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Re: So, is COUNT(*) fast now?

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: So, is COUNT(*) fast now?
Date: 2011-10-21 17:18:19
Message-ID: 25287.1319217499@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> That's a bit disappointing - it's now more than a third faster to do
> the sequential scan, even though the sequential scan has to touch six
> times as many blocks (at scale factor 20, index is 43 MB, table is 256
> MB) all of which are in cache.  Of course, touching that many fewer
> blocks does have some advantages if there is concurrent activity on
> the system, but it still seems unfortunate that the ratio of runtime
> to blocks touched is more than 8x higher for the index-only case.

I don't know why you'd imagine that touching an index is free, or even
cheap, CPU-wise.  The whole point of the index-only optimization is to
avoid I/O.  When you try it on a case where there's no I/O to be saved,
*and* no shared-buffers contention to be avoided, there's no way it's
going to be a win.

			regards, tom lane

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