Re: optimizer_index_cost_adj and optimizer_index_caching

From: Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk>
Date: Sat, 6 Mar 2004 07:19:01 -0000
Message-ID: <036101c4034b$4cb33520$7002a8c0@Primary>

Notes in-line

Regards

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

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• Original Message ----- From: "Wolfgang Breitling" <breitliw_at_centrexcc.com>

: 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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Received on Sat Mar 06 2004 - 01:29:10 CST