How important is gathering statistics on SYS objects?

Hi,
How important is gathering statistics on Data dictionary tables and other X$ tables in SYS schema. Is it bad to keep the statistics. Recently our Sr.DBA has deleted all the SYS schema stats telling that it will inversely affect the DB performance. Is it true?
Regards
Satish

Hi Satish,
*10g:*
A new DBA task in Oracle Database 10g is to generate statistics on data dictionary objects which are contained in the SYS schema. The stored procedures dbms_stats.gather_database_stats and dbms_stats.gather_schema_stats can be used to gather the SYS schema stats. Here is an example of using dbms_stats.gather_schema_stats to gather data dictionary statistics:
EXEC dbms_stats.gather_schema_stats(’SYS’);
*9i*
While it is supported in 9.2 to gather statistics on the data dictionary and fixed views, doing so isn't the norm.
There is a bug fixed only in 10gR2 (not expected to be back-ported to 9.2) that caused this error. The fix is – don’t generate statistics against SYS – especially not the Fixed tables.
For this query, let's see if we can get a better plan by removing statistics or by getting better statistics, or if we need to do something else to tune it. Take the SYS statistics as before, but with gather_fixed => false.
I would like for you to test first by deleting the statistics on these two X$ tables and see how the query runs (elapsed time, plan).
delete_table_stats('SYS','X$KQLFXPL');
delete_table_stats('SYS','X$KGLOB');
Then you can take statistics on them using gather_table_stats and check again (elapsed time, plan).
gather_table_stats('SYS','X$KQLFXPL');
gather_table_stats('SYS','X$KGLOB');
The issue with this is that the contents of these fixed views, particularly x$kqlfxpl, can change dramatically. Gathering fixed object statistics may help now and cause problems later as the contents change.
Warning, this is a bit dangerous due to latch contention, see the following note. I've supported a couple of very busy systems that were completely halted for a time due to latch contention on x$kglob due to monitoring queries (particularly on v$open_cursors).
Note.4339128.8 Ext/Pub Bug 4339128 - Heavy latch contention from queries against library cache views.
Hope this answers your question . . .
Donald K. Burleson
Oracle Press author
Author of "Oracle Tuning: The Definitive Reference":
http://www.dba-oracle.com/bp/s_oracle_tuning_book.htm

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0
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Oracle related stuff blog:
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code{
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How important is gathering statistics on SYS objects?

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