组合分区表创建索引之后最好收集索引的统计信息

本实验基于Oracle10g

先建立一个组合分区表

create table t_sub(id number, data varchar2(20))

partition by range(id)subpartition by hash(data)

(partition p1 values less than (100)

(subpartition p1_s_1,

subpartition p1_s_2

),

partition p2 values less than (maxvalue)

(subpartition p2_s_1,

subpartition p2_s_2

)

);

插入数据

BEGIN

FOR i in 1..200 loop

insert into t_sub values(i,i||'A');

end loop;

END;

SQL> create index i_id on t_sub(id) local nologging;

Index created

SQL> select a.index_name,

2 decode(partitioned,'YES',b.partition_name,'NO')

3 partition,b.subpartition_name subpartition_name,b.num_rows,

4 b.distinct_keys,b.num_rows/b.distinct_keys

5 avg_row_per_key,b.distinct_keys/b.num_rows SELECTIVITY,

6 b.clustering_factor from dba_indexes

7 a,dba_ind_statistics b where a.owner=b.owner

8 and a.index_name=b.index_name and a.owner='&owner' and

9 a.index_name='&index_name';

INDEX_NAME PARTITION SUBPARTITI NUM_ROWS DISTINCT_KEYS AVG_ROW_PER_KEY SELECTIVITY CLUSTERING_FACTOR

---------- ---------- ---------- ---------- ------------- --------------- ----------- -----------------

I_ID P2 P2_S_2 58 58 1 1 1

I_ID P2 P2_S_1 43 43 1 1 1

I_ID P1 P1_S_2 44 44 1 1 1

I_ID P1 P1_S_1 55 55 1 1 1

I_ID P2 101 58
1.7413793103448 0.574257425 2

I_ID P1 99 55 1.8 0.555555555 2

I_ID 200 58 3.4482758620689 0.29 4

7 rows selected

我们都知道，创建索引的时候会自动收集统计信息，以前创建索引如果要收集统计信息还需要加上compute statistics，不过现在不需要了，已经默认会收集。从查询看出，子分区的统计信息 是准确的， 不过分区，以及全局的统计信息 是不准确 的。

注意看 distinct_keys。我插入的数据都没有重复的，不过Oracle却统计出有重复数据。

好了现在我收集统计信息

SQL> BEGIN

2 DBMS_STATS.GATHER_TABLE_STATS(ownname => 'ROBINSON',

3 tabname => 'T_SUB',

4 estimate_percent => DBMS_STATS.AUTO_SAMPLE_SIZE,

5 method_opt => 'for all columns size repeat',

6 degree => DBMS_STATS.AUTO_DEGREE,

7 granularity => 'ALL',

8 cascade=>TRUE

9 );

10 END;

11 /

PL/SQL procedure successfully completed

再次查询 统计信息

SQL> select a.index_name,

2 decode(partitioned,'YES',b.partition_name,'NO')

3 partition,b.subpartition_name subpartition_name,b.num_rows,

4 b.distinct_keys,b.num_rows/b.distinct_keys

5 avg_row_per_key,b.distinct_keys/b.num_rows SELECTIVITY,

6 b.clustering_factor from dba_indexes

7 a,dba_ind_statistics b where a.owner=b.owner

8 and a.index_name=b.index_name and a.owner='&owner' and

9 a.index_name='&index_name';

INDEX_NAME PARTITION SUBPARTITI NUM_ROWS DISTINCT_KEYS AVG_ROW_PER_KEY SELECTIVITY CLUSTERING_FACTOR

---------- ---------- ---------- ---------- ------------- --------------- ----------- -----------------

I_ID P2 P2_S_2 58 58 1 1 1

I_ID P2 P2_S_1 43 43 1 1 1

I_ID P1 P1_S_2 44 44 1 1 1

I_ID P1 P1_S_1 55 55 1 1 1

I_ID P2 101 101 1 1 2

I_ID P1 99 99 1 1 2

I_ID 200 200 1 1 4

7 rows selected

现在准确了。