Re: Query tuning: partitioning, DISTINCT ON, and indexing

On Thu, Jun 20, 2013 at 10:14 PM, Maciek Sakrejda <m(dot)sakrejda(at)gmail(dot)com>wrote:

> On Thu, Jun 20, 2013 at 9:13 PM, bricklen <bricklen(at)gmail(dot)com> wrote:
>
>>
>> On Thu, Jun 20, 2013 at 6:24 PM, Maciek Sakrejda <m(dot)sakrejda(at)gmail(dot)com>wrote:
>>
>>> SELECT
>>> DISTINCT ON (type) ts, type, details
>>> FROM
>>> observations
>>> WHERE
>>> subject = '...'
>>> ORDER BY
>>> type, ts DESC;
>>>
>>
>> First thing: What is your "work_mem" set to, and how much RAM is in the
>> machine? If you look at the plan, you'll immediately notice the "external
>> merge Disk" line where it spills to disk on the sort. Try setting your
>> work_mem to 120MB or so (depending on how much RAM you have, # concurrent
>> sessions, complexity of queries etc)
>>
>
> Good call, thanks, although the in-mem quicksort is not much faster:
>
>
> QUERY PLAN
>
>
> -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
> Unique (cost=471248.30..489392.67 rows=3 width=47) (actual
> time=32002.133..32817.474 rows=3 loops=1)
> Buffers: shared read=30264
> -> Sort (cost=471248.30..480320.48 rows=3628873 width=47) (actual
> time=32002.128..32455.950 rows=3628803 loops=1)
> Sort Key: public.observations.type, public.observations.ts
> Sort Method: quicksort Memory: 381805kB
> Buffers: shared read=30264
> -> Result (cost=0.00..75862.81 rows=3628873 width=47) (actual
> time=0.026..1323.317 rows=3628803 loops=1)
> Buffers: shared read=30264
> -> Append (cost=0.00..75862.81 rows=3628873 width=47)
> (actual time=0.026..978.477 rows=3628803 loops=1)
> Buffers: shared read=30264
> ...
>
> the machine is not nailed down, but I think I'd need to find a way to
> drastically improve the plan to keep this in Postgres. The alternative is
> probably caching the results somewhere else: for any given subject, I only
> need the latest observation of each type 99.9+% of the time.
>

Here are some pages that might help for what details to provide:
https://wiki.postgresql.org/wiki/Server_Configuration
https://wiki.postgresql.org/wiki/Slow_Query_Questions

Did you try an index on (type, ts desc) ? I don't have much else to add at
this point, but maybe after posting some more server and table (parent and
child) details someone will have an answer for you.