用union替換or (適用於索引列)
通常情況下, 用union替換where子句中的or將會起到較好的效果. 對索引列使用or將造成全表掃瞄. 注意,以上規則只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, 那就需要返回記錄最少的索引列寫在最前面.
注意: where key1 = 10 (返回最少記錄) or key2 = 20 (返回最多記錄)
oracle 內部將以上轉換為
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 返回一條記錄 , b = 1 返回1003條記錄)
sql> select from unionvsor /1st test*/
2 where a = 1003 or b = 1;
1003 rows selected.
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)
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 /2nd test*/
2 where b = 1 or a = 1003 ;
1003 rows selected.
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)
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 /3rd test*/
2 where a = 1003
3 union
4 select * from unionvsor
5 where b = 1;
1003 rows selected.
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)
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
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