Suppose we have a query plan with table
tblX being accessed via the
ref access method:
tblX.key_part1 = expr1 AND tblX.key_part2 = expr2 AND ...
Before performing an index lookup, we determine whether any of
NULL. If it is, we don't perform the
lookup, but rather immediately return that the matching tuple
is not found.
This optimization reuses the
attribute produced by the early
filtering code (see
Section 188.8.131.52, “Early NULLs Filtering”). The check
itself is located in the function