RE: Why CBO choose wrong way?

From: info [mailto:info@xxxxxxxxxxxxxx] 
Sent: Saturday, May 28, 2005 10:18
To: 'zhai_jingmin@xxxxxxxxx'; 'oracle-l@xxxxxxxxxxxxx'
Subject: RE: Why CBO choose wrong way?
the CBO thinks it saves an expensive(?) sort this way, to satisfy the order by
clause.
why do you have optimizer_mode set to FIRST_ROWS? and even if you need it, then
you should at least use the flavors with a trailing number. FIRST_ROWS is only
maintained for backward compatibility, and is based on heuristics rather than
statistics. and why do you have optimizer_index_cost_adj set to 30?

I would start tuning from the default CBO settings ...

hope this helps, kind regards,

Lex.
 
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-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On
Behalf Of Zhai Jingmin
Sent: Saturday, May 28, 2005 04:45
To: oracle-l@xxxxxxxxxxxxx
Subject: Why CBO choose wrong way?

Hi,list
   My query is
 
    select a.* from peak_present a,present b
    where a.present_id=b.id
    and a.peak_id=:1
    order by a.id;
 
The  table a has 39M rows ,and table b is very small,I have a index on
a.peak_id,all the statistics are current.The problem is the CBO choose following
execution plan and the index on peak_id is not used .If I delete the 'order by'
line,theCBO can use the index on peak_id column.Actually the peak_id column is
almost unique.The query is very fast if it can use the index on peak_id
column.Why the CBO order the table so early before filter out right rows? My
optimizer* parameters list:
 
optimizer_dynamic_sampling           integer     2
optimizer_features_enable            string      10.1.0.4
optimizer_index_caching              integer     0
optimizer_index_cost_adj             integer     30
optimizer_mode                       string      FIRST_ROWS
db_file_multi_block_read_count   16



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