Optimization of sub-query conditions for Oracle query optimization

Environment: Oracle 11g

The existing A and B tables are associated with the A01 field, and the data in table A is not available in B. SQL as follows

Select Count (1 from (select A.*, (selectCount(1) from    where b.a01=as fromwhere flag=0 

Because flag is a virtual field, the index causes the SQL to execute particularly slowly, and the total number of 310W data is about 2 seconds.

Use NOT exists to optimize SQL as follows

Select Count (1fromwherenoexists(select1 from where a.a01=b.b01)

Using not exists to go through the index, the execution takes about 0.2 seconds

Optimization of sub-query conditions for Oracle query optimization