| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Thom Brown <thom(at)linux(dot)com> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Jeff Davis <pgsql(at)j-davis(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, Greg Stark <stark(at)mit(dot)edu>, Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: COUNT(*) and index-only scans |
| Date: | 2011-11-19 16:08:02 |
| Message-ID: | 17571.1321718882@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Thom Brown <thom(at)linux(dot)com> writes:
> So is there a chance of getting bitmap index-only scans?
Don't hold your breath. It seems like a huge increment of complexity
for probably very marginal gains. The point of a bitmap scan (as
opposed to regular indexscan) is to reduce heap accesses by combining
visits to the same page; but it's not clear how useful that is if
you're not making heap accesses at all.
Robert's sketch of how this could work, full of don't-know-how-to-do-this
as it was, still managed to omit a whole lot of reasons why it wouldn't
work. Notably the fact that the index AM API for bitmap scans is to
return a bitmap, not index-tuple data; and trying to do the latter would
break a lot of the performance advantages that exist now for bitmap
scans.
regards, tom lane
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