Reuse the function descriptor
The access method can store the function descriptor in
user-data memory for use in multiple executions of the same UDR. For
example, the access method stores the function descriptor so that
it can repeat a WHERE-clause function on each index entry.
Important: The database server assigns a PER_COMMAND duration
to the function descriptor. The access method cannot change the duration
of the original function descriptor, but can store a copy of it as
part of the PER_STATEMENT user data to which the table descriptor
points. Any access-method purpose function can obtain the function
descriptor because they all have access to the table descriptor.
If the access method uses FastPath to execute support functions, the am_open purpose function can store the function descriptor in PER_STATEMENT memory. For example, a CREATE INDEX statement causes the database server to call the am_insert purpose function iteratively. To execute the support function or functions that build an index, each iteration of am_insert can retrieve the support-function descriptor from the table descriptor.
For information about user data, see Store data in shared memory.