通常情況下, 用UNION替換WHERE子句中的OR将會起到較好的效果. 對索引列使用OR将造成全表掃描. 注意, 以上規則隻針對多個索引列有效. 如果有column沒有被索引, 查詢效率可能會因為你沒有選擇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 ”
如果你堅持要用OR, 那就需要傳回記錄最少的索引列寫在最前面.
注意:
WHERE KEY1 = 10 (傳回最少記錄)
OR KEY2 = 20 (傳回最多記錄)
ORACLE 内部将以上轉換為
WHERE KEY1 = 10 AND
((NOT KEY1 = 10) AND KEY2 = 20)
譯者按: 下面的測試資料僅供參考: (a = 1003 傳回一條記錄 , b = 1 傳回1003 條記錄) SQL> select * from unionvsor 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 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 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 68 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1003 rows processed SQL> select * from unionvsor 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 68 SQL*Net roundtrips to/from client 1 sorts (memory) 0 sorts (disk) 1003 rows processed 用UNION 的效果可以從consistent gets 和 SQL*NET 的資料交換量的減少看出 37. 用IN 來替換OR下面的查詢可以被更有效率的語句替換:
低效:
SELECT….
FROM LOCATION
WHERE LOC_ID = 10
OR LOC_ID = 20
OR LOC_ID = 30
高效
SELECT…
FROM LOCATION
WHERE LOC_IN IN (10,20,30);
譯者按: 這是一條簡單易記的規則,但是實際的執行效果還須檢驗,在ORACLE8i 下,兩者的執行路徑似乎是相同的. 38. 避免在索引列上使用IS NULL 和IS NOT NULL避免在索引中使用任何可以為空的列,ORACLE将無法使用該索引 .對于單列索引,如果列包含空值,索引中将不存在此記錄. 對于複合索引,如果每個列都為空,索引中同樣不存在此記錄. 如果至少有一個列不為空,則記錄存在于索引中.
舉例:
如果唯一性索引建立在表的A列和B列上, 并且表中存在一條記錄的A,B值為(123,null) , ORACLE将不接受下一條具有相同A,B值(123,null)的記錄(插入). 然而如果
所有的索引列都為空,ORACLE将認為整個鍵值為空而
空不等于空. 是以你可以插入1000
條具有相同鍵值的記錄,當然它們都是空!
因為空值不存在于索引列中,是以WHERE子句中對索引列進行空值比較将使ORACLE停用該索引.
舉例:
低效: (索引失效)
SELECT …
FROM DEPARTMENT
WHERE DEPT_CODE
IS NOT NULL;高效: (索引有效)
SELECT …
FROM DEPARTMENT
WHERE DEPT_CODE
>=0;