Thank you for sharing your insights and the link to the presentation.
We used to gather the statistics with AUTO daily, but it took too much time
because the synopses had to be calculated whenever the optimizer decided to
create new histograms. Then we switched to AUTO on weekends and REPEAT daily.
This seems to work good for this particular application - none of the
performance issues we had so far was related to the histogram sizes. Of course,
it’s not intended to be a general recommendation.
If we started from scratch, it would be worthwhile considering no histograms as
a default and add them only where they’re really needed.
Best regards,
Nenad
From: Neil Chandler <neil_chandler@xxxxxxxxxxx>
Sent: Freitag, 5. März 2021 19:46
To: Noveljic Nenad <nenad.noveljic@xxxxxxxxxxxx>; ORACLE-L
(oracle-l@xxxxxxxxxxxxx) <oracle-l@xxxxxxxxxxxxx>; w1ti@xxxxxxxxxxx
Subject: Re: stale partition statistics
Using METHOD_OPT of "FOR ALL COLUMNS SIZE REPEAT" and nothing will change,
until you get an additional distinct value in a Frequency or Top Frequency
column. Then it will change and become a far less useful histogram (even if you
only have a very few distinct values), which will be constructed using
"samples" leading to the potential for plan instability.
If you want histogram stability from 12.1 onward (and most people should -
definitely if you have partitioned tables), look and see what you have on the
table currently and put that explicitly into the method opt. I cannot recommend
strongly enough moving away from using "REPEAT". I'd rather use AUTO (and I
don't like AUTO)
exec dbms_stats.set_table_prefs(schema,table,'METHOD_OPT',
'FOR ALL COLUMNS SIZE 1
FOR COLUMNS SIZE 254 col1, col2
FOR COLUMNS SIZE 2000 col7, col20, col35');
You might even identify columns which have a little over 254 distinct values
and change the histogram to be the cheap and accurate Frequency-type.
If you look on my blog there's a low quality video of me talking about this
sort of thing (Database Statistics – When It’s
Harder<https://chandlerdba.com/oracle-database-statistics-when-its-harder/> ) :
https://chandlerdba.com/presentations/
Fortunately the camera is mostly pointed at the screen and not me.
regards
Neil Chandler
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