exists and not EXISTS usage in Oracle

Http://blog.sina.com.cn/s/blog_601d1ce30100cyrb.html

There are two simple examples to illustrate the efficiency issues of "exists" and "in"

1) SELECT * from T1 where exists (select 1 from T2 where t1.a=t2.a);

When the T1 data volume is small and the T2 data is very large, the query efficiency of T1<<T2 1) is high.

2) SELECT * from T1 where t1.a in (select t2.a from T2);

T1 data volume is very large and T2 data amount of hours, T1>>T2, 2) query efficiency is high.

exists and not EXISTS usage in Oracle