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@sss.pgh.pa.us> wrote:

Robert James <srobertjames@gmail.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