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Tuesday, July 24, 2007

How to Gather Manually Statistics

Oracle: 10gr1
OS: Windows 32bit

Statistics Gathering Using Sampling

The statistics-gathering operations can utilize sampling to estimate statistics. Sampling is an important technique for gathering statistics. Gathering statistics without sampling requires full table scans and sorts of entire tables. Sampling minimizes the resources necessary to gather statistics.

Sampling is specified using the ESTIMATE_PERCENT argument to the DBMS_STATS procedures. Oracle Corporation recommends setting the ESTIMATE_PERCENT parameter of the DBMS_STATS gathering procedures to DBMS_STATS.AUTO_SAMPLE_SIZE to maximize performance gains while achieving necessary statistical accuracy. AUTO_SAMPLE_SIZE lets Oracle determine the best sample size necessary for good statistics, based on the statistical property of the object. Because each type of statistics has different requirements, the size of the actual sample taken may not be the same across the table, columns, or indexes.


Parallel Statistics Gathering

The statistics-gathering operations can run either serially or in parallel. The degree of parallelism can be specified with the DEGREE argument to the DBMS_STATS gathering procedures. Parallel statistics gathering can be used in conjunction with sampling. Oracle Corporation recommends setting the DEGREE parameter to DBMS_STATS.AUTO_DEGREE. This setting allows Oracle to choose an appropriate degree of parallelism based on the size of the object and the settings for the parallel-related init.ora parameters.

Note that certain types of index statistics are not gathered in parallel, including cluster indexes, domain indexes, and bitmap join indexes.
Statistics on Partitioned Objects

For partitioned tables and indexes, DBMS_STATS can gather separate statistics for each partition, as well as global statistics for the entire table or index. Similarly, for composite partitioning, DBMS_STATS can gather separate statistics for subpartitions, partitions, and the entire table or index. The type of partitioning statistics to be gathered is specified in the GRANULARITY argument to the DBMS_STATS gathering procedures.

Depending on the SQL statement being optimized, the optimizer can choose to use either the partition (or sub partition) statistics or the global statistics. Both types of statistics are important for most applications, and Oracle Corporation recommends setting the GRANULARITY parameter to AUTO to gather both types of partition statistics.

Column Statistics and Histograms

When gathering statistics on a table, DBMS_STATS gathers information about the data distribution of the columns within the table. The most basic information about the data distribution is the maximum value and minimum value of the column. However, this level of statistics may be insufficient for the optimizer's needs if the data within the column is skewed. For skewed data distributions, histograms can also be created as part of the column statistics to describe the data distribution of a given column.

Histograms are specified using the METHOD_OPT argument of the DBMS_STATS gathering procedures. Oracle Corporation recommends setting the METHOD_OPT to FOR ALL COLUMNS SIZE AUTO. With this setting, Oracle automatically determines which columns require histograms and the number of buckets (size) of each histogram. You can also manually specify which columns should have histograms and the size of each histogram.

Determining Stale Statistics

Statistics must be regularly gathered on database objects as those database objects are modified over time. In order to determine whether or not a given database object needs new database statistics, Oracle provides a table monitoring facility. This monitoring is enabled by default when STATISTICS_LEVEL is set to TYPICAL or ALL. Monitoring tracks the approximate number of INSERTs, UPDATEs, and DELETEs for that table, as well as whether the table has been truncated, since the last time statistics were gathered. The information about changes of tables can be viewed in the USER_TAB_MODIFICATIONS view. Following a data-modification, there may be a few minutes delay while Oracle propagates the information to this view. Use the DBMS_STATS.FLUSH_DATABASE_MONITORING_INFO procedure to immediately reflect the outstanding monitored information kept in the memory.

The GATHER_DATABASE_STATS or GATHER_SCHEMA_STATS procedures gather new statistics for tables with stale statistics when the OPTIONS parameter is set to GATHER STALE or GATHER AUTO. If a monitored table has been modified more than 10%, then these statistics are considered stale and gathered again.

OwnName => 'SCOTT'
,Granularity => 'ALL'
,Options => 'GATHER'
,Gather_Temp => TRUE
,DEGREE => 8
,No_Invalidate => FALSE);

PL/SQL procedure successfully completed.
ownname => 'SCOTT',
estimate_percent => dbms_stats.auto_sample_size,
degree => dbms_stats.auto_degree,
cascade => TRUE

PL/SQL procedure successfully completed.


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