Re: Need advice to avoid ORDER BY

On 2013-04-05 00:38, Merlin Moncure wrote:
> On Thu, Apr 4, 2013 at 4:32 PM, Condor <condor@stz-bg.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


Hm,
I only can say: Thank You!
Your solution is work, but Im now a little confused. I has a index
CREATE INDEX ON mytable (valids) USING BTREE (valids) and the
query to find valids = 0 tooks 137 ms.

Why, your solution is worked ? Yes, it's worked.


Cheers,
Condor