Re: [PERFORM] Query running slow for only one specific id. (Postgres 9.3) version

On Fri, Jun 5, 2015 at 2:54 PM, Sheena, Prabhjot <
Prabhjot(dot)Singh(at)classmates(dot)com> wrote:

> explain analyze SELECT max(last_update_date) AS last_update_date FROM
> btdt_responses WHERE registration_id = 8718704208 AND response != 4;
>
>
> QUERY PLAN
>
>
> --------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>
> Result (cost=2902.98..2903.01 rows=1 width=0) (actual
> time=86910.730..86910.731 rows=1 loops=1)
>
> InitPlan 1 (returns $0)
>
> -> Limit (cost=0.57..2902.98 rows=1 width=8) (actual
> time=86910.725..86910.725 rows=1 loops=1)
>
> -> Index Scan Backward using btdt_responses_n5 on
> btdt_responses (cost=0.57..6425932.41 rows=2214 width=8) (actual
> time=86910.723..86910.723 rows=1 loops=1)
>
> Index Cond: (last_update_date IS NOT NULL)
>
> Filter: ((response <> 4) AND (registration_id =
> 8718704208::bigint))
>
> Rows Removed by Filter: 52145434
>
> Total runtime: 86910.766 ms
>

The issue here is the "Row Removed by Filter", you are filtering out more
than 52M rows, so the index is not being much effective.

What you want for this query is a composite index on (registration_id,
last_update_date). And if the filter always include `response <> 4`, then
you can also create a partial index with that (unless it is not very
selective, then it might not be worthy it).

Regards,
--
Matheus de Oliveira
Analista de Banco de Dados
Dextra Sistemas - MPS.Br nível F!
www.dextra.com.br/postgres