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Re: Slow update with simple query

From: Ragnar <gnari(at)hive(dot)is>
To: Arnaud Lesauvage <thewild(at)freesurf(dot)fr>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Slow update with simple query
Date: 2006-12-13 11:20:03
Message-ID: 1166008803.6369.94.camel@localhost.localdomain (view raw or flat)
Thread:
Lists: pgsql-performance
On miĆ°, 2006-12-13 at 11:51 +0100, Arnaud Lesauvage wrote:
> 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 ?

I guess so. are you sure about the index on t1.uid?
what are the column definitions for t1.uid and t2.uid ?
are they the same ?
you should ba able to get a plan similar to:
Merge Join  (cost=0.00..43.56 rows=1000 width=11)
   Merge Cond: ("outer".uid = "inner".uid)
   ->  Index Scan using t1i on t1  (cost=0.00..38298.39 rows=2000035
width=10)
   ->  Index Scan using t2i on t2  (cost=0.00..26.73 rows=1000 width=5)

what postgres version are you using ?

gnari




> 
> 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
> 
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