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Re: Odd out of memory problem.

From: Andrew Dunstan <andrew(at)dunslane(dot)net>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Odd out of memory problem.
Date: 2012-03-26 16:26:19
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers

On 03/26/2012 12:11 PM, Tom Lane wrote:
> Andrew Dunstan<andrew(at)dunslane(dot)net>  writes:
>> On 03/26/2012 11:18 AM, Tom Lane wrote:
>>> Could we see EXPLAIN output for this query?
>> Currently it shows:
>> Limit  (cost=19443025.87..19443025.89 rows=10 width=8
>>     ->   Sort  (cost=19443025.87..19446451.29 rows=1370168 width=8)
>>           Sort Key: (max(pageno))
>>           ->   GroupAggregate  (cost=18537785.99..19413417.03 rows=1370168
>> width=8)
>>                 ->   Sort  (cost=18537785.99..18823953.97 rows=114467192
>> width=8)
>>                       Sort Key: loid
>>                       ->   Seq Scan on ldata  (cost=0.00..1651163.92
>> rows=114467192 width=8)
>> The table might have been analysed since I ran the query, though.
> That plan should not create a tuple hash table, so I think it's almost
> certain that the plan changed.  It might be interesting to remove the
> pg_statistic rows for the table and then see what plan you get.

Yeah, that gets us:

Limit  (cost=2223492.78..2223492.81 rows=10 width=8)
   ->  Sort  (cost=2223492.78..2223493.28 rows=200 width=8)
         Sort Key: (max(pageno))
         ->  HashAggregate  (cost=2223485.96..2223488.46 rows=200 width=8)
               ->  Seq Scan on ldata  (cost=0.00..1651154.64 
rows=114466264 width=8)

>> To answer Hans' question, we have seen the problem in other contexts. We
>> first noticed this problem in a failure to restore large objects when
>> running pg_restore.
> [ scratches head... ]  I don't understand how or why pg_restore would be
> executing such a query.

It's not. I was explaining that we have seen memory failures in *other* 
contexts, not just this query. The restore fails after many hours on a 
call to lo_write().



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