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DBMS_STATS METHOD_OPT default behaviour changed in 10g. Be careful … January 4, 2008

Posted by Richard Foote in Index statistics, Oracle Cost Based Optimizer, Oracle General, Performance Tuning, Richard's Musings.

A question on the OTN forum has prompted me to quickly knock up a demo on the possible dangers of the default behaviour in 10g with regard to the METHOD_OPT option in DBMS_STATS.

When collecting statistics with DBMS_STATS in 9i, the default value of METHOD_OPT was ‘FOR ALL COLUMNS SIZE 1′. This basically says to Oracle please only collect basic column statistics (min, max, distinct values etc.), do not collect histograms on these columns. For columns that are evenly distributed and for columns that are not referenced in SQL statements, this is perfectly adequate. If a column was unevenly distributed and detrimentally impacted the CBO’s costings of an execution plan, then one could generate histograms for those particular columns separately.

However, this default behaviour changed in 10g and IMHO this change is possibly the most significant and problematic difference when migrating to 10g.

The new default value of METHOD_OPT with 10g is ‘FOR ALL COLUMNS SIZE AUTO’. This basically means that Oracle will automatically decide for us which columns need histograms and which columns don’t based on what it considers to be the distribution of values within a column and based on the “workload” associated with the table (basically are there any SQL statements running in the database referencing columns which might need histograms for those statements to be costed correctly).

This sounds like an ideal scenario, just let Oracle work it out for us.

However, the problem is that Oracle in many cases doesn’t do a particularly good job at determining when it should generate a histogram and when it shouldn’t. In fact, the likelihood is that Oracle will actually generate many many many unnecessary histograms while still missing out on some columns that should have them.

In environments with few tables and with few users executing few distinct SQL statements, the impact of some unnecessary histograms may be minimal. However in environments with many tables and columns (potentially many thousands) with many users executing many different SQL statements, the ramifications of potentially suddenly having thousands of additional histograms can be disastrous.

Note also that by having a histogram, Oracle changes the manner in which the DENSITY statistic for a column is calculated (as stored in DBA_TAB_COLUMNS). This is often used by Oracle to determine the selectivity of predicates so the impact of suddenly having additional unnecessary histograms can be wider and more significant than one might initially imagine.

Of course, the impact on the shared_pool and the row_cache and it’s associated latches in particular can be extremely ugly indeed if suddenly Oracle had to deal with thousands of new histograms when parsing statements.

This silly little demo, “Dangers of default METHOD_OPT behaviour in 10g“,  creates a simple little table with three columns. The first column has an outlier value and as previously discussed here, a histogram might be required to correctly cost range scans. The second column is perfectly distributed, it has 10 distinct values with 100,000 occurrences of each. The third column is also perfectly distributed but it’s a special example in that it has only 1 distinct value.

As you can see by the results of the demo, Oracle has got it wrong one way or the other in varying degrees in all three examples. It hasn’t created a histogram when it was needed and created histograms when they weren’t needed, impacting the Density column statistics as a result.

My advice. Just be very careful when using the default method_opt ‘FOR ALL COLUMNS SIZE AUTO’ behaviour in 10g.


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