Tom Lane wrote:
> mark(at)mark(dot)mielke(dot)cc writes:
>> Reading 1/4, for a larger table, has a good chance of being faster than
>> reading 4/4 of the table. :-)
>
> Really?
>
> If you have to hit one tuple out of four, it's pretty much guaranteed
> that you will need to fetch every heap page.
I think it's not uncommon to have data that is more clustered than expected.
In my case it shows up often in GIS data; where often a query
that only accesses 1/4 of the rows in the table really will
only access about 1/4 of the pages in the table -- for example
when analyzing Texas or the Southwest US.