Re: [GENERAL] Aggregate query on large tables

On 04/09/2017 08:05 AM, Job wrote:
> Hi,
>
> i have a table with about 400 millions of rows and i need to build some aggregate function for reporting.
> I noticed that query performances are slowing down, even though indexes are present.
>
> Query is simple (i make an example, my table is in italian language):
> select a,sum(b) from table where a=x and c=y group by a
>
> a is a varchar
> b is an integer
>
> x and y are two field i use for filter results.
>
> I tried to create different indexes to try to speed up performance:
> index1 (a)
> index2 (c)
> index3 (a,c)
>
> I noticed, with query planner, that the mulfi-field index is not used.
> Postgresql 9.6.1 still use scan without indexes.

At some point doing a scan over a table is better then using an index,
as index use incurs a lookup cost. Unfortunately there is no way to
determine what is actually happening in your case without more information.

So:

1) Table schema(language does not matter, the names are just tags).

2) The actual query run through EXPLAIN ANALYZE.

>
> I obtain significant improvements only if i create a materialized view with aggregated data.
>
> But i would like to avoid - if possible - creating (and mantaining) the materialized view.
>
> Than you!
> /F
>


--
Adrian Klaver
adrian.klaver@aklaver.com