Date: Tue, 5 Dec 2017 13:43:28 -0300
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Rodrigo Rosenfeld Rosas <rr.rosas@gmail.com>
Cc: pgsql-performance@lists.postgresql.org
Subject: Re: Extremely slow DELETE with cascade foreign keys
Message-ID: <20171205164328.6t4w3abfpzp54y2y@alvherre.pgsql>
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Rodrigo Rosenfeld Rosas wrote:

> explain analyze delete from field_values where transaction_id=226;
> QUERY PLAN
> ---------------------------------------------------------------------------------------------------------------------------------------------------------------
>  Delete on field_values  (cost=0.43..257.93 rows=481 width=6) (actual
> time=367375.805..367375.805 rows=0 loops=1)
>    ->  Index Scan using index_field_values_on_transaction_id on
> field_values  (cost=0.43..257.93 rows=481 width=6) (actual time=0.223..4.216
> rows=651 loops=1)
>          Index Cond: (transaction_id = 226)
>  Planning time: 0.234 ms
>  Execution time: 367375.882 ms
> (5 registros)
> 
> Time: 367377,085 ms (06:07,377)

Normally this is because you lack indexes on the referencing columns, so
the query that scans the table to find the referencing rows is a
seqscan.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services