From: | Matteo Beccati <php(at)beccati(dot)com> |
---|---|
To: | pgsql-performance(at)postgresql(dot)org |
Subject: | ORDER BY and LIMIT not propagated on inherited tables / UNIONs |
Date: | 2005-09-02 10:20:57 |
Message-ID: | 43182789.4070307@beccati.com |
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Lists: | pgsql-performance |
Hi,
I'm using inherited tables to partition some data which can grow very
large. Recently I discovered that a simple query that on a regular table
would use an index was instead using seq scans (70s vs a guessed 2s).
The well known query is:
SELECT foo FROM bar ORDER BY foo DESC LIMIT 1
(The same applies for SELECT MIN(foo) FROM bar using 8.1)
The query plan generated when running the query on a table which has
inheritance forces the planner to choose a seq_scan for each table.
Wouldn't be a good thing to also promote ORDER BYs and LIMITs to each
subscan (like WHERE does)?
I needed a quick solution, so I wrote a function which looks each
inherited table separately and my problem is partially solved, but I
think that a (hopefully) little change in the optimizer could be advisable.
Attached are some EXPLAIN ANALYZE outputs of my suggestion.
Best regards
--
Matteo Beccati
http://phpadsnew.com
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Attachment | Content-Type | Size |
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order_limit_inheritance.txt | plain/text | 12.0 KB |
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