> -----Original Message-----
> From: cluster
>
> If I disable the nested loops, the query becomes *much* slower.
>
> A thing that strikes me is the following. As you can see I have the
> constraint: q.status = 1. Only a small subset of the data set
> has this status. I have an index on q.status but for some
> reason this is not used. Instead the constraint are ensured
> with a "Filter: (q.status = 1)"
> in an index scan for the primary key in the "q" table. If the
> small subset having q.status = 1 could be isolated quickly
> using an index, I would expect the query to perform better. I
> just don't know why the planner doesn't use the index on q.status.
>
What version of Postgres are you using? Do you know what your
join_collapse_limit is set to?
You might be able to force it to scan for questions with a status of 1 first
to see if it helps by changing the FROM clause to:
FROM posts p, question_tags qt, (SELECT * FROM questions WHERE status = 1
OFFSET 0) q
Dave
