Re: The cost of visibillity testing? (gin-search)

Heikki Linnakangas <heikki.linnakangas@enterprisedb.com> writes:
> On 21.12.2010 21:25, Jesper Krogh wrote:
>> Or is a Bitmap Heap Scan simply 3 times faster than a Seq-scan for
>> visibillity-testing?

> It certainly shouldn't be.

>> What have I missed in the logic?

> Perhaps you have a lot of empty space or dead tuples that don't match 
> the query in the table, which the sequential scan has to grovel through, 
> but the bitmap scan skips? What does EXPLAIN ANALYZE of both queries say?

Another possibility is that the seqscan is slowed by trying to operate
in a limited number of buffers (the buffer strategy stuff).
        regards, tom lane