Beginning Oracle Database 11g Administration From Novice to Professional

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CHAPTER 17 SQL TUNING
It is my firm belief that most scheduled statistics- gathering jobs do not cause much
harm only because (most) changes in the statistics were insignificant as far as the
optimizer is concerned—meaning that it was an exercise in futility. 6
There are some statistics about your data that can be left unchanged for a long time,
possibly forever; there are some statistics that need to be changed periodically; and
there are some statistics that need to be changed constantly....The biggest problem is
that you need to understand the data. 7
Beginning with Oracle 10g, statistics are automatically collected by a job called
that runs during a nightly maintenance window; this default strategy
may work for many databases. The procedures in the package can be used
to create a custom strategy. Complete details of the package can be found in
Oracle Database 11g PL/SQL Packages and Types Reference, which can be freely searched
and downloaded at . Here is a representative list of the available
procedures:
and procedures can be used to enable and disable the
job, respectively. The procedure can be used to run the
job immediately.
procedures can be used to manually gather statistics. For
example, can be used to gather statistics for a single table, and
can be used to gather statistics for all the indexes and tables in
a schema.
procedures can be used to delete statistics.
procedures can be used to copy the statistics to a special table.
can be used to import desired statistics. For example, statistics can
be exported from a production database and imported into a development database;
this ensures that both databases use the same query execution plans.
job retains previous statistics. The procedures
can be used to restore statistics from a previous point in time. The default
retention period used by the is 31 days; this can be changed by
using the procedure.
procedures can be used to lock statistics and prevent them from
being overwritten. Oddly enough, locking statistics can be a useful practice not
only when data is static, but also when it is volatile.
6. Wolfgang Breitling in the Journal of the Northern California Oracle Users Group
7. Jonathan Lewis in the Journal of Northern California Oracle Users Group