ORACLE SQL性能优化系列 第 36 页 共 57 页
0 SELECT STATEMENT Optimizer=CHOOSE 1 0 TABLE ACCESS (BY INDEX ROWID) OF 'EMP' 2 1 INDEX (RANGE SCAN) OF 'EMPNO' (UNIQUE)
SQL> select * from emp where empno <= 1; no rows selected Execution Plan
---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE 1 0 TABLE ACCESS (BY INDEX ROWID) OF 'EMP' 2 1 INDEX (RANGE SCAN) OF 'EMPNO' (UNIQUE)
两者的效率完全一样，也许这符合作者关于” 在某些时候, ORACLE优化器会自动将NOT转化成相对应的关系操作符” 的观点．
35.用>=替代>
如果DEPTNO上有一个索引, 高效:
SELECT * FROM EMP
WHERE DEPTNO >=4 低效:
ORACLE SQL性能优化系列 第 37 页 共 57 页
SELECT * FROM EMP WHERE DEPTNO >3
两者的区别在于, 前者DBMS将直接跳到第一个DEPT等于4的记录而后者将首先定位到DEPTNO=3的记录并且向前扫描到第一个DEPT大于3的记录.
36.用UNION替换OR (适用于索引列)
通常情况下, 用UNION替换WHERE子句中的OR将会起到较好的效果. 对索引列使用OR将造成全表扫描. 注意, 以上规则只针对多个索引列有效. 如果有column没有被索引, 查询效率可能会因为你没有选择OR而降低. 在下面的例子中, LOC_ID 和REGION上都建有索引. 高效:
SELECT LOC_ID , LOC_DESC , REGION FROM LOCATION WHERE LOC_ID = 10 UNION
SELECT LOC_ID , LOC_DESC , REGION FROM LOCATION
WHERE REGION = “MELBOURNE” 低效:
SELECT LOC_ID , LOC_DESC , REGION FROM LOCATION
WHERE LOC_ID = 10 OR REGION = “MELBOURNE”
如果你坚持要用OR, 那就需要返回记录最少的索引列写在最前面.
ORACLE SQL性能优化系列 第 38 页 共 57 页
注意:
WHERE KEY1 = 10 (返回最少记录) OR KEY2 = 20 (返回最多记录)
ORACLE 内部将以上转换为 WHERE KEY1 = 10 AND
((NOT KEY1 = 10) AND KEY2 = 20)
译者按:
下面的测试数据仅供参考: (a = 1003 返回一条记录 , b = 1 返回1003条记录)
SQL> select * from unionvsor /*1st test*/ 2 where a = 1003 or b = 1; 1003 rows selected. Execution Plan
---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE 1 0 CONCATENATION
2 1 TABLE ACCESS (BY INDEX ROWID) OF 'UNIONVSOR' 3 2 INDEX (RANGE SCAN) OF 'UB' (NON-UNIQUE) 4 1 TABLE ACCESS (BY INDEX ROWID) OF 'UNIONVSOR' 5 4 INDEX (RANGE SCAN) OF 'UA' (NON-UNIQUE) Statistics
---------------------------------------------------------- 0 recursive calls 0 db block gets
ORACLE SQL性能优化系列 第 39 页 共 57 页
144 consistent gets 0 physical reads 0 redo size
63749 bytes sent via SQL*Net to client 7751 bytes received via SQL*Net from client 68 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1003 rows processed
SQL> select * from unionvsor /*2nd test*/ 2 where b = 1 or a = 1003 ; 1003 rows selected. Execution Plan
---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE 1 0 CONCATENATION
2 1 TABLE ACCESS (BY INDEX ROWID) OF 'UNIONVSOR' 3 2 INDEX (RANGE SCAN) OF 'UA' (NON-UNIQUE) 4 1 TABLE ACCESS (BY INDEX ROWID) OF 'UNIONVSOR' 5 4 INDEX (RANGE SCAN) OF 'UB' (NON-UNIQUE) Statistics
---------------------------------------------------------- 0 recursive calls 0 db block gets 143 consistent gets 0 physical reads 0 redo size
63749 bytes sent via SQL*Net to client 7751 bytes received via SQL*Net from client
ORACLE SQL性能优化系列 第 40 页 共 57 页
68 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1003 rows processed
SQL> select * from unionvsor /*3rd test*/ 2 where a = 1003 3 union
4 select * from unionvsor 5 where b = 1; 1003 rows selected. Execution Plan
---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE 1 0 SORT (UNIQUE) 2 1 UNION-ALL
3 2 TABLE ACCESS (BY INDEX ROWID) OF 'UNIONVSOR' 4 3 INDEX (RANGE SCAN) OF 'UA' (NON-UNIQUE) 5 2 TABLE ACCESS (BY INDEX ROWID) OF 'UNIONVSOR' 6 5 INDEX (RANGE SCAN) OF 'UB' (NON-UNIQUE) Statistics
---------------------------------------------------------- 0 recursive calls 0 db block gets 10 consistent gets 0 physical reads 0 redo size
63735 bytes sent via SQL*Net to client 7751 bytes received via SQL*Net from client