Re: VACUUM ANALYZE slows down query

werner fraga wrote:

>Certain queries on my database get slower after
>running a VACUUM ANALYZE. Why would this happen, and
>how can I fix it?
>
>I am running PostgreSQL 7.4.2 (I also seen this
>problem on v. 7.3 and 8.0)
>
>Here is a sample query that exhibits this behaviour
>(here the query goes from 1 second before VACUUM
>ANALYZE to 2 seconds after; there are other queries
>that go from 20 seconds before to 800 seconds after):
>
>
>
First, try to attach your explain analyze as a textfile attachment,
rather than inline to prevent wrapping and make it easier to read.

Second, the problem is that it *is* getting a more accurate estimate of
the number of rows that are going to be returned, compare:

Plan 1:


>-----------------------------------------------------------------------------------------------------------------------------------------------------------------
> Nested Loop Left Join  (cost=3974.74..48055.42
>rows=79 width=8) (actual time=359.751..1136.165
>rows=1518 loops=1)
>
>
The planner was expecting 79 rows, but was actually getting 1518.

Plan 2:


>-----------------------------------------------------------------------------------------------------------------------------------------------------------------------
> Merge Left Join  (cost=18310.59..29162.44 rows=1533
>width=8) (actual time=1886.942..2183.774 rows=1518
>loops=1)
>
>
It is predicting 1533 rows, and found 1518, a pretty good guess.

So the big issue is why does the planner think that a nested loop is
going to be more expensive than a merge join. That I don't really know.
I'm guessing some parameters like random_page_cost could be tweaked, but
I don't really know the criteria postgres uses for merge joins vs nested
loop joins.

>Thanks for any assistance.
>
>Walt
>
>
Hopefully someone can help a little better. In the mean time, you might
want to resend with an attachment. I know I had trouble reading your
explain analyze.

John
=:->