Cost/Time Anomaly
- From: "blr_dba" <deepak_blr@xxxxxxxxxxx>
- To: "'ORACLE-L'" <oracle-l@xxxxxxxxxxxxx>
- Date: Sat, 30 Dec 2006 00:33:22 +0530
Hi Gurus,
Am stuck in a tuning problem and need your expertise to get rid of the issue.
I have 3 huge tables(~20M rows each) and many small look up tables joined in a
query as follows...
Assume:
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
BT1.id=BT2.id(+) and
BT1.id=BT3.id(+) and
BT1.id=ST1.id(+) and
BT1.id=ST2.id(+) and
BT1.id=ST3.id(+) and
BT1.id=ST4.id(+) and
BT1.id=ST5.id(+) and
BT1.id=ST6.id(+);
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?
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