Re: AUTO_SAMPLE_SIZE is every row
Date: Wed, 17 Jan 2018 20:34:10 +0000
Message-ID: <MM1P12301MB1658AB74CCE090805705B3A9A5E90_at_MM1P12301MB1658.GBRP123.PROD.OUTLOOK.COM>
If you're running Oracle 12 you probably don't want to change from auto_sample_size, and if you're running 11g you might want to change it for only a few cases. In either case you could decide to gather basic stats using auto_sample_size then use a second call to gather histograms for a few columns.
The critical issue is that gathering stats for a specified sample size makes Oracle use a count(distinct ) on every column, and that (a) very expensive for a reasonable sample size or (b) highly inaccurate for a small sample size. 11g onwards auto_sample_size means 100% but using an "approximate_ndv" mechanism that doesn't have to do the expensive "distinct", so it's much more efficient than the older method and also generally more accurate than any "cheap" sample size.
12c also uses 100% with a very clever algorithm for frequency and top-N frequency histograms IF you've specified auto_sample_size, and that means "perfect" histograms almost free of charge. 12c uses a small sample for hybrid histograms, and 11g uses a small sample size for both frequency and height-balanced histograms if you specify auto_sample_size and that can very easily lead to unstable and inaccurate histograms.
Regards
Jonathan Lewis
From: oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> on behalf of Matt Adams <MAdams_at_equian.com> Sent: 17 January 2018 16:19:35
To: oracle-l_at_freelists.org
Subject: AUTO_SAMPLE_SIZE is every row
A previous DBA set up all our stats analyzation jobs to be of the form.
BEGIN DBMS_STATS.GATHER_SCHEMA_STATS(ownname => 'FRED', estimate_percent => DBMS_STATS.AUTO_SAMPLE_SIZE, cascade => TRUE ); END;
But for every table I can see, the SAMPLE_SIZE is the same as NUM_ROWS, which is the same as the actual number of rows in the table.
I read somewhere that actual value of AUTO_SAMPLE_SIZE can vary from release to release (and patch to patch), but why on earth would it always be every row in the table?
I’m getting ready to change the estimate percentage to something more reasonable. Just wondering if there is something I’m missing somewhere that is influencing this behavior.
Matt
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