Invoking Purpose Functions

When an SQL statement or oncheck command specifies a virtual table, the database server executes one or more access-method purpose functions. A single SQL command might involve a combination of the following purposes:

• Open a connection, file, or smart large object
• Create a table
• Scan and select data
• Insert, delete, or update data
• Drop a table
• Close the connection, file, or smart large object

A single oncheck request requires at least the following actions:

• Open a connection, file, or smart large object
• Check the integrity of a table
• Close the connection, file, or smart large object

For information about which purpose functions the database server executes for specific commands, refer to Purpose-Function Flow.

The example in Table 6 specifies only the am_getnext purpose for the super_access access method. A SELECT statement on a virtual-table that uses super_access initiates the following database server actions:

1. Gets the function name for am_getnext that the super_access entry in sysams specifies; in this case get_next_record()
2. Gets the external file name of the executable from the get_next_record() entry in the sysprocedures catalog

   The CREATE FUNCTION statement on page *** assigns the executable file as follows.

   Operating System	External Executable-File Name
   UNIX or Linux	$INFORMIXDIR/extend/am_lib.bld(get_next_record)
   Windows	%INFORMIXDIR%\extend\am_lib.bld(get_next_record)

3. Allocates memory for the descriptors that the database server passes by reference through get_next_record() to the access method
4. Executes the am_getnext purpose function, get_next_record()