两者之间的性能(在oracle中)是否有区别
Select * from Table1 T1 Inner Join Table2 T2 On T1.ID = T2.ID
和
Select * from Table1 T1, Table2 T2 Where T1.ID = T2.ID
?
不!相同的执行计划,请看下面两个表:
CREATE TABLE table1 ( id INT, name VARCHAR(20) ); CREATE TABLE table2 ( id INT, name VARCHAR(20) );
使用内部联接的查询的执行计划:
-- with inner join EXPLAIN PLAN FOR SELECT * FROM table1 t1 INNER JOIN table2 t2 ON t1.id = t2.id; SELECT * FROM TABLE (DBMS_XPLAN.DISPLAY); -- 0 select statement -- 1 hash join (access("T1"."ID"="T2"."ID")) -- 2 table access full table1 -- 3 table access full table2
以及使用WHERE子句的查询的执行计划。
-- with where clause EXPLAIN PLAN FOR SELECT * FROM table1 t1, table2 t2 WHERE t1.id = t2.id; SELECT * FROM TABLE (DBMS_XPLAN.DISPLAY); -- 0 select statement -- 1 hash join (access("T1"."ID"="T2"."ID")) -- 2 table access full table1 -- 3 table access full table2