Create statistics
Since Oracle 8i the Cost Based Optimizer (CBO) is the preferred
optimizer for Oracle.
In order to make good use of the CBO, you need to create statistics for
the data in the database. There are several options to create statistics.
Analyze command
The ANALYZE command is available for all versions of Oracle, however to
obtain faster and better
statistics use the procedures supplied - in 7.3.4 and 8.0
DBMS_UTILITY.ANALYZE_SCHEMA,
and in 8i and above - DBMS_STATS.GATHER_SCHEMA_STATS
The analyze table can be used to create statistics for 1 table, index
or cluster.
Syntax:
ANALYZE table tableName {compute|estimate|delete) statistics options
ANALYZE table indexName {compute|estimate|delete) statistics options
ANALYZE cluster clusterName {compute|estimate|delete) statistics options
Code examples
ANALYZE table scott compute statistics;
ANALYZE table scott estimate statistics sample 25 percent;
ANALYZE table scott estimate statistics sample 1000 rows;
analyze index sc_idx compute statistics;
analyze index sc_idx validate structure;
DBMS_UTILITY.ANALYZE_SCHEMA
With DBMS_UTILITY.ANALYZE_SCHEMA you can gather all the statistics for
all the tables,
clusters and indexes of a schema.
Code examples
exec DBMS_UTILITY.ANALYZE_SCHEMA('SCOTT','COMPUTE');
exec DBMS_UTILITY.ANALYZE_SCHEMA('SCOTT','ESTIMATE', estimate_rows =>
1000);
exec DBMS_UTILITY.ANALYZE_SCHEMA('SCOTT','ESTIMATE', estimate_percent
=> 25);
exec DBMS_UTILITY.ANALYZE_SCHEMA('SCOTT','DELETE');
Note: It's also possible to analyze the whole database with the
DBMS_UTILITY.ANALYZE_DATABASE('COMPUTE'); command.
DBMS_STATS.GATHER_SCHEMA_STATS
From Oracle 8i the DBMS_STATS package is the preferred method
Oracle list a number of benefits to using it including parallel
execution, long term storage of statistics
and transfer of statistics between servers. Once again, it follows a
similar format to the other methods:
Syntax:
exec DBMS_STATS.GATHER_SCHEMA_STATS(ownname,estimate_percent, block_sample
, method_opt,degree,granularity,cascade,stattab, statid,options,statown
,no_invalidate, gather_temp,gather_fixed);
Code examples:
exec
DBMS_STATS.GATHER_SCHEMA_STATS('SCOTT',DBMS_STATS.AUTO_SAMPLE_SIZE);
exec
DBMS_STATS.GATHER_SCHEMA_STATS(ownname=>'SCOTT'
,estimate_percent=>DBMS_STATS.AUTO_SAMPLE_SIZE);
EXEC DBMS_STATS.gather_schema_stats(ownname => 'SCOTT',
estimate_percent => 25);
EXEC DBMS_STATS.gather_table_stats('SCOTT', 'EMPLOYEES');
EXEC DBMS_STATS.gather_index_stats('SCOTT', 'EMPLOYEES_PK');
exec DBMS_STATS.DELETE_SCHEMA_STATS('SCOTT');
Note: It's also possible to gather statistics for the whole database
with the
DBMS_STATS.gather_database_stats; command.
Transfering statistics between database.
It can be very handy to use production statistics on your development
database,
so that you can forecast the optimizer behavior.
You can do this the following way:
1. Create the statistics table.
exec DBMS_STATS.CREATE_STAT_TABLE(ownname =>'SCHEMA_NAME'
,stat_tab => 'STATS_TABLE'
, tblspace => 'STATS_TABLESPACE');
Example:
exec DBMS_STATS.CREATE_STAT_TABLE(ownname =>'SYSTEM',stat_tab =>
'STATS_TABLE');
2. Export statistics to statistics table
EXEC
DBMS_STATS.EXPORT_SCHEMA_STATS('ORIGINAL_SCHEMA'
,'STATS_TABLE',NULL,'SYSTEM');
3. Import statistics into the data dictionary.
exec
DBMS_STATS.IMPORT_SCHEMA_STATS('NEW_SCHEMA','STATS_TABLE',NULL,'SYSTEM');
4. Drop the statistics table.
exec DBMS_STATS.DROP_STAT_TABLE('SYSTEM','STATS_TABLE');
原文地址：
http://www.oradev.com/create_statistics.jsp