UNION messing up sorting WAS: psql performance

[Joseph Shraibman <jks@xxxxxxxxxxxxxxx>]

Alvaro Herrera wrote:

> > Incidentally when I did that I only got back one row.  What's up with that?
>
>
> Try with "union all" instead of plain union.
Talk about serendipity.  The problem I've been struggling with for the 
last few hours has been why my query wasn't producing sorted output even 
though I put in an ORDER BY and the EXPLAIN shows that it is ordering. 
The DISTINCT implied by the UNION must have been messing up the sorting.

The docs say 
(http://www.postgresql.org/docs/7.4/interactive/sql-select.html#SQL-UNION):

select_statement is any SELECT statement without an ORDER BY, LIMIT, or 
FOR UPDATE clause. (ORDER BY and LIMIT can be attached to a 
subexpression if it is enclosed in parentheses.



... but I *did* put my SELECTs in parentheses.  This is either a bug in 
pg or a serious ommision from the docs.

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