Re: 9.2 and index only scans

2012/8/26 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
> Jeff Janes <jeff(dot)janes(at)gmail(dot)com> writes:
>> On Sun, Aug 26, 2012 at 8:02 AM, Thomas Kellerer <spam_eater(at)gmx(dot)net> wrote:
>>> Should the following setup qualify for an index scan?
>
>> ... Also, your filler is highly compressible, which means the table is
>> much smaller than you might think.
>
> Yeah. I see something like 100 rows per page with this example; the
> heap is 935 pages, the index 276, which makes things about a wash I/O
> wise when you assume that random reads from the index will cost 4x what
> sequential reads from the heap will.
>
> You can force an index scan to occur anyway by setting enable_seqscan to
> zero. When I do that, I see an estimated cost that is marginally more
> than for the seqscan, and the actual runtime is too. I'm not sure I'd
> put a whole lot of stock in that considering the example is small enough
> to be fully cached, but it does show that index-only scans aren't a
> magic bullet.

is possible use seqscan for index? When index is small - and can be
smaller than related table.

Regards

Pavel

>
> regards, tom lane
>
>
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