Date: Fri, 28 Dec 2018 09:32:05 -0600
From: Justin Pryzby <pryzby@telsasoft.com>
To: =?utf-8?B?bmVzbGnFn2Fo?= demirci <neslisah.demirci@gmail.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Query Performance Issue
Message-ID: <20181228153205.GM30382@telsasoft.com>
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On Thu, Dec 27, 2018 at 10:25:47PM +0300, neslişah demirci wrote:
> Have this explain analyze output :
> 
> *https://explain.depesz.com/s/Pra8a <https://explain.depesz.com/s/Pra8a>*

Row counts are being badly underestimated leading to nested loop joins:
|Index Scan using product_content_recommendation_main2_recommended_content_id_idx on product_content_recommendation_main2 prm (cost=0.57..2,031.03 ROWS=345 width=8) (actual time=0.098..68.314 ROWS=3,347 loops=1)
|Index Cond: (recommended_content_id = 3371132)
|Filter: (version = 1)

Apparently, recommended_content_id and version aren't independent condition,
but postgres thinks they are.

Would you send statistics about those tables ? MCVs, ndistinct, etc.
https://wiki.postgresql.org/wiki/Slow_Query_Questions#Statistics:_n_distinct.2C_MCV.2C_histogram

I think the solution is to upgrade (at least) to PG10 and CREATE STATISTICS
(dependencies).

https://www.postgresql.org/docs/10/catalog-pg-statistic-ext.html
https://www.postgresql.org/docs/10/sql-createstatistics.html
https://www.postgresql.org/docs/10/planner-stats.html#PLANNER-STATS-EXTENDED
https://www.postgresql.org/docs/10/multivariate-statistics-examples.html

Justin