Question: I want to understand when to export and import by dbms_stats statistics and learn when it is a good idea to export and import statistics.
Answer: Importing and exporting statistics for the CBO and the systems stats (external system statistics for CPU, I/O. etc) and useful in a variety of areas:
- Export production into test to make test systems “look like” large systems for execution plan generation”.
- Export/imports can be used to control execution plans by “freezing execution plans”.
- Statistics are used as a backup before re-analyzing a schema.
- System stats can be moved to a smaller server to make it appear as if Oracle is executing on a large fast server.
You can batch together entire import export job under these scenarios:
-System stats: When migrating to a new server, you can export the old system statistics to ensure consistent execution plans until you are ready to use the "real" system stats.
- Systems reverse: Conversely, you can migrate system stats from production to test to make a tiny server appear to be a larger server. This will not improve SQL execution speed, but developers will see the same execution plans that they would see in production:
- Backup stats: Before making any production change to the CBO stats with dbms_stats, take a full schema backup and an backup of your dbms_stats system stats. Remember, the primary reason for re-analyzing stats is to change SQL execution plans.
For example, here we export production table stats and backport them to the test database to make it appear to be a larger table:
exec dbms_stats.create_stat_table ( ownname => user , stattab => ‘temp_stat‘ ) ;
exec dbms_stats.export_table_stats ( ownname => user , stattab => ‘temp_stat‘, tabname => ‘mytable‘, statid => ‘stats03252011‘) ;
FTP stats to to new database by exporting the table temp_stat table
exec dbms_stats.import_table_stats ( ownname => user , stattab => ‘temp_stat‘, tabname => ‘customer_fact‘ )
Oracle recommends that you collect and save CBO statistics and swap them whenever you see a major change in workloads, such as a database that runs in OLTP mode during the day and processes batch jobs at night. The dbms_stats export and import utilities are perfect for matching your statistics to a changing workload.
You can use the Oracle dbms_stats and export utilities to migrate schema statistics from your PROD instance to your TEST instance, so that your developers will be able to do more-realistic execution-plan tuning of new SQL before it‘s migrated into PROD.
Here are the steps:
Step 1: Create the stats_table:
exec dbms_stats.create_stat_table(
ownname => ‘SYS‘,
stattab => ‘prod_stats‘, -
tblspace => ‘SYSTEM‘);
Step 2: Gather the statistics with gather_system_stats. In this dbms_stats example, we compute histograms on all indexed columns:
DBMS_STATS.gather_schema_stats( ownname=>’<schema>’, estimate_percent=>dbms_stats.auto_sample_size cascade=>TRUE, method_opt=>’FOR ALL COLUMNS SIZE AUTO’)
Step 3: Export the stats to the prod_stats table using export_system_stats::
exec dbms_stats.export_schema_stats(
ownname => ‘SYS‘,
stattab => ‘prod_stats‘);
Step 4: Export the stats to the prod_stats table using exp:
exp scott/tiger file=prod_stats.dmp log=stats.log tables=prod_stats rows=yes
Step 5: copy the export file (e.g. FTP) over to the production server:
ftp -i prodserv . . .
Step 6: Backup the existing production statistics:
exec dbms_stats.create_stat_table(
ownname => ‘SYS‘,
stattab => ‘test_stats‘, -
tblspace => ‘SYSTEM‘);
DBMS_STATS.gather_schema_stats(
ownname=>’<schema>’,
estimate_percent=>dbms_stats.auto_sample_size
cascade=>TRUE,
method_opt=>’FOR ALL COLUMNS SIZE AUTO’)
Step 7: Delete the existing production schema stats
exec dbms_stats.delete_schema_stats(ownname=>‘<schema>’);
Step 8: Import the stats:
EXEC DBMS_STATS.import_schema_stats(‘TEST’,‘STATS_TABLE’,NULL,’TEST’);
Step 9: We can now use the import_system_stats procedure in Oracle dbms_stats to overlay the existing CBO statistics from the smaller TEST instance:
dbms_stats.import_system_stats(‘STATS_TO_MOVE‘);
Simple Usage for DBMS_STATS:
Using procedures in DBMS_STATS package one can backup statistics and restore them.Assumes the user scott already has access to execute DBMS_STATS and using “GRANT EXECUTE ON dbms_stats TO scott;” as sysdba one can grant execute access to DBMS_STATS.
