RE: Cost/Time Anomaly

  • From: "Richard J. Goulet" <rgoulet@xxxxxxxxxx>
  • To: "ORACLE-L" <oracle-l@xxxxxxxxxxxxx>
  • Date: Tue, 2 Jan 2007 12:32:06 -0500

This stinks like a star schema of one sort or another, therefore is
star_join enabled in this database?

Dick Goulet, Senior Oracle DBA

45 Bartlett St  Marlborough, Ma 01752, USA
Tel.: 508.573.1978 |Fax:  508.229.2019 | Cell:508.742.5795 




From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx] On Behalf Of blr_dba
Sent: Friday, December 29, 2006 2:03 PM
Subject: Cost/Time Anomaly

Hi Gurus,
Am stuck in a tuning problem and need your expertise to get rid of the

I have 3 huge tables(~20M rows each) and many small look up tables
joined in a query as follows... 

Big tables : BT1, BT2, BT3 
Small tables: ST1, ST2, ST3, ST4, ST5, ST6 

select * from BT1, BT2, BT3, ST1, ST2, ST3, ST4, ST5, ST6 
where and and and and and and and; 

The CBO is using hash joins and the cost is too high (400K) and we are
having a lots of "direct path write waits". 

I tried to remove the outer joins for the small lookup tables by using
sclar sub-queries. The cost reduced drastically (10K) but the overall
execution time got increased. 

Badly need your expertise to get rid of this issue. 

Also would like to know even if the CBO cost is less in the second case,
why the overall execution time is more. Is n't the cost inversly
proportional to the time taken to execute the query? 

GIF image

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