From: Richard Huxton <dev@archonet.com>
To: smiley2211 <smiley2211@yahoo.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Query is taking 5 HOURS to Complete on 8.1 version
Date: Tue, 03 Jul 2007 20:18:11 +0100
Message-ID: <468AA0F3.6030207@archonet.com> (raw)
In-Reply-To: <11418557.post@talk.nabble.com>
References: <11416966.post@talk.nabble.com>
<11418557.post@talk.nabble.com>
smiley2211 wrote:
> Here is the EXPLAIN after I changed some conf file - now I am running another
> EXPLAIN ANALYZE which may take 5 or more hours to complete :,(
>
> effective_cache = 170000
Why has effective_cache changed from 80,000 to 170,000 - have you
stopped running some other application?
> enable_seqscan = on
> enable _bitmapscan = on
Why were these disabled before? What were you trying to achieve? What
has now changed?
> QUERY PLAN
You still haven't supplied the query. However, looking at the explain
I'd guess there's a lot of sorting going on? You might want to increase
work_mem just for this query:
SET work_mem = ...;
SELECT ...
However, that's just a blind guess because you haven't supplied the
absolutely vital information:
1. The query
2. An idea of how many rows are in the relevant tables
3. The "I have vacuumed and analysed recently" disclaimer
4. The explain analyse (which you are running - good, make sure you save
a copy of it somwhere).
Even then it'll be difficult to get a quick answer because it looks like
a large query. So - you can speed things along by looking for oddities
yourself.
The explain analyse will have two values for "rows" on each line, the
predicted and the actual - look for where they are wildly different. If
the planner is expecting 2 matches and seeing 2000 it might make the
wrong choice. You can usually cut down the large query to test just this
section. Then you might want to read up about "ALTER TABLE ... SET
STATISTICS" - that might give the planner more to work with.
The other thing to look for is the time. The explain analyse has two
figures for "actual time". These are startup and total time for that
node (if "loops" is > 1 then multiply the time by the number of loop
iterations). It might be there are one or two nodes that are taking a
long time and we can find out why then.
HTH
--
Richard Huxton
Archonet Ltd
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To: pgsql-performance@postgresql.org
Cc: dev@archonet.com, smiley2211@yahoo.com
Subject: Re: Query is taking 5 HOURS to Complete on 8.1 version
In-Reply-To: <468AA0F3.6030207@archonet.com>
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