public inbox for [email protected]
help / color / mirror / Atom feedFrom: Tom Lane <[email protected]>
To: Jim Nasby <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: Regression from 9.4-9.6
Date: Sun, 08 Oct 2017 15:34:51 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
List-Unsubscribe: <mailto:[email protected]?body=unsub%20pgsql-performance>
Jim Nasby <[email protected]> writes:
> I've got a query that's regressed from 9.4 to 9.6. I suspect it has
> something to do with the work done around bad plans from single-row
> estimates.
Why has this indexscan's cost estimate changed so much?
>> -> Index Scan using bdata_filed_departuretime on bdata_forks (cost=0.57..50807.51 rows=1 width=36) (actual time=979.381..3207.777 rows=508 loops=1)
>> -> Index Scan using bdata_filed_departuretime on bdata_forks (cost=0.57..14894236.06 rows=1 width=36) (actual time=892.664..3025.653 rows=508 loops=1)
I think the reason it's discarding the preferable plan is that, with this
huge increment in the estimated cost getting added to both alternatives,
the two nestloop plans have fuzzily the same total cost, and it's picking
the one you don't want on the basis of some secondary criterion.
regards, tom lane
--
Sent via pgsql-performance mailing list ([email protected])
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-performance
reply
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: [email protected]
Cc: [email protected], [email protected]
Subject: Re: Regression from 9.4-9.6
In-Reply-To: <[email protected]>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by agora; see mirroring instructions
for how to clone and mirror all data and code used for this inbox