Re: Question on Explain : Index Scan

perfecto, thank you for the explanation.

- Deepak

On Thu, Oct 21, 2010 at 3:20 AM, Mathieu De Zutter <mathieu(at)dezutter(dot)org>wrote:

> On Thu, Oct 21, 2010 at 3:47 AM, DM <dm(dot)aeqa(at)gmail(dot)com> wrote:
> > I was hoping the optimizer would do a join using index scan.
> >
> > Could some one please explain me why its not doing an index scan rather
> than
> > sequential scan .
>
>
> A index scan would be probably slower here because you're asking for a
> lot of rows. A lot of rows means a lot of I/O, and an index scan is
> more I/O intensive (since it has to read the index too). If you limit
> the result (by being more selective in your where clause, just like
> you do in the first two queries), postgres will most likely switch to
> index scan.
>
> You can see for yourself if index-scan would be faster in your case by
> running the following command before "explain (analyze)":
>
> set enable_seqscan = off;
>
> BTW, try to use explain analyze instead of explain, that way you'll
> see the actual timings too instead of just the planner estimates.
>
> Kind regards,
> Mathieu
>