Re: Evaluation of secondary sort key.

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.
-- 
David Fetter <david@fetter.org> http://fetter.org/
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