Create function
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See the dedicated 'User-defined subprograms and anonymous blocks' section.
User-defined functions are part of a larger area of functionality. See this major section:
Synopsis
Use the CREATE FUNCTION
statement to create a function in a database.
Syntax
create_function ::= CREATE [ OR REPLACE ] FUNCTION subprogram_name (
[ arg_decl_with_dflt [ , ... ] ] )
{ RETURNS data_type
| RETURNS TABLE ( { column_name data_type }
[ , ... ] ) }
{ unalterable_fn_attribute
| alterable_fn_only_attribute
| alterable_fn_and_proc_attribute } [ ... ]
arg_decl_with_dflt ::= arg_decl [ { DEFAULT | = } expression ]
arg_decl ::= [ formal_arg ] [ arg_mode ] arg_type
subprogram_signature ::= arg_decl [ , ... ]
unalterable_fn_attribute ::= WINDOW
| LANGUAGE lang_name
| AS subprogram_implementation
lang_name ::= SQL | PLPGSQL | C
subprogram_implementation ::= ' sql_stmt_list '
| ' plpgsql_block_stmt '
| ' obj_file ' [ , ' link_symbol ' ]
sql_stmt_list ::= sql_stmt ; [ sql_stmt ... ]
alterable_fn_and_proc_attribute ::= SET run_time_parameter
{ TO value
| = value
| FROM CURRENT }
| RESET run_time_parameter
| RESET ALL
| [ EXTERNAL ] SECURITY
{ INVOKER | DEFINER }
alterable_fn_only_attribute ::= volatility
| on_null_input
| PARALLEL parallel_mode
| [ NOT ] LEAKPROOF
| COST int_literal
| ROWS int_literal
volatility ::= IMMUTABLE | STABLE | VOLATILE
on_null_input ::= CALLED ON NULL INPUT
| RETURNS NULL ON NULL INPUT
| STRICT
parallel_mode ::= UNSAFE | RESTRICTED | SAFE
'Regard the 'RETURNS' clause as mandatory.
The general introduction to the topic of user-defined subprograms explains that, for historical reasons, you can create a function, when a creating a procedure is the proper choice, by omitting the RETURNS
clause and by giving it OUT
or INOUT
arguments.
Yugabyte recommends that you don't exploit this freedom.
For this reason, the RETURNS
clause is shown as mandatory in the create_function syntax rule (though the PostgreSQL documentation shows it as optional).
Yugabyte further recommends that you avoid declaring out
or inout
arguments for a function.
When the purpose is to return more than just a single scalar value, you should create a dedicated composite type for the purpose and use this to declare the function's return value.
'create function' and the 'subprogram_signature' rule.
When you write aCREATE FUNCTION
statement, you will already have decided what formal arguments it will have—i.e. for each, what will be its name, mode, data type, and optionally its default value. When, later, you alter or drop a function, you must identify it. You do this, in ALTER FUNCTION
and DROP FUNCTION
, typically by specifying just its subprogram_call_signature. You are allowed to use the full subprogram_signature. But this is unconventional. Notice that the subprogram_signature does not include the optional specification of default values; and you cannot mention these when you alter or drop a function. The distinction between the subprogram_signature and the subprogram_call_signature is discussed carefully in the section Subprogram overloading.
Semantics
-
The meanings of the various function attributes are explained in the section Subprogram attributes.
-
A function, like other schema objects such as a table, inevitably has an owner. You cannot specify the owner explicitly when a function is created. Rather, it's defined implicitly as what the built-in current_user function returns when it's invoked in the session that creates the function. This user must have the usage privilege on the function's schema, its argument data types, and its return data type. You (optionally) specify the function's schema and (mandatorily) its name within its schema as the argument of the subprogram_name rule.
-
If a function with the given name, schema, and argument types already exists then
CREATE FUNCTION
will draw an error unless theCREATE OR REPLACE FUNCTION
variant is used. -
Functions with different subprogram_call_signatures can share the same subprogram_name. (The same holds for procedures.) See section Subprogram overloading.
