Re: Understanding sequential versus index scans.

Is there anyway to tell Postgres "Run these two queries, and union their
results, but don't change the plan as to a UNION - just run them
separately"?
Something seems funny to me that running a UNION should be twice as slow as
running the two queries one after the other.

On Sun, Jul 19, 2009 at 8:10 PM, Robert James <srobertjames(at)gmail(dot)com>wrote:

> 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
>>
>
>