From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: So, is COUNT(*) fast now? |
Date: | 2011-10-22 21:15:20 |
Message-ID: | 16683.1319318120@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Fri, Oct 21, 2011 at 10:57 PM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
>> If count(*) could cause the index-only scan to happen in physical
>> order of the index, rather than logical order, that might be a big
>> win. Both for all in memory and for not-all-in-memory.
> That's an interesting point. I sort of assumed that would only help
> for not-all-in-memory, but maybe not. The trouble is that I think
> there are some problematic concurrency issues there.
Yeah. We managed to make physical-order scanning work for VACUUM
because it's okay if VACUUM sometimes sees the same index tuple twice;
it'll just make the same decision about (not) deleting it. That will
not fly for regular querying.
regards, tom lane
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