Re: AUTO_SAMPLE_SIZE is every row

From: Neil Chandler <neil_chandler_at_hotmail.com>
Date: Wed, 17 Jan 2018 17:48:01 +0000
Message-ID: <HE1PR1001MB12437D945AC5A34EEAC9D25085E90_at_HE1PR1001MB1243.EURPRD10.PROD.OUTLOOK.COM>

Matt,

That is correct.

"estimate_percent => DBMS_STATS.AUTO_SAMPLE_SIZE" is the default setting from Oracle 11G and it perform a full table scan to determine the table statistics very accurately.

If you are on Oracle 12C, this has the added advanage that the 2 new histogram types - Hybrid and Top Frequency - are allowed to be created. This is a very good thing as the hybrid histograms are significantly better than the height balanced histograms they replace. Additionally, the gathering of Frequency and Top Frequency histograms is effectivley free when using the defaults as it allows Oracle to use a "HyperLogLog" algorythm to get the approvimate_ndv values in a single pass. This also makes those histograms hugely accurate and stable compared to histograms created by performing adaptive sampling of data.

I would recommend that you do not change from this default unless you have a good, *proven* reason for the change.

(Whether you want lots of histograms on your data is a very different question)

regards

Neil Chandler

Database Guy

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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
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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Received on Wed Jan 17 2018 - 18:48:01 CET