Re: too low optimizer_index_cost_adj causing bizarre index choice

I believe that Waleed's response is essentially correct. The numbers still don't make sense, but ...

1. 250,000 blocks with the mbrc = 64. The t/s cost (according to my observations on 9.0.1 and 8.1.7) should be based on an adjusted mbrc of 15.5

        250,000 / 15.5 = 16,000

b) Using an index to hit every block, Oracle is

    able to determine from data clustering stats     that many consecutive index leaf values will     hit a single data block, therefore can estimate     the single read-count via index as 250,000 -     but you have told it to reduce this by a factor     of 1/100 - for a total of 2,500.

Unfortunately for the theory, both your costs are 4,924 - so the T/S cost is close to the traditional "table blocks / simple mbrc", and index cost is too high be a factor of 2, which may be due to some internal bitmap index clustering fudge factor - some of the critical bitmap index numbers apparently used to be hard-coded constants. Possibly it just means that your bitmap column has two values (and of course I haven't allowed a count for the number of leaf blocks in the index !).

Details notwithstanding - I suspect that calculations like the above are the reason why Oracle decided that a full indexed path was cheaper than a scan.

Jonathan Lewis
http://www.jlcomp.demon.co.uk

Author of:
Practical Oracle 8i: Building Efficient Databases

Next Seminar - Australia - July/August
http://www.jlcomp.demon.co.uk/seminar.html

Host to The Co-Operative Oracle Users' FAQ http://www.jlcomp.demon.co.uk/faq/ind_faq.html

-----Original Message-----
To: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com> Date: 09 May 2002 22:16
choice

|Hi Johnathan,
|
|here is the skinny:
|
|db_file_multiblock_read_count = 64
|number of rows=15m
|blocks=251071
|empty_blocks=0
|db_block_size=16384
|
|total plan cost=4924
|tablescan cost = 4924
|
|Jack
|
|

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Author: Jonathan Lewis
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