From: | Jayadevan M <Jayadevan(dot)Maymala(at)ibsplc(dot)com> |
---|---|
To: | Mladen Gogala <mgogala(at)vmsinfo(dot)com> |
Cc: | "pgsql-novice(at)postgresql(dot)org" <pgsql-novice(at)postgresql(dot)org>, pgsql-novice-owner(at)postgresql(dot)org |
Subject: | Re: Partitions and the optimizer. |
Date: | 2010-08-31 03:35:32 |
Message-ID: | OFDEFF461F.C16B2587-ON65257790.00136ED5-65257790.0013BDA5@ibsplc.com |
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Thread: | |
Lists: | pgsql-novice |
Hello,
> Optimizer doesn't behave well when the tables are partitioned. Here is
> the problem:
>
> news=# explain
> select max("document#") from moreover_documents_y2010m08;
>
> QUERY
> PLAN
>
------------------------------------------------------------------------------------------------------------------------------------------------------
> Result (cost=0.15..0.16 rows=1 width=0)
> InitPlan 1 (returns $0)
> -> Limit (cost=0.00..0.15 rows=1 width=8)
> -> Index Scan Backward using pk_moreover_documents_y2010m08
> on moreover_documents_y2010m08 (cost=0.00..2169871.61 rows=14615132
> width=8)
> Filter: ("document#" IS NOT NULL)
> (5 rows)
>
> Time: 31.191 ms
> news=# explain
> select max("document#") from moreover_documents;
> QUERY
> PLAN
>
-----------------------------------------------------------------------------------------------------------------------
> Aggregate (cost=4227019.40..4227019.41 rows=1 width=8)
> -> Append (cost=0.00..4145103.32 rows=32766432 width=8)
> -> Seq Scan on moreover_documents (cost=0.00..10.20 rows=20
> width=8)
> -> Seq Scan on moreover_documents_y2010m06 moreover_documents
> (cost=0.00..236523.53 rows=1856853 width=8)
> -> Seq Scan on moreover_documents_y2010m07 moreover_documents
> (cost=0.00..2073778.27 rows=16294327 width=8)
> -> Seq Scan on moreover_documents_y2010m08 moreover_documents
> (cost=0.00..1834740.32 rows=14615132 width=8)
> -> Seq Scan on moreover_documents_y2010m09 moreover_documents
> (cost=0.00..10.20 rows=20 width=8)
> -> Seq Scan on moreover_documents_y2010m10 moreover_documents
> (cost=0.00..10.20 rows=20 width=8)
> -> Seq Scan on moreover_documents_y2010m11 moreover_documents
> (cost=0.00..10.20 rows=20 width=8)
> -> Seq Scan on moreover_documents_y2010m12 moreover_documents
> (cost=0.00..10.20 rows=20 width=8)
> -> Seq Scan on moreover_documents_y2011m01 moreover_documents
> (cost=0.00..10.20 rows=20 width=8)
> (11 rows)
>
> Time: 31.961 ms
> news=#
>
> I have several partitions for the table, 3 of them are loaded. When I
> search for the maximum value of the primary key column on a single
> partition, the access path is as it should be, using the PK index. Each
> of the partitions has the same PK. When I try selecting the maximum on
> the entire table, the indexes on the partitions are not used, Postgres
> does the full scan instead. The difference in timing is drastic:
> news=# select max("document#") from moreover_documents_y2010m08;
> max
> ------------
> 1175107508
> (1 row)
>
> Time: 56.778 ms
> news=# select max("document#") from moreover_documents;
> max
> ------------
> 1175107508
> (1 row)
>
> Time: 200490.228 ms
> news=#
>
> --
I am a PostgreSQL novice myself. So please try my suggestions only if you
have enough time to experiment.
If you try
select max("document#") from (
select "document#" from moreover_documents_y2010m08
union
select "document#" from moreover_documents_xxx
)
would that improve performance?
Regards,
Jayadevan
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