Josh Berkus <josh@agliodbs.com> writes:
> The Bitmap index allows the query executor to use several indexes on
> the same operation, comparing them and selecting rows where they
> "overlap" like a Venn diagram.
Note that what Josh is describing is not really a distinct index type,
but a different way of using an index: that is, you pull candidate tuple
locations from several indexes and intersect or union those sets before
you go to the heap. In principle this works whatever the index access
methods are.
I believe that the term "bitmap index" is also used with a different
meaning wherein it actually does describe a particular kind of on-disk
index structure, with one bit per table row.
IMHO building in-memory bitmaps (the first idea) is a very good idea to
pursue for Postgres. I'm not at all sold on on-disk bitmap indexes,
though ... those I suspect *are* sufficiently replaced by partial
indexes.
regards, tom lane