Re: performance of bitmap scans in nested loop joins

"Sergey E. Koposov" <math@sai.msu.ru> writes:
> And I coadded the "flat profiles" of first two (index scan) queries and 
> compared it with the flat profile of bitmap scan: 

Thanks, I had been thinking of doing that same calculation but hadn't
got round to it yet.  It looks like the bitmap case is actually a little
ahead on buffer access (as you'd expect) and btree work (which is
surprising because it ought to be dead even; are these numbers very
repeatable?).  Where we are losing is mostly on the actual manipulation
of the bitmaps (particularly hash_seq_search which is done in
tbm_begin_iterate; and it looks like memory allocation for the bitmap
hashtables is nontrivial too).  I had already had a TODO item to look
into speeding up hash_seq_search ... will see what I can find.
        regards, tom lane