Re: [9.2devel] why it doesn't do index scan only?

2011/10/9 Tom Lane <tgl@sss.pgh.pa.us>:
> Pavel Stehule <pavel.stehule@gmail.com> writes:
>> 2011/10/9 Thom Brown <thom@linux.com>:
>>> On 9 October 2011 04:35, Pavel Stehule <pavel.stehule@gmail.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
>