Oralce 索引聚簇因子

--聚合因子试验准备:

--分别建两张有序和无序的表
CREATE TABLE T_COLOCATED ( ID NUMBER, COL2 VARCHAR2(100) );
BEGIN
        FOR I IN 1 .. 100000
        LOOP
            INSERT INTO T_COLOCATED(ID,COL2)
            VALUES (I, RPAD(DBMS_RANDOM.RANDOM,95,'*') );
        END LOOP;
END;
/

ALTER TABLE T_COLOCATED ADD CONSTRAINT PK_T_COLOCATED PRIMARY KEY(ID);

CREATE TABLE T_DISORGANIZED
     AS
    SELECT ID,COL2
    FROM T_COLOCATED
    ORDER BY COL2;

ALTER TABLE T_DISORGANIZED ADD CONSTRAINT PK_T_DISORG PRIMARY KEY (ID);


--分别分析两张表的聚合因子层度
                                                         
SELECT INDEX_NAME,                                                         
              BLEVEL,                                                          
              LEAF_BLOCKS,                                                     
              NUM_ROWS,                                                        
              DISTINCT_KEYS,                                                   
              CLUSTERING_FACTOR                                                
         FROM USER_IND_STATISTICS                                              
        WHERE TABLE_NAME IN( 'T_COLOCATED','T_DISORGANIZED');    


INDEX_NAME                         BLEVEL LEAF_BLOCKS   NUM_ROWS DISTINCT_KEYS CLUSTERING_FACTOR
------------------------------ ---------- ----------- ---------- ------------- -----------------
PK_T_COLOCATED                          1         208     100000        100000              1469
PK_T_DISORG                             1         208     100000        100000             99932
--首先观察有序表的查询性能



执行并比较性能差异
select /*+index(t)*/ * from  t_colocated t  where id>=20000 and id<=40000;

执行计划
----------------------------------------------------------
Plan hash value: 4204525375

----------------------------------------------------------------------------------------------
| Id  | Operation                   | Name           | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |                | 21104 |  1339K|   389   (1)| 00:00:05 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T_COLOCATED    | 21104 |  1339K|   389   (1)| 00:00:05 |
|*  2 |   INDEX RANGE SCAN          | PK_T_COLOCATED | 21104 |       |    53   (2)| 00:00:01 |
----------------------------------------------------------------------------------------------

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

   2 - access("ID">=20000 AND "ID"<=40000)

Note
-----
   - dynamic sampling used for this statement


统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
       2986  consistent gets
          0  physical reads
          0  redo size
    2293678  bytes sent via SQL*Net to client
      15048  bytes received via SQL*Net from client
       1335  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
      20001  rows processed

再观察无序表的查询性能
select /*+index(t)*/ * from  t_disorganized t  where id>=20000 and id<=40000;

已用时间:  00: 00: 09.75

执行计划
----------------------------------------------------------
Plan hash value: 4204525375

----------------------------------------------------------------------------------------------
| Id  | Operation                   | Name           | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |                | 21104 |  1339K|   389   (1)| 00:00:05 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T_COLOCATED    | 21104 |  1339K|   389   (1)| 00:00:05 |
|*  2 |   INDEX RANGE SCAN          | PK_T_COLOCATED | 21104 |       |    53   (2)| 00:00:01 |
----------------------------------------------------------------------------------------------

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

   2 - access("ID">=20000 AND "ID"<=40000)

Note
-----
   - dynamic sampling used for this statement


统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
       2986  consistent gets
          0  physical reads
          0  redo size
    2293678  bytes sent via SQL*Net to client
      15048  bytes received via SQL*Net from client
       1335  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
      20001  rows processed

--Oracle文档对聚簇因子的解释
Indicates the amount of order of the rows in the table based on the values of the index.
If the value is near the number of blocks, then the table is very well ordered. In this case, the index entries in a single leaf block tend to point to rows in the same data blocks.
If the value is near the number of rows, then the table is very randomly ordered. In this case, it is unlikely that index entries in the same leaf block point to rows in the same data blocks.