Re: optimizer_index_cost_adj and optimizer_index_caching
- From: "Jonathan Lewis" <jonathan@xxxxxxxxxxxxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Sat, 6 Mar 2004 07:19:01 -0000
Notes in-line
Regards
Jonathan Lewis
http://www.jlcomp.demon.co.uk
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----- Original Message -----
From: "Wolfgang Breitling" <breitliw@xxxxxxxxxxxxx>
: That is how I interpret it too, which is why I prefer using system_stats
: over changing o_i_c_a. There is a subtle difference between the way
o_i_c_a
: and system_stats achieve the apparently same goal:
: o_i_c_a lowers the cost of index accesses compared to the
baseline
: whereas
: system_stats increase the cost of full scans compared to the
baseline
Very good point
:
: I have not figured out yet if that can lead to differences in plan
: compositions or if it is guaranteed that the relative costs of all plan
: components remain the same when comparing a plan stemming from having
: o_i_c_a=25 (i.e. single reads cost are 1/4 of multi read costs) vs. having
: system statistics where mreadtim = 4*sreadtim.
:
In theory it might - It would depend
on how Oracle handles the the "plus 1"
for tablescans in the new calculation.
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