| Well, with very large tables, 100% sample size is completely
| unacceptable. Full table scan, with or without distinct, can take
| forever. However, for such tables one can set table preferences, which
| would fix the situation.
For any sufficiently large table using dbms_stats to gather stats is (almost
certainly) a mistake anyway.
In your case "sufficiently large" should probably be interpreted as "a table
for which a full tablescan is too much work" - others may have a more subtle
interpretation (though with a similar intent).
Regards
Jonathan Lewis
________________________________________
From: oracle-l-bounce@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx> on behalf
of Mladen Gogala <gogala.mladen@xxxxxxxxx>
Sent: 26 January 2018 13:16:57
To: oracle-l@xxxxxxxxxxxxx
Subject: Re: AUTO_SAMPLE_SIZE is every row
Well, with very large tables, 100% sample size is completely
unacceptable. Full table scan, with or without distinct, can take
forever. However, for such tables one can set table preferences, which
would fix the situation.
On 01/17/2018 03:34 PM, Jonathan Lewis wrote:
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
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