Re: Understanding sequential versus index scans.

UNION was better, but still 5 times as slow as either query done
individually.
set enable_seqscan=off didn't help at all - it was totally ignored
Is there anything else I can do?

On Sun, Jul 19, 2009 at 7:47 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Robert James <srobertjames(at)gmail(dot)com> writes:
> > Hi. I notice that when I do a WHERE x, Postgres uses an index, and when
> I
> > do WHERE y, it does so as well, but when I do WHERE x OR y, it
> > doesn't.
>
> It can use indexes for OR conditions, but not for arbitrary OR
> conditions...
>
> > select * from dict
> > where
> > word in (select substr('moon', 0, generate_series(3,length('moon')))) --
> > this is my X above
> > OR word like 'moon%' -- this is my Y above
>
> ... and that one is pretty arbitrary. You might have some luck with
> using a UNION instead, viz
>
> select * from dict where X
> union all
> select * from dict where Y
>
> regards, tom lane
>