From: | David Fetter <david(at)fetter(dot)org> |
---|---|
To: | Jesper Krogh <jesper(at)krogh(dot)cc> |
Cc: | pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: Evaluation of secondary sort key. |
Date: | 2011-04-09 16:17:10 |
Message-ID: | 20110409161710.GA1464@fetter.org |
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Thread: | |
Lists: | pgsql-hackers |
On Sat, Apr 09, 2011 at 03:22:14PM +0200, Jesper Krogh wrote:
> This seems like a place where there is room for improvement.
>
> 2011-04-09 15:18:08.016 testdb=# select id from test1 where id < 3
> order by id;
> id
> ----
> 1
> 2
> (2 rows)
>
> Time: 0.328 ms
> 2011-04-09 15:18:11.936 testdb=# CREATE or Replace FUNCTION
> testsort(id integer) returns integer as $$ BEGIN perform
> pg_sleep(id); return id; END; $$ language plpgsql;
> CREATE FUNCTION
> Time: 12.349 ms
> 2011-04-09 15:18:22.138 testdb=# select id from test1 where id < 3
> order by id,testsort(id);
> id
> ----
> 1
> 2
> (2 rows)
>
> Time: 3001.896 ms
>
> It seems strange that there is a need to evaluate testsort(id) at
> all in this case.
How would PostgreSQL know that sorting by id leaves no ambiguity for
the next key to address?
Cheers,
David.
--
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