关于dbms_stats取代analyze

　　自从Oracle8.1.5引入dbms_stats包，Experts们便推荐使用dbms_stats取代analyze。 理由如下

　　dbms_stats可以并行分析

　　dbms_stats有自动分析的功能(alter table monitor )

　　analyze 分析统计信息的不准确some times

　　1,2好理解，且第2点实际上在VLDB中是最吸引人的；3以前比较模糊，看了metalink236935.1 解释，analyze在分析Partition表的时候，有时候会计算出不准确的Global statistics .

　　原因是，dbms_stats会实在的去分析表全局统计信息（当指定参数）；而analyze是将表分区（局部）的statistics 汇总计算成表全局statistics ,可能导致误差。

drop table T3;

CREATE TABLE T3(i number, p number,sp number)PARTITION BY RANGE(p)
SUBPARTITION BY HASH(sp) SUBPARTITIONS 2 (PARTITION q1 VALUES LESS THAN(3) TABLESPACE USERS,
PARTITION q2 VALUES LESS THAN(MAXVALUE) TABLESPACE USERS);

declare
i number;
begin
for i in 1..100000 loop
insert into T3 values(i,mod(i,7), mod(i,8));
if( mod(i, 10000) = 0) then commit; end if;
end loop;
for i in 1..50000 loop
insert into T3 values(i,mod(i,7), mod(i,8)+5);
if( mod(i, 10000) = 0) then commit; end if;
end loop;
end;

/ 如上建立一个分区表

SQL> exec DBMS_STATS.GATHER_TABLE_STATS('ORACLE','T3',granularity => 'ALL');

PL/SQL procedure successfully completed.

SQL> select GLOBAL_STATS from dba_tables where table_name='T3';

GLO
---
YES 计算了Global Statistics

SQL> select COLUMN_NAME,NUM_DISTINCT,DENSITY,SAMPLE_SIZE,AVG_COL_LEN from user_TAB_COL_STATISTICS where table_name='T3';

COLUMN_NAME NUM_DISTINCT DENSITY SAMPLE_SIZE AVG_COL_LEN
------------------------------ ------------ ---------- ----------- -----------
I 100000 .00001 150000 5
P 7 .142857143 150000 3
SP 13 .076923077 150000 3
SQL> analyze table t3 delete statistics;

Table analyzed.

SQL> select GLOBAL_STATS from dba_tables where table_name='T3';

GLO
---
YES 看来analyze不能删除dbms_stats的统计信息？

SQL> analyze table t3 compute statistics;

Table analyzed.

SQL> select COLUMN_NAME,NUM_DISTINCT,DENSITY,SAMPLE_SIZE,AVG_COL_LEN from user_TAB_COL_STATISTICS where table_name='T3';

COLUMN_NAME NUM_DISTINCT DENSITY SAMPLE_SIZE AVG_COL_LEN
------------------------------ ------------ ---------- ----------- -----------
I 100000 .00001 150000 5
P 7 .142857143 150000 3
SP 13 .076923077 150000 3

　　没有变，还是一样。 Confused ?

SQL> select GLOBAL_STATS from dba_tables where table_name='T3';

GLO
---
YES
SQL> exec dbms_stats.DELETE_TABLE_STATS('oracle','t3');

PL/SQL procedure successfully completed.

SQL> select GLOBAL_STATS from dba_tables where table_name='T3';

GLO
---
NO 《 ----- 有效果了

SQL> analyze table t3 compute statistics;

Table analyzed.

SQL> select COLUMN_NAME,NUM_DISTINCT,DENSITY,SAMPLE_SIZE,AVG_COL_LEN from user_TAB_COL_STATISTICS where table_name='T3';

COLUMN_NAME NUM_DISTINCT DENSITY SAMPLE_SIZE AVG_COL_LEN
------------------------------ ------------ ---------- ----------- -----------
I 150000 6.6667E-06 4
P 7 .142857143 2
SP 8 .125 2

SQL> select GLOBAL_STATS from dba_tables where table_name='T3';

GLO
---
NO

　　可见，sometimes，analyze出错了。当CBO只需要partition的统计信息的时候还好，当使用表上的全局统计信息的时候，有可能产生不正确的执行计划。