Oracle11gR2-聚簇因子浅析

创建表t1,t2

SQL> conn n1/n1
Connected.
SQL>
SQL>
SQL> create table t1 as select trunc(rownum/100) id ,object_name from all_objects where rownum<1000 ;

Table created.

SQL> create table t2 as select mod(rownum,100) id ,object_name from all_objects where rownum<1000;

Table created.

创建索引

SQL> create index inx_t1 on t1(id);

Index created.

SQL> create index inx_t2 on t2(id);

Index created.

SQL> exec dbms_stats.gather_table_stats(null,'T1',CASCADE=>true);

PL/SQL procedure successfully completed.

SQL> exec dbms_stats.gather_table_stats(null,'T2',CASCADE=>true);

PL/SQL procedure successfully completed.

SQL> conn / as sysdba
Connected.

比较执行计划 查询t1走的索引范围扫描,t2确走的全表扫描

SQL> set autotrace trace exp stat
SQL> set linesize 300
SQL> select * from n1.t1 where id =2;

100 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 2808986199

--------------------------------------------------------------------------------------
| Id  | Operation                   | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |        |   100 |  2100 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T1     |   100 |  2100 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | INX_T1 |   100 |       |     1   (0)| 00:00:01 |
--------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ID"=2)

Statistics
----------------------------------------------------------
		  0  recursive calls
		  0  db block gets
		 18  consistent gets
		  0  physical reads
		  0  redo size
	   4386  bytes sent via SQL*Net to client
		589  bytes received via SQL*Net from client
		  8  SQL*Net roundtrips to/from client
		  0  sorts (memory)
		  0  sorts (disk)
		100  rows processed

SQL> select * from n1.t2 where id  =2;

10 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 1513984157

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |    10 |   210 |     4   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| T2   |    10 |   210 |     4   (0)| 00:00:01 |
--------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("ID"=2)

Statistics
----------------------------------------------------------
		  1  recursive calls
		  0  db block gets
		  6  consistent gets
		  0  physical reads
		  0  redo size
		842  bytes sent via SQL*Net to client
		523  bytes received via SQL*Net from client
		  2  SQL*Net roundtrips to/from client
		  0  sorts (memory)
		  0  sorts (disk)
		 10  rows processed

查看聚簇因子,可以看到t2 CLUSTERING_FACTOR很高,说明数据分布很散

SQL> conn n1/n1
Connected.
SQL> set linesize 300
SQL> col table_name format a30
SQL> col index_name format a30
SQL> select i.table_name,i.index_name,i.CLUSTERING_FACTOR,t.blocks,i.NUM_ROWS from
  2  user_tables t,user_indexes i where t.table_name=i.table_name and t.table_name in
  3  ('T1','T2');

TABLE_NAME                     INDEX_NAME                     CLUSTERING_FACTOR     BLOCKS   NUM_ROWS
------------------------------ ------------------------------ ----------------- ---------- ----------
T1                             INX_T1                                         4          7        999
T2                             INX_T2                                       400          7        999

解决方案:按照索引的顺序重新导入数据！