-
CREATE OR REPLACE FUNCTION
doesn't change permissions that have been granted on an existing function. To use this statement, the current user must own the function, or be a member of the role that owns it. -
In contrast, if you drop and then recreate a function, the new function is not the same entity as the old one. So you will have to drop existing objects that depend upon the old function. (Dropping the function
CASCADE
achieves this.) Alternatively,ALTER FUNCTION
can be used to change most of the attributes of an existing function. -
The languages supported by default are
SQL
,PLPGSQL
andC
.
'Create function' grants 'execute' to 'public'.
Execute is granted automatically to public when you create a new function. This is very unlikely to be want you want—and so this behavior presents a disguised security risk. Yugabyte recommends that your standard practice be to revoke this privilege immediately after creating a function.You cannot set the 'depends on extension' attribute with 'create function'.
A function's depends on extension attribute cannot be set usingCREATE [OR REPLACE] FUNCTION
. You must use ALTER FUNCTION
to set it.
Scalar functions and table functions
Use the appropriate variant of the RETURN
clause to create either a scalar function or a table function. Notice that scalar can denote not just an atomic value of data types like int, numeric, text, and so on (and domains based on those data types); it can also denote a single composite value like that of a user-defined type.
Scalar function example
Try this:
create schema s;
create type s.x as (i int, t text);
create function s.f(i in int, t in text)
returns x
security invoker
set search_path = pg_catalog, pg_temp
language plpgsql
as $body$
begin
return (i*2, t||t)::s.x;
end;
$body$;
WITH c as (select s.f(42, 'dog') as v)
select
(v).i, (v).t
FROM c;
This is the result:
i | t
----+--------
84 | dogdog
Table function example:
Try this
create table s.t(k serial primary key, v text);
insert into s.t(v) values ('dog'), ('cat'), ('frog');
create function s.f()
returns table(z text)
security definer
set search_path = pg_catalog, pg_temp
language plpgsql
as $body$
begin
z := 'Starting content of t '; return next;
z := '----------------------'; return next;
for z in (select v from s.t order by k) loop
return next;
end loop;
begin
insert into s.t(v) values ('mouse');
exception
when string_data_right_truncation then
z := ''; return next;
z := 'string_data_right_truncation caught'; return next;
end;
insert into s.t(v) values ('bird');
z := ''; return next;
z := 'Finishing content of t'; return next;
z := '----------------------'; return next;
for z in (select v from s.t order by k) loop
return next;
end loop;
end;
$body$;
\t on
select z from s.f();
\t off
This is the result:
Starting content of t
----------------------
dog
cat
frog
string_data_right_truncation caught
Finishing content of t
----------------------
dog
cat
frog
bird
This kind of table function provides a convenient way to produce an arbitrarily formatted report that can easily be spooled to a file. This is because the select output is easily accessible (in ysqlsh) on stdout—and it's correspondingly easily accessible in client-side programming languages that do SQL like say, Python. In contrast, the output from raise info is tricky to capture (and definitely very hard to interleave in proper sequence with select results) because it comes on stderr.
Always name the formal arguments and write the function's body last.
YSQL inherits from PostgreSQL the ability to specify the arguments only by listing their data types and to reference them in the body using the positional notation $1
, $2
, and so on. The earliest versions of PostgreSQL didn't allow named parameters. But the version that YSQL is based on does allow this. Your code will be very much easier to understand if you use named arguments like the example does.
The syntax rules allow you to write the alterable and unalterable attributes in any order, like this:
create function s.f(i in int, t in text)
returns s.x
security invoker
as $body$
begin
return (i*2, t||t)::s.x;
end;
$body$
set search_path = pg_catalog, pg_temp
language plpgsql;
Yugabyte recommends that you avoid exploiting this freedom and choose a standard order where, especially, you write the body last. For example, it helps readability to specify the language immediately before the body. (Try the \sf meta-command for a function that you created. It always shows the source text last, no matter what order your create [or replace] used.) Following this practice will allow you to review, and discuss, your code in a natural way by distinguishing, informally, between the function header (i.e. everything that comes before the body) and the implementation (i.e. the body).