To: pgsql-performance@postgresql.org
From: Thomas Kellerer <spam_eater@gmx.net>
Subject: Re: PostgreSQL Parallel Processing !
Date: Fri, 27 Jan 2012 10:52:44 +0100
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sridhar bamandlapally, 27.01.2012 05:31:
> SQL> explain plan for select * from hr.emp ;
> Explained.
> PLAN
> --------------------------------------------------------------------------
> | Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
> --------------------------------------------------------------------------
> |   0 | SELECT STATEMENT  |      |  7444K|   944M| 16077   (4)| 00:03:13 |
> |   1 |  TABLE ACCESS FULL| EMP  |  7444K|   944M| 16077   (4)| 00:03:13 |
> --------------------------------------------------------------------------
> *WITH PARALLEL Option*
> SQL> explain plan for select /*+parallel(emp,4)*/ * from hr.emp ;
> Explained.
> PLAN
> ---------------------------------------------------------------------------------
> | Id  | Operation            | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
> ---------------------------------------------------------------------------------
> |   0 | SELECT STATEMENT     |          |  7444K|   944M|  4442   (3)| 00:00:54 |
> |   1 |  PX COORDINATOR      |          |       |       |            |          |
> |   2 |   PX SEND QC (RANDOM)| :TQ10000 |  7444K|   944M|  4442   (3)| 00:00:54 |
> |   3 |    PX BLOCK ITERATOR |          |  7444K|   944M|  4442   (3)| 00:00:54 |
> |   4 |     TABLE ACCESS FULL| EMP      |  7444K|   944M|  4442   (3)| 00:00:54 |
> ---------------------------------------------------------------------------------
>
> In the above plan ( WITH PARALLEL Option )
> 1. "Cost" has been nearly reduced to 1/4th
> 2. "CPU" has been reduced
> 3. "Time" has been nearly reduced to 1/3rd

I have *never* seen the "time" column in the explain plan output come anywhere near the actual execution time in Oracle.