I'm curious whether there will be any difference in running the statistics
gather step mentioned below
BEGIN
DBMS_STATS.GATHER_SCHEMA_STATS(ownname => 'FRED',
estimate_percent => DBMS_STATS.AUTO_SAMPLE_SIZE,
cascade => TRUE );
END;
Vs. this:
BEGIN
DBMS_STATS.GATHER_SCHEMA_STATS(ownname => 'FRED',
,Estimate_Percent => 100,
,Method_Opt => 'FOR ALL COLUMNS SIZE AUTO',
cascade => TRUE);
END;
I know there was some discussion of Estimate_Percent => 100 being slower than
Estimate_Percent => NULL, but I haven't found much difference in performance.
Thanks,
Steve
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Jonathan Lewis
Sent: Wednesday, January 17, 2018 2:34 PM
To: oracle-l@xxxxxxxxxxxxx; MAdams@xxxxxxxxxx
Subject: [External] Re: AUTO_SAMPLE_SIZE is every row
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@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx> on behalf
of Matt Adams <MAdams@xxxxxxxxxx>
Sent: 17 January 2018 16:19:35
To: oracle-l@xxxxxxxxxxxxx
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
**** This communication may contain privileged and/or confidential information.
If you are not the intended recipient, you are hereby notified that disclosing,
copying, or distributing of the contents is strictly prohibited. If you have
received this message in error, please contact the sender immediately and
destroy any copies of this document. ****
--
https://urldefense.proofpoint.com/v2/url?u=http-3A__www.freelists.org_webpage_oracle-2Dl&d=DwIFEA&c=LkAXfnqL6_MvrMPL5JzdE3Ild0DUTpmjbCJvMv5_TcQ&r=p64P693r52tzs7tJCmFvOg&m=i5zhUfuV_Mr7DVlATPFD-PqtVgyXYlKK5fWIUBIrQ7c&s=-SaofzmrHOf_WDgErh7uwwA6IFckntL5REoU84b1lFY&e=
--
//www.freelists.org/webpage/oracle-l
