Re: Need advice to avoid ORDER BY

On Thu, Apr 4, 2013 at 4:32 PM, Condor <condor(at)stz-bg(dot)com> wrote:
> Hello,
>
> I have one query in my postgresql 9.2.3 that took 137 ms to me executed and
> looking a way
> what I can do to optimize it. I have one table generated numbers from 1 to 1
> 000 000 and
> I need to get first free id, meanwhile id's when is taken can be free
> (deleted data and id
> is free for next job). Table is simple:
>
>
> id serial,
> jobid text,
> valids int default 0
>
> (Yes, I have index).
>
>
> my query is: SELECT jobid FROM mytable WHERE valids = 0 ORDER BY id ASC
> LIMIT 1
>
> I need the first id only.
>
> My question is: Is there a way how I can avoid using ORDER BY to receive the
> first
> free id from mytable ?

well, you can (via EXISTS()), but you can really optimize this with
partial index.

CREATE INDEX ON mytable (id) WHERE valids = 0;

then,

SELECT jobid FROM mytable WHERE valids = 0 ORDER BY id ASC LIMIT 1;

should return in zero time since btree indexes can optimize order by
expressions and the partial index will bypass having to wade through
the rows you don't want.

merlin