create_function.hlp

关系型数据库 Postgresql 6.5.2

.pgaw:Help.f.t insert end \
"Synopsis" {bold} "

CREATE FUNCTION name ( \[ ftype \[, ...\] \] )
RETURNS rtype
AS definition
LANGUAGE 'langname'
" {code} "

name" {italic} "
	The name of a function to create. 

" {} "ftype" {italic} "
	The data type of function arguments. 

" {} "rtype" {italic} "
	The return data type. 

" {} "definition" {italic} "
	A string defining the function; the meaning depends on the language. It may be an internal function name, the path to an object file, an SQL query, or text in a procedural language. 

" {} "langname" {italic} "
	may be 'C', 'sql', 'internal' or 'plname', where 'plname' is the name of a created procedural language. See CREATE LANGUAGE for details. 

" {} "Outputs" {bold} "

CREATE
	This is returned if the command completes successfully. 

CREATE FUNCTION allows a Postgres user to register a function with a database. Subsequently, this user is treated as the owner of the function. 

" {} "Notes:" {italic} "Refer to the chapter on functions in the PostgreSQL Programmer's Guide for further information. 

Use " {} "DROP FUNCTION" {link drop_function} " to drop user-defined functions. 

Postgres allows function \"overloading\"; that is, the same name can be used for several different functions so long as they have distinct argument types. This facility must be used with caution for INTERNAL and C-language functions, however. 

Two INTERNAL functions cannot have the same C name without causing errors at link time. To get around that, give them different C names (for example, use the argument types as part of the C names), then specify those names in the AS clause of CREATE FUNCTION. If the AS clause is left empty then CREATE FUNCTION assumes the C name of the function is the same as the SQL name. 

For dynamically-loaded C functions, the SQL name of the function must be the same as the C function name, because the AS clause is used to give the path name of the object file containing the C code. In this situation it is best not to try to overload SQL function names. It might work to load a C function that has the same C name as an internal function or another dynamically-loaded function --- or it might not. On some platforms the dynamic loader may botch the load in interesting ways if there is a conflict of C function names. So, even if it works for you today, you might regret overloading names later when you try to run the code somewhere else. 

" {} "Usage" {bold} "

To create a simple SQL function: 
" {} "
CREATE FUNCTION product_price(int4) RETURNS float8 AS
	'SELECT price FROM products where id = \$1'
LANGUAGE 'sql';

SELECT product_price(314) AS answer;

answer 
------
15.25
" {code} "

To create a C function, calling a routine from a user-created shared library. This particular routine calculates a check digit and returns TRUE if the check digit in the function parameters is correct. It is intended for use in a CHECK contraint. 
" {} "
CREATE FUNCTION ean_checkdigit(bpchar, bpchar) RETURNS bool
AS '/usr1/proj/bray/sql/funcs.so' LANGUAGE 'c';

CREATE TABLE product (
	id        char(8) PRIMARY KEY,
	eanprefix char(8) CHECK (eanprefix ~ '\[0-9\]{2}-\[0-9\]{5}')
	REFERENCES brandname(ean_prefix),
	eancode   char(6) CHECK (eancode ~ '\[0-9\]{6}'),
	CONSTRAINT ean    CHECK (ean_checkdigit(eanprefix, eancode))
);
" {code}