Re: PostgreSQL Parallel Processing !

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.