Re: Slow query when using ORDER BY *and* LIMIT

Does anyone have any suggestions for my problem? (I have to wonder if
I'm somehow just not getting peoples attention or what. This is my
second question this week on a public mailing list that has gotten
exactly 0 replies)

Jonathan

On 7/5/2011 8:18 PM, Jonathan wrote:
> I have a query that uses ORDER BY and LIMIT to get a set of image data
> rows that match a given tag. When both ORDER BY and LIMIT are included
> for some reason the planner chooses a very slow query plan. Dropping one
> or the other results in a much faster query going from 4+ seconds -> 30
> ms. Database schema, EXPLAIN ANALYZE and other information can be found
> at http://pgsql.privatepaste.com/31113c27bf Is there a way to convince
> the planner to use the faster plan when doing both ORDER BY and LIMIT
> without using SET options or will I need to disable the slow plan
> options to force the planner to use the fast plan?
>
> I found some stuff in the mailing list archives that looks related but I
> didn't see any fixes. Apparently the planner hopes the merge join will
> find the LIMIT # of rows fairly quickly but instead it winds up scanning
> almost the entire table.