Robert Haas <> writes:
> On Fri, Oct 21, 2011 at 10:57 PM, Jeff Janes <> 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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