Re: Creating Histograms
- From: Wolfgang Breitling <breitliw@xxxxxxxxxxxxx>
- To: oracle-l@xxxxxxxxxxxxx
- Date: Wed, 21 Jul 2004 13:40:03 -0600
You create histograms on individual columns with gather_table_stats.
Gathering histograms is a very selective process - both, on which column(s)
and with what number of buckets. It is not something you ought to do with a
broad stroke like gather_schema_stats. One size (pun intended) certainly
does not fit all.
Once you gathered the histograms in this way for the columns where it
benefits performance, you can use method_opt=>'for all columns size repeat'
in gather_schema_stats to re-gather the histograms - according to the
documentation, I have not verified that myself.
All that provided you insist on doing the regular gather_schema_stats
russian roulette.
The values for all (analyzed) tables in dba_histograms are OK. Capturing
min and max column values can be viewed as a 1-bucket histogram (bounded by
lowest and highest column value), which is what the default
method_opt=>'for all columns size 1' also implies - a histogram of size one
for all columns..
PS. An easy way to find which columns are used in predicates, and one which
Oracle uses when you use method_opt=>'... size auto', is to query
sys.col_usage$.
Regards
Wolfgang Breitling
Centrex Consulting Corporation
http://www.centrexcc.com
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