Create statisticsSince 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 {computeestimatedelete) statistics optionsANALYZE table indexName {computeestimatedelete) statistics optionsANALYZE cluster clusterName {computeestimatedelete) statistics optionsCode 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 tableEXEC 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');
