On 03/26/2012 12:11 PM, Tom Lane wrote:
> Andrew Dunstan<andrew@dunslane.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)
SortKey: (max(pageno)) -> HashAggregate (cost=2223485.96..2223488.46 rows=200 width=8) -> Seq
Scanon 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().
cheers
andrew
