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

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.

regards, tom lane