Oracle确定过期的统计信息

经过一段时间，随着数据库对象被修改，必须定期搜集统计信息。为了确定数据库对象需要新的数据库统计信息，oracle数据库提供了一

youmust regularly gather statistics on database objects as thesedatabase objects are modified over time. to determine whether agiven database object needs new database statistics, oracledatabase provides a table monitoring facility. this monitoring isenabled by default when statistics_level is set totypical orall.

经过一段时间，随着数据库对象被修改，，必须定期搜集统计信息。为了确定数据库对象需要新的数据库统计信息，oracle数据库提供了一个表监控特性。当STATISTICS_LEVEL设置为TYPICAL或ALL时，表监控特性默认是启动的。

Monitoring tracks the approximate number of INSERTs,UPDATEs, andDELETEs for that table andwhether the table has been truncated since the last time statisticswere gathered. You can access information about changes of tablesin theUSER_TAB_MODIFICATIONS view. Following adata-modification, there may be a few minutes delay while OracleDatabase propagates the information to this view. Use theDBMS_STATS.FLUSH_DATABASE_MONITORING_INFO procedure toimmediately reflect the outstanding monitored information kept inthe memory.

表监控特性跟踪从最后一次统计搜集后，表的insert、update、delete操作的近似数，和表是否被truncate。可以通过查询USER_TAB_MODIFICATIONS视图获得关于表变化的信息。数据修改后，通过USER_TAB_MODIFICATIONS获取修改信息可能有一些延时。使用DBMS_STATS.FLUSH_DATABASE_MONITORING_INFO立即将延时信息保持到内存中。

TheGATHER_DATABASE_STATS orGATHER_SCHEMA_STATS procedures gather new statisticsfor tables with stale statistics when theOPTIONSparameter is set to GATHER STALE orGATHERAUTO. If a monitored table has been modified more than10%, then these statistics are considered stale and gatheredagain.

当属性OPTIONS设置为GATHER STALEor GATHERAUTO，GATHER_DATABASE_STATS或GATHER_SCHEMA_STATS为有过期统计信息的表搜集新的统计信息，如果一个监控的表修改超过了10%，则统计信息被认为过期，需再次搜集。

部分实验如下：

SQL> select * from test01;

A B
---------- ----------
21 10
9 10
22 10
23 10
24 10
25 10
1 1
2 2
44 44
55 55

10 rowsselected. ------表数据10行

SQL> select * fromuser_tab_modifications;

no rowsselected ------实验前已对测试库做了搜集统计信息，此时显示为空

SQL> insert into test01 values(66,66);

1 row created.

SQL> commit;

Commit complete.

SQL> select * from user_tab_modifications;

no rowsselected ------表的修改未及时显示

SQL> exec DBMS_STATS.FLUSH_DATABASE_MONITORING_INFO; ------将修改的信息keep到内存

PL/SQL procedure successfully completed.

SQL> selecttable_name,INSERTS,UPDATES,DELETES,truncated fromuser_tab_modifications;

TABLE_NAME INSERTS UPDATES DELETES TRU
------------------------------ ---------- ---------- -------------
TEST01 10 0 NO ------ 1 代表刚才的1条insert

为更好的展现监控信息，接着进行update、delete、truncate操作演示：

SQL> update test01 set a=77 where b=66;

1 row updated.

SQL> commit;

Commit complete.

SQL> selecttable_name,INSERTS,UPDATES,DELETES,truncated fromuser_tab_modifications;

TABLE_NAME INSERTS UPDATES DELETES TRU
------------------------------ ---------- ---------- -------------
TEST01 1 0 0 NO

SQL> exec DBMS_STATS.FLUSH_DATABASE_MONITORING_INFO;

PL/SQL procedure successfully completed.

SQL> selecttable_name,INSERTS,UPDATES,DELETES,truncated fromuser_tab_modifications;

TABLE_NAME INSERTS UPDATES DELETES TRU
------------------------------ ---------- ---------- -------------
TEST01 1 1 0 NO

SQL> delete from test01 where a=77;

1 row deleted.

SQL> commit;

Commit complete.

SQL> selecttable_name,INSERTS,UPDATES,DELETES,truncated fromuser_tab_modifications;

TABLE_NAME INSERTS UPDATES DELETES TRU
------------------------------ ---------- ---------- -------------
TEST01 1 1 0 NO

SQL> exec DBMS_STATS.FLUSH_DATABASE_MONITORING_INFO;

PL/SQL procedure successfully completed.

SQL> selecttable_name,INSERTS,UPDATES,DELETES,truncated fromuser_tab_modifications;

TABLE_NAME INSERTS UPDATES DELETES TRU
------------------------------ ---------- ---------- -------------
TEST01 1 1 1 NO

SQL> truncate table test01;

Table truncated.

SQL> selecttable_name,INSERTS,UPDATES,DELETES,truncated fromuser_tab_modifications;

TABLE_NAME INSERTS UPDATES DELETES TRU
------------------------------ ---------- ---------- -------------
TEST01 1 1 1 NO

SQL> exec DBMS_STATS.FLUSH_DATABASE_MONITORING_INFO;

PL/SQL procedure successfully completed.

SQL> selecttable_name,INSERTS,UPDATES,DELETES,truncated fromuser_tab_modifications;

TABLE_NAME INSERTS UPDATES DELETES TRU
------------------------------ ---------- ---------- -------------
TEST01 1 1 11YES

对表test01执行搜集统计信息，监控表的信息会清除：

SQL> execdbms_stats.gather_table_stats(null,'test01');

PL/SQL procedure successfully completed.

SQL> selecttable_name,INSERTS,UPDATES,DELETES,truncated fromuser_tab_modifications;

no rows selected