– create table to backup statistics, ownname is statistics table owner and stattab is statistics table name
SQL> execute dbms_stats.create_stat_table(ownname= ‘scott‘, stattab= ‘backup_stats‘);
– procedure to export statistics, exports statistics scott.test into scott.backup_stats, cascade=>true means it will export index statistics too
SQL> exec dbms_stats.export_table_stats(ownname=>‘scott‘, tabname=>‘test‘, statown=>‘scott‘, stattab=>‘backup_stats‘, cascade=>true);
– import table stats
SQL> exec dbms_stats.import_table_stats(ownname=>‘scott‘, tabname=>‘test1‘, statown=>‘scott‘, stattab=>‘backup_stats‘, cascade=>true);
– drop statistics table
SQL> execute dbms_stats.drop_stat_table(ownname= ‘scott‘, stattab= ‘backup_stats‘);
-----------------------------------------------------------------------------------------------------------------------------------------------------------------
10G中可以使用dbms_stats.restore_table_stats恢复表老的统计信息，这功能还是挺不错的。
因为统计信息的变化有时会引起某些语句执行计划变差，这时恢复原有统计信息，常常会有效果。当然分析前备份老的统计信息，始终是个好习惯。
查询统计信息保存时间，当然这个时间不一定能保证：
SQL> select DBMS_STATS.GET_STATS_HISTORY_RETENTION from dual;
GET_STATS_HISTORY_RETENTION
---------------------------
31
|
也可以通过execute DBMS_STATS.ALTER_STATS_HISTORY_RETENTION (45) 来修改这个保存时间。
查询统计信息能恢复到哪个时间点：
SQL> select DBMS_STATS.GET_STATS_HISTORY_AVAILABILITY from dual; GET_STATS_HISTORY_AVAILABILITY ---------------------------------------------------------------------- 20-NOV-11 10.08.13.843271000 PM +08:00 |
小测试：
当前统计信息：
SQL> SELECT COLUMN_NAME,NUM_DISTINCT,NUM_BUCKETS,
2 HISTOGRAM,DENSITY,LOW_VALUE,
3 HIGH_VALUE,LAST_ANALYZED
4 FROM DBA_TAB_COL_STATISTICS a
5 WHERE a.TABLE_NAME = ‘T1‘
6 and a.owner = ‘SYS‘;
COLUMN_NAM NUM_DISTINCT NUM_BUCKETS HISTOGRAM DENSITY LOW_VALUE HIGH_VALUE LAST_ANALYZED
---------- ------------ ----------- ---------- ---------- --------------- --------------- ----------------
ID 108046 100 HEIGHT BAL .000015395 C3020104 C30C512D 2011-12-22 14:53
ANCED
NAME 7775 100 HEIGHT BAL .001328021 2F3130303065386 73756E2F7574696 2011-12-22 14:53
ANCED 4315F4C696E6B65 C2F427564646869
64486173684D617 737443616C656E6
056616C75654974 46172
|
恢复到分析之前：
SQL> select to_timestamp (‘2011-12-22 14:52‘,‘yyyy-mm-dd hh24:mi‘) from dual;
TO_TIMESTAMP(‘2011-12-2214:52‘,‘YYYY-MM-DDHH24:MI‘)
---------------------------------------------------------------------------
22-DEC-11 02.52.00.000000000 PM
SQL> BEGIN
2 DBMS_STATS.RESTORE_TABLE_STATS(
3 ownname => ‘SYS‘,
4 tabname => ‘T1‘,
5 as_of_timestamp => ‘22-DEC-11 02.52.00.000000000 PM‘
6 );
7 END;
8 /
PL/SQL procedure successfully completed.
SQL> SELECT COLUMN_NAME,NUM_DISTINCT,NUM_BUCKETS,
2 HISTOGRAM,DENSITY,LOW_VALUE,
3 HIGH_VALUE,LAST_ANALYZED
4 FROM DBA_TAB_COL_STATISTICS a
5 WHERE a.TABLE_NAME = ‘T1‘
6 and a.owner = ‘SYS‘;
COLUMN_NAM NUM_DISTINCT NUM_BUCKETS HISTOGRAM DENSITY LOW_VALUE HIGH_VALUE LAST_ANALYZED
---------- ------------ ----------- ---------- ---------- --------------- --------------- ----------------
ID 118045 100 HEIGHT BAL .000013915 C102 C30C512E 2011-12-22 14:45
ANCED
NAME 11656 100 HEIGHT BAL .00104712 2F3130343866633 73756E2F746F6F6 2011-12-22 14:45
ANCED 9355F5772617054 C732F747265652F
6F6B656E5772617 5768696C6553746
0546F6B656E496E 174656D656E74
|
http://www.dba-oracle.com/t_export_import_cbo_optimizer_statistics_dbms_stats.htm
http://blog.csdn.net/tianlesoftware/article/details/4668723
http://www.dba-oracle.com/oracle_tips_dbms_stats1.htm
http://www.dba-oracle.com/art_builder_histo.htm
http://www.datadisk.co.uk/html_docs/oracle/sql_optimization.htm
http://www.dba-oracle.com/t_dbms_stats_gather_fixed_object_stats.htm