Hi,
Oracle, eventhough is a timestamp based database, stores only one
version for each row in the table and the rest of the versions of the
same-row(which might have got created due to updates) in a separate
place called undo log. In postgres, all the versions are stored in the
table. So the table would be bigger than it is in Oracle. Try doing a
Vacuum Full and fire the query. You might save some seconds.
I think we will definitely address this problem in 8.4.
Thanks,
Gokul.
On Nov 6, 2007 8:07 PM, SHARMILA JOTHIRAJAH <sharmi_jo@xxxxxxxxx> wrote:
>
> I understand that. But why is that when oracle is given a hint to do full
> table scan instead of using index to get the count, it is still faster than
> postgres when both has the same explain plan? Oracle takes 34 sec and
> postgres takes 1 m10 sec . Is there anything that can be done in postgresql
> for speeding this up?
>
> Oracle --select /*+ full(foo1) */ count(*) from foo1
>
> >
> > OPERATION OBJECT ACCESS_PREDICATES
> > FILTER_PREDICATES
> > ----------------------- ------------------ --------------------
> >
> > SELECT STATEMENT () (null) (null) (null)
> >
> > SORT (AGGREGATE) (null) (null) (null)
> > TABLE ACCESS (FULL) foo (null) (null)
>
> postgresql --Select count(*) from foo
>
> > This table has 29384048 rows and is indexed on foo_id
> >
> > The tables are vacuumed and the explain plan for postgresql is
> >
> > QUERY PLAN
> >
> >
> > ------------------------------------------
> > Aggregate (cost=1194020.60..1194020.61 rows=1 width=0) (actual
> > time=68797.280..68797.280 rows=1 loops=1)
> >
> > -> Seq Scan on foo (cost=0.00..1120560.48 rows=29384048 width=0)
> > (actual time=0.232..60657.948 rows=29384048 loops=1)
> > Total runtime: 68797.358 ms
>
> Thanks again
> sharmila
>
>
>
> ----- Original Message ----
> From: Pavel Stehule <pavel.stehule@xxxxxxxxx>
> To: SHARMILA JOTHIRAJAH <sharmi_jo@xxxxxxxxx>
> Cc: pgsql-general@xxxxxxxxxxxxxx
> Sent: Tuesday, November 6, 2007 9:11:02 AM
> Subject: Re: Postgresql simple query performance question
>
> Hello
>
> PostgreSQL doesn't use index for COUN(*)
>
> http://www.varlena.com/GeneralBits/18.php
> http://sql-info.de/en/postgresql/postgres-gotchas.html#1_7
>
> Regards
> Pavel Stehule
>
> On 06/11/2007, SHARMILA JOTHIRAJAH <sharmi_jo@xxxxxxxxx> wrote:
> >
> > Hi
> > We are in the process of testing for migration of our database from Oracle
> > to Postgresql.
> > I hava a simple query
> >
> > Select count(*) from foo
> > This table has 29384048 rows and is indexed on foo_id
> >
> > The tables are vacuumed and the explain plan for postgresql is
> >
> > QUERY PLAN
> >
> >
> > ------------------------------------------
> > Aggregate (cost=1194020.60..1194020.61 rows=1 width=0) (actual
> > time=68797.280..68797.280 rows=1 loops=1)
> >
> > -> Seq Scan on foo (cost=0.00..1120560.48 rows=29384048 width=0)
> > (actual
> > time=0.232..60657.948 rows=29384048 loops=1)
> > Total runtime: 68797.358 ms
> >
> >
> >
> > The explain plan for oracle is
> >
> > OPERATION OBJECT ACCESS_PREDICATES
> > FILTER_PREDICATES
> > ------------------- ------------------------ --------------------
> > --------------------
> > SELECT STATEMENT () (null) (null)
> (null)
> >
> > SORT (AGGREGATE) (null) (null)
> (null)
> >
> > INDEX (FULL SCAN) foo_IDX_ID (null) (null)
> >
> > Oracle uses index for count(*) query in this case
> > This query in Oracle takes only 5 sec and in postgresql it takes 1 min
> > 10sec
> >
> > The same query in oracle without the index and full table scan(like in
> > postgresql) has the
> >
> > explain plan like this and it takes 34 sec.
> >
> > select /*+ full(foo1) */ count(*) from foo1
> >
> > OPERATION OBJECT ACCESS_PREDICATES
> > FILTER_PREDICATES
> > ----------------------- ------------------ --------------------
> > --------------------
> > SELECT STATEMENT () (null) (null) (null)
> >
> > SORT (AGGREGATE) (null) (null) (null)
> > TABLE ACCESS (FULL) foo (null) (null)
> >
> >
> > In short the query "Select count(*) from foo" takes the following time:
> > Postgresql - 1m 10 sec
> > Oracle(index scan) - 5 sec
> > Oracle (full table scan) - 34 sec
> >
> > How can I speed up this query in postgresql ? The other postgres settings
> > are
> >
> > postgresql
> >
> > max_connections = 100
> > shared_buffers = 50000
> > temp_buffers = 5000
> > work_mem = 16384
> > maintenance_work_mem = 262144
> > fsync = on
> > wal_sync_method = fsync
> > effective_cache_size = 300000
> > random_page_cost = 4
> > cpu_tuple_cost = 0.01
> > cpu_index_tuple_cost = 0.001
> > cpu_operator_cost = 0.0025
> >
> > Are there any tuning that need to be done in the OS or database side? I
> had
> > attached the iostat and vmstat results of postgresql
> >
> > Thanks
> >
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>
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