Hemant, histograms provide input to CBO, which creates an execution plan. For what decision can histogram on an unidexed column be useful? Such histogram can be used to decide between sort/merge join or hash join. The resulting difference in execution speed can be dramatic. Mladen Gogala Sr. Oracle DBA 1500 Broadway, 6th floor New York, NY 10036 (212) 329-5201 www.vmsinfo.com The Leader in Integrated Media Intelligence Solutions > -----Original Message----- > From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l- > bounce@xxxxxxxxxxxxx] On Behalf Of Hemant K Chitale > Sent: Thursday, December 28, 2006 11:12 AM > To: christian.antognini@xxxxxxxxxxxx > Cc: oracle-l@xxxxxxxxxxxxx > Subject: re "CBO - A Configuration Roadmap" -- Histograms on Non- > Indexed Columns > > > Christian, > > I was reading your document "CBO - A Configuration Roadmap" . > > You write : > "Histograms are essential for all columns referenced in WHERE > clauses > that contain > skewed data. Notice that they are useful on non-indexed columns as > well! For simplicity > use SIZE SKEWONLY. If it takes too much time try SIZE AUTO6. If it's > still too slow or > the chosen number of buckets is not good (or the needed histogram > isn't created at all), > manually specify the list of columns." > > Can you explain the second sentence about Histograms on non-indexed > columns as well ? > How would they be useful ? I would understand Histograms on > non-indexed columns > as providing _me_ information about the data in those columns and > which could allow > me to make a judgement call as to whether I should index the > columns. How does the > optimizer use Histograms on non-indexed columns ? > > If you do not mind [as you do post frequently at ORACLE-L], I have > CC'd this to ORACLE-L > trusting that your reply might also be of interest to others on the > list. > > > Hemant K Chitale > http://web.singnet.com.sg/~hkchital > > > -- > //www.freelists.org/webpage/oracle-l -- //www.freelists.org/webpage/oracle-l