a database implementation (data structure) for particular queries?

	I wonder if there's a database implementation (or data
	structure, or an index) well suited for queries like the
	following:

    SELECT * FROM foo
        WHERE (a1     IN (a1a, ..., a1P)
               AND a2 IN (a2a, ..., a2Q)
               ...
               AND aN IN (aNa, ..., aNZ));

	where a1, ..., aN are all but one of the table columns, and
	P, Q, ..., Z may vary (and are not necessarily equal to each
	other.)

	To note is that the amount of INSERT's into the structure is
	non-negligible when compared to the number of SELECT's.

	Also, I'd be interested in a quick way to compute an estimate
	(an upper bound) of the number of rows to be returned from the
	query above.

	TIA.

-- 
FSF associate member #7257