Curt Sampson <cjs@cynic.net> writes:
> On Wed, 7 Aug 2002, Tom Lane wrote:
>> Also, the main downside of this approach is that the bitmap could
>> get large --- but you could have some logic that causes you to fall
>> back to plain sequential scan if you get too many index hits.
> Well, what I was thinking of, should the list of TIDs to fetch get too
> long, was just to break it down in to chunks.
But then you lose the possibility of combining multiple indexes through
bitmap AND/OR steps, which seems quite interesting to me. If you've
visited only a part of each index then you can't apply that concept.
Another point to keep in mind is that the bigger the bitmap gets, the
less useful an indexscan is, by definition --- sooner or later you might
as well fall back to a seqscan. So the idea of lossy compression of a
large bitmap seems really ideal to me. In principle you could seqscan
the parts of the table where matching tuples are thick on the ground,
and indexscan the parts where they ain't. Maybe this seems natural
to me as an old JPEG campaigner, but if you don't see the logic I
recommend thinking about it a little ...
regards, tom lane