Re: slow joining very large table to smaller ones

Tom Lane wrote:
> John A Meinel <john(at)arbash-meinel(dot)com> writes:
>
>>What I don't understand is that the planner is actually estimating that
>>joining against the new table is going to *increase* the number of
>>returned rows.
>
>
> It evidently thinks that incidentid in the k_r table is pretty
> nonunique. We really need to look at the statistics data to
> see what's going on.
>
> regards, tom lane
>

Okay, sure. What about doing this, then:

EXPLAIN ANALYZE
SELECT recordtext FROM eventactivity
JOIN (SELECT DISTINCT incidentid FROM k_r JOIN k_b USING (incidentid)
WHERE k_r.id = ?? AND k_b.id = ??)
USING (incidentid)
;

Since I assume that eventactivity is the only table with "recordtext",
and that you don't get any columns from k_r and k_b, meaning it would be
pointless to get duplicate incidentids.

I may be misunderstanding what the query is trying to do, but depending
on what is in k_r and k_b, is it possible to use a UNIQUE INDEX rather
than just an index on incidentid?

There is also the possibility of
EXPLAIN ANALYZE
SELECT recordtext FROM eventactivtity
JOIN (SELECT incidentid FROM k_r WHERE k_r.id = ??
UNION SELECT incidentid FROM k_b WHERE k_b.id = ??)
USING (incidentid)
;

But both of these would mean that you don't actually want columns from
k_r or k_b, just a unique list of incident ids.

But first, I agree, we should make sure the pg_stats values are reasonable.

John
=:->