| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> |
| Cc: | Jesper Krogh <jesper(at)krogh(dot)cc>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: The cost of visibillity testing? (gin-search) |
| Date: | 2010-12-21 23:37:52 |
| Message-ID: | 8036.1292974672@sss.pgh.pa.us |
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Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)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
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