From: | Jim Nasby <Jim(dot)Nasby(at)BlueTreble(dot)com> |
---|---|
To: | David Kubečka <kubecka(dot)dav(at)gmail(dot)com>, <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Weirdly pesimistic estimates in optimizer |
Date: | 2015-03-06 01:58:01 |
Message-ID: | 54F909A9.8000102@BlueTreble.com |
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Lists: | pgsql-hackers |
On 2/28/15 12:01 PM, David Kubečka wrote:
> With 'random_fk_dupl':
> -> Index Scan using facts_fk_idx on facts (cost=0.42..5.75
> rows=100 width=15) (actual time=0.009..0.117 rows=98 loops=100)
> With 'random_fk_uniq':
> -> Index Scan using facts_fk_idx on facts (cost=0.42..214.26
> rows=100 width=15) (actual time=0.007..0.109 rows=98 loops=100)
>
> I have read the optimizer README file and also looked briefly at the
> code, but this seems to be something not related to particular
> implementation of algorithm (e.g. nested loop). Perhaps it's the way how
> cost estimates are propagated down (or sideways? that would be weird...)
> the query tree. But I am really not sure, since this is my first time
> lookng at the optimizer code base. I should also add that I have
> reproduced this behaviour for all versions of Pg from 9.2 up to current
> devel.
This got answered on one of the other lists, right?
--
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com
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