Locate purpose functions

The SQL statements that register a purpose function and an access method create records in the system catalog, which the database server consults to locate a purpose function.

As the access-method developer, you write the purpose functions and register them with the CREATE FUNCTION statement. When you register a purpose function, the database server puts a description of it in the sysprocedures system catalog table.

For example, assume that you write a get_next_record() function that performs the tasks of the am_getnext purpose function. Assume that as user informix, you register the get_next_record() function. Depending on the operating system, you use one of the following statements to register the function.

For UNIX, use the following statement:

CREATE FUNCTION get_next_record(pointer,pointer,pointer)
RETURNS int
WITH (NOT VARIANT)
EXTERNAL NAME "$INFORMIXDIR/extend/am_lib.bld(get_next_record)"
LANGUAGE C

For Windows, use the following statement:

CREATE FUNCTION get_next_record (pointer,pointer,pointer)
RETURNS int
WITH (NOT VARIANT)
EXTERNAL NAME "%INFORMIXDIR%\extend\am_lib.bld(get_next_record)"
LANGUAGE C

The get_next_record() declaration has three generic pointer arguments to conform with the prototype of the am_getnext purpose function.

As a result of the CREATE FUNCTION statement, the sysprocedures system catalog table includes an entry with values that are similar to the example in the following table.

Table 1. Partial **sysprocedures** entry
Column name	Value
procname	`get_next_record`
owner	informix
procid	`163`
numargs	`3`
externalname	`$INFORMIXDIR/extend/am_lib.bld(get_next_record)` (on UNIX)
langid	`1` (Identifies C in the syslanguages system catalog table)
paramtypes	`pointer,pointer,pointer`
variant	`f` (Indicates false or nonvariant)

You then register the access method with a CREATE PRIMARY ACCESS_METHOD statement to inform the database server what function from sysprocedures to execute for each purpose.

The following example registers the super_access access method and identifies get_next_record() as the am_getnext purpose function.

CREATE PRIMARY ACCESS_METHOD super_access
(AM_GETNEXT = get_next_record)

The super_access access method provides only one purpose function. If user informix executes the CREATE PRIMARY ACCESS_METHOD, the sysams system catalog table has an entry similar to Table 2.

Table 2. Partial sysams entry
Column name	Value
am_name	`super_access`
am_owner	`informix`
am_id	`100` (Unique identifier that the database server assigns)
am_type	`P`
am_sptype	`A`
am_getnext	`163` (Matches the procid value in the sysprocedures system catalog table entry for get_next_record())