Message-ID: <457FDB1E.6040801@freesurf.fr>
Date: Wed, 13 Dec 2006 11:51:10 +0100
From: Arnaud Lesauvage <thewild@freesurf.fr>
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To: pgsql-performance@postgresql.org
Subject: Slow update with simple query
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Hi list !

I am running a query to update the boolean field of a table based on
another table's fields.

The query is (changed names for readability):
UPDATE t1
SET booleanfield = (t2.field1 IN ('some', 'other') AND t2.field2 = 'Y')
FROM t2
WHERE t1.uid = t2.uid

t2.uid is the PRIMARY KEY.
t2 only has ~1000 rows, so I think it fits fully in memory.
t1 as ~2.000.000 rows.
There is an index on t1.uid also.

The explain (sorry, not explain analyze available yet) is :

Hash Join  (cost=112.75..307410.10 rows=2019448 width=357)
   Hash Cond: ("outer".uid= "inner".uid)
   ->  Seq Scan on t1 (cost=0.00..261792.01 rows=2033001 width=340)
   ->  Hash  (cost=110.20..110.20 rows=1020 width=53)
         ->  Seq Scan on t2  (cost=0.00..110.20 rows=1020 width=53)

My query has been running for more than 1.5 hour now, and it is still running.
Nothing else is running on the server.
There are two multicolumn-indexes on this column (both are 3-columns indexes). One of them has a 
functional column (date_trunc('month', datefield)).

Do you think the problem is with the indexes ?

The hardware is not great, but the database is on a RAID1 array, so its not bad either.
I am surprised that it takes more than 3 seconds per row to be updated.

Thanks for your opinion on this !

--
Arnaud