Qualification by the database server
The optimizer does not create a qualification descriptor
if the cost for the access method to qualify rows exceeds the cost
for a full table scan. If the database server does not construct a
qualification descriptor, the mi_scan_quals() function
returns a NULL-valued pointer.
Important: The mi_scan_quals() function
returns a NULL-valued pointer to indicate that a qualification descriptor
does not exist. In response to the NULL-valued pointer, the access
method creates a row from each source record.