Oralce索引聚簇因子
Sql代码
--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.