From: | Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Thom Brown <thom(at)linux(dot)com>, depesz(at)depesz(dot)com, pgsql-general(at)postgresql(dot)org |
Subject: | Re: [9.2devel] why it doesn't do index scan only? |
Date: | 2011-10-09 17:02:05 |
Message-ID: | CAFj8pRDp_QxpWV+1mVSrvCSwVMMi1uu75K-LmAHHmsZcy40q1Q@mail.gmail.com |
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2011/10/9 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
> Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> writes:
>> 2011/10/9 Thom Brown <thom(at)linux(dot)com>:
>>> On 9 October 2011 04:35, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> wrote:
>>>> It has a sense - index only scan it is faster (and significantly
>>>> faster) on wider tables - or tables with strings where TOAST is not
>>>> active. Maybe there is a some issue because on thin tables is slower
>>>> (and I expect a should be faster everywhere).
>
>>> No, that's my point, I re-tested it on a table with just 2 int
>>> columns, and the results are roughly the same. I added all the
>>> columns to make it expensive to fetch the column being queried.
>
>> then I don't understand
>
> Are you sure you've remembered to vacuum the test table? I get results
> like yours (ie, no speed benefit for index-only scan) if the table
> doesn't have its visibility-map bits set.
it should be - I didn't do VACUUM
Regards
Pavel
>
> regards, tom lane
>
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