL SAFE
      indicates that the function is safe to run in parallel mode without
      restriction.
     
      Functions should be labeled parallel unsafe if they modify any database
      state, or if they make changes to the transaction such as using
      sub-transactions, or if they access sequences or attempt to make
      persistent changes to settings (e.g. setval).  They should
      be labeled as parallel restricted if they access temporary tables,
      client connection state, cursors, prepared statements, or miscellaneous
      backend-local state which the system cannot synchronize in parallel mode
      (e.g.  setseed cannot be executed other than by the group
      leader because a change made by another process would not be reflected
      in the leader).  In general, if a function is labeled as being safe when
      it is restricted or unsafe, or if it is labeled as being restricted when
      it is in fact unsafe, it may throw errors or produce wrong answers
      when used in a parallel query.  C-language functions could in theory
      exhibit totally undefined behavior if mislabeled, since there is no way
      for the system to protect itself against arbitrary C code, but in most
      likely cases the result will be no worse than for any other function.
      If in doubt, functions should be labeled as UNSAFE, which is
      the default.
     
COST execution_costA positive number giving the estimated execution cost for the function, in units of cpu_operator_cost. If the function returns a set, this is the cost per returned row. If the cost is not specified, 1 unit is assumed for C-language and internal functions, and 100 units for functions in all other languages. Larger values cause the planner to try to avoid evaluating the function more often than necessary.
ROWS result_rowsA positive number giving the estimated number of rows that the planner should expect the function to return. This is only allowed when the function is declared to return a set. The default assumption is 1000 rows.
SUPPORT support_functionThe name (optionally schema-qualified) of a planner support function to use for this function. See Section 37.11 for details. You must be superuser to use this option.
configuration_parametervalue
       The SET clause causes the specified configuration
       parameter to be set to the specified value when the function is
       entered, and then restored to its prior value when the function exits.
       SET FROM CURRENT saves the value of the parameter that
       is current when CREATE FUNCTION is executed as the value
       to be applied when the function is entered.
      
       If a SET clause is attached to a function, then
       the effects of a SET LOCAL command executed inside the
       function for the same variable are restricted to the function: the
       configuration parameter's prior value is still restored at function exit.
       However, an ordinary
       SET command (without LOCAL) overrides the
       SET clause, much as it would do for a previous SET
       LOCAL command: the effects of such a command will persist after
       function exit, unless the current transaction is rolled back.
      
See SET and Chapter 19 for more information about allowed parameter names and values.
definitionA string constant defining the function; the meaning depends on the language. It can be an internal function name, the path to an object file, an SQL command, or text in a procedural language.
It is often helpful to use dollar quoting (see Section 4.1.2.4) to write the function definition string, rather than the normal single quote syntax. Without dollar quoting, any single quotes or backslashes in the function definition must be escaped by doubling them.
obj_file, link_symbol
       This form of the AS clause is used for
       dynamically loadable C language functions when the function name
       in the C language source code is not the same as the name of
       the SQL function. The string obj_file is the name of the shared
       library file containing the compiled C function, and is interpreted
       as for the LOAD command.  The string
       link_symbol is the
       function's link symbol, that is, the name of the function in the C
       language source code.  If the link symbol is omitted, it is assumed to
       be the same as the name of the SQL function being defined.  The C names
       of all functions must be different, so you must give overloaded C
       functions different C names (for example, use the argument types as
       part of the C names).
      
       When repeated CREATE FUNCTION calls refer to
       the same object file, the file is only loaded once per session.
       To unload and
       reload the file (perhaps during development), start a new session.
      
Refer to Section 37.3 for further information on writing functions.
PostgreSQL allows function overloading; that is, the same name can be used for several different functions so long as they have distinct input argument types. Whether or not you use it, this capability entails security precautions when calling functions in databases where some users mistrust other users; see Section 10.3.
    Two functions are considered the same if they have the same names and
    input argument types, ignoring any OUT
    parameters.  Thus for example these declarations conflict:
CREATE FUNCTION foo(int) ... CREATE FUNCTION foo(int, out text) ...
Functions that have different argument type lists will not be considered to conflict at creation time, but if defaults are provided they might conflict in use. For example, consider
CREATE FUNCTION foo(int) ... CREATE FUNCTION foo(int, int default 42) ...
    A call foo(10) will fail due to the ambiguity about which
    function should be called.
   
    The full SQL type syntax is allowed for
    declaring a function's arguments and return value.  However,
    parenthesized type modifiers (e.g., the precision field for
    type numeric) are discarded by CREATE FUNCTION.
    Thus for example
    CREATE FUNCTION foo (varchar(10)) ...
    is exactly the same as
    CREATE FUNCTION foo (varchar) ....
   
