Re: Extensions and/or Multi Column Indexes

From: Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk>
Date: Wed, 9 Jan 2019 10:36:37 +0000
Message-ID: <CWXP265MB04082030918F00974BCFD197A58B0_at_CWXP265MB0408.GBRP265.PROD.OUTLOOK.COM>

If you have a multi-column index and use equality on all columns of that index (whether in a join or in a simple table access) the optimizer will use the distinct_keys value for the index in cardinality calculations in all the cases where it could otherwise use extended stats.

There are many cases, however, where the distinct_keys AND extended stats cease to be valid options as far as the optimizer is concerned. i've written various notes about them on my lock (search for "column groups").

Possibly the only cse where you would want a column group that is an exact match for the index is when you have a histogram on one (or more) of the columns in the index. Oracle will not use the distinct_keys in that case. If you want it to do so you have to drop the histogram on the base column(s) or create a column group for the index and generate a historgam on the column group.

Regards
Jonathan Lewis

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From: oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> on behalf of Martin Klier - Performing Databases GmbH <martin.klier_at_performing-db.com> Sent: 09 January 2019 10:25:19
To: Oracle-L Freelists
Subject: CBO: Extensions and/or Multi Column Indexes

Hi listers,

happy new year to all of you!

I was wondering if it is useful to still have manually created extended statistics (to enhance cardinality calculation for logically connected columns) and multi-column indexes of the same columns at the same time. I need the two- or three-column indexes for query performance reasons anyway, so leaving them out is not an option.

My question is, does it make sense to manually create extended statstics for two columns, that are already part of a two-column index? My field of using this stuff is versions 11.2, 12.1, 12.2 and 18.3 on Linux.

Background: I can't see an extension in dba_stat_extensions if I do not have the extended stats created, so I was thinking, if the CBO can calculate the correct cardinality from the index / index stats, I could spare my stats jobs the effort to calculate many of my virtual column stats.

Thank you in advance, and please don't laugh if I am way off the path... :)

Best regards
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Martin Klier // Performing Databases GmbH Managing Partner // Senior DB Consultant Oracle ACE Director

martin.klier_at_performing-db.com // https://www.performing-databases.com
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http://www.freelists.org/webpage/oracle-l Received on Wed Jan 09 2019 - 11:36:37 CET