Re: [BUGS] BUG #14562: Query optimization when sorting multipleUNIQUE columns

This is not a bug -- please don't post feature requests to pgsql-bugs.
pgsql-general would be better.

On Tue, Feb 21, 2017 at 11:58 AM,  <james@emerton.info> wrote:
> postgres=# EXPLAIN SELECT * FROM test ORDER BY key, id;
>                           QUERY PLAN
> ---------------------------------------------------------------
>  Sort  (cost=66.83..69.33 rows=1000 width=20)
>    Sort Key: key, id
>    ->  Seq Scan on test  (cost=0.00..17.00 rows=1000 width=20)
> (3 rows)
>
> postgres=# EXPLAIN SELECT * FROM test ORDER BY key;
>                                   QUERY PLAN
> ------------------------------------------------------------------------------
>  Index Scan using test_key_key on test  (cost=0.28..49.27 rows=1000
> width=20)
> (1 row)
>
>
> It seems that these two queries are effectively identical, but the query
> planner makes significantly different choices. In our application there are
> several additional tables joined and the multiple column sort version is
> over two orders of magnitude slower.

What happens when the "key" column here has NULL values?

-- 
Peter Geoghegan


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