    When replacing an existing function with CREATE OR REPLACE
    FUNCTION, there are restrictions on changing parameter names.
    You cannot change the name already assigned to any input parameter
    (although you can add names to parameters that had none before).
    If there is more than one output parameter, you cannot change the
    names of the output parameters, because that would change the
    column names of the anonymous composite type that describes the
    function's result.  These restrictions are made to ensure that
    existing calls of the function do not stop working when it is replaced.
   
    If a function is declared STRICT with a VARIADIC
    argument, the strictness check tests that the variadic array as
    a whole is non-null.  The function will still be called if the
    array has null elements.
   
Here are some trivial examples to help you get started. For more information and examples, see Section 37.3.
CREATE FUNCTION add(integer, integer) RETURNS integer
    AS 'select $1 + $2;'
    LANGUAGE SQL
    IMMUTABLE
    RETURNS NULL ON NULL INPUT;
Increment an integer, making use of an argument name, in PL/pgSQL:
CREATE OR REPLACE FUNCTION increment(i integer) RETURNS integer AS $$
        BEGIN
                RETURN i + 1;
        END;
$$ LANGUAGE plpgsql;
Return a record containing multiple output parameters:
CREATE FUNCTION dup(in int, out f1 int, out f2 text)
    AS $$ SELECT $1, CAST($1 AS text) || ' is text' $$
    LANGUAGE SQL;
SELECT * FROM dup(42);
You can do the same thing more verbosely with an explicitly named composite type:
CREATE TYPE dup_result AS (f1 int, f2 text);
CREATE FUNCTION dup(int) RETURNS dup_result
    AS $$ SELECT $1, CAST($1 AS text) || ' is text' $$
    LANGUAGE SQL;
SELECT * FROM dup(42);
   Another way to return multiple columns is to use a TABLE
   function:
CREATE FUNCTION dup(int) RETURNS TABLE(f1 int, f2 text)
    AS $$ SELECT $1, CAST($1 AS text) || ' is text' $$
    LANGUAGE SQL;
SELECT * FROM dup(42);
   However, a TABLE function is different from the
   preceding examples, because it actually returns a set
   of records, not just one record.
  
SECURITY DEFINER Functions Safely
    Because a SECURITY DEFINER function is executed
    with the privileges of the user that owns it, care is needed to
    ensure that the function cannot be misused.  For security,
    search_path should be set to exclude any schemas
    writable by untrusted users.  This prevents
    malicious users from creating objects (e.g., tables, functions, and
    operators) that mask objects intended to be used by the function.
    Particularly important in this regard is the
    temporary-table schema, which is searched first by default, and
    is normally writable by anyone.  A secure arrangement can be obtained
    by forcing the temporary schema to be searched last.  To do this,
    write pg_temp as the last entry in search_path.
    This function illustrates safe usage:
CREATE FUNCTION check_password(uname TEXT, pass TEXT)
RETURNS BOOLEAN AS $$
DECLARE passed BOOLEAN;
BEGIN
        SELECT  (pwd = $2) INTO passed
        FROM    pwds
        WHERE   username = $1;
        RETURN passed;
END;
$$  LANGUAGE plpgsql
    SECURITY DEFINER
    -- Set a secure search_path: trusted schema(s), then 'pg_temp'.
    SET search_path = admin, pg_temp;
    This function's intention is to access a table admin.pwds.
    But without the SET clause, or with a SET clause
    mentioning only admin, the function could be subverted by
    creating a temporary table named pwds.
   
    Before PostgreSQL version 8.3, the
    SET clause was not available, and so older functions may
    contain rather complicated logic to save, set, and restore
    search_path.  The SET clause is far easier
    to use for this purpose.
   
    Another point to keep in mind is that by default, execute privilege
    is granted to PUBLIC for newly created functions
    (see Section 5.7 for more
    information).  Frequently you will wish to restrict use of a security
    definer function to only some users.  To do that, you must revoke
    the default PUBLIC privileges and then grant execute
    privilege selectively.  To avoid having a window where the new function
    is accessible to all, create it and set the privileges within a single
    transaction.  For example:
   
BEGIN; CREATE FUNCTION check_password(uname TEXT, pass TEXT) ... SECURITY DEFINER; REVOKE ALL ON FUNCTION check_password(uname TEXT, pass TEXT) FROM PUBLIC; GRANT EXECUTE ON FUNCTION check_password(uname TEXT, pass TEXT) TO admins; COMMIT;
   A CREATE FUNCTION command is defined in the SQL standard.
   The PostgreSQL version is similar but
   not fully compatible.  The attributes are not portable, neither are the
   different available languages.
  
   For compatibility with some other database systems,
   argmode can be written
   either before or after argname.
   But only the first way is standard-compliant.
  
   For parameter defaults, the SQL standard specifies only the syntax with
   the DEFAULT key word.  The syntax
   with = is used in T-SQL and Firebird.