Re: Optimizing select count query which often takes over 10 seconds

Hello -

On Thu, Jan 24, 2013 at 3:39 PM, Alban Hertroys <haramrae(at)gmail(dot)com> wrote:
> On 24 January 2013 10:57, Alexander Farber <alexander(dot)farber(at)gmail(dot)com>
> wrote:
>>
>> # explain analyze select count(id) from (
>> select id,
>> row_number() over(partition by yw order by money
>> desc) as ranking
>> from pref_money
>> ) x
>> where x.ranking = 1 and id='OK452217781481';
>> QUERY PLAN
>>
>> -------------------------------------------------------------------------------------------------------------------------------------------
>> Aggregate (cost=63694.22..63694.23 rows=1 width=82) (actual
>> time=4520.719..4520.719 rows=1 loops=1)
>> -> Subquery Scan x (cost=48519.10..63694.19 rows=11 width=82)
>> (actual time=4470.620..4520.710 rows=6 loops=1)
>> Filter: ((x.ranking = 1) AND ((x.id)::text =
>> 'OK452217781481'::text))
>> -> WindowAgg (cost=48519.10..57190.58 rows=433574 width=26)
>> (actual time=4293.315..4491.652 rows=429803 loops=1)
>> -> Sort (cost=48519.10..49603.03 rows=433574
>> width=26) (actual time=4293.306..4352.544 rows=429803 loops=1)
>> Sort Key: pref_money.yw, pref_money.money
>> Sort Method: external sort Disk: 15856kB
>
>
> It's sorting on disk. That's not going to be fast. Indeed, it's taking
> nearly all the time the query takes (4.4s for this step out of 4.5s for the
> query).

I've noticed that too, but what
does "sorting on disk" mean?

I have a lot of RAM (32 GB) ,
should I increase work_mem even more?
(it is currenlty 32 MB)

Regards
Alex