How the columns retrive from datebase when u r using joins

Question

Hi,
I am Seetharamaiah.c working in Xansa India Limited. I got a doubt . Pls clarify my doubt when we r retriving the data from database, how the colums retrieve when we r using self join. What exactly internally happen.
Thanking u,
Seetharamaiah.C

Answer 1

use set autotrace on
SQL> set autotrace on
SQL> select dname,ename
from dept d,emp e
where d.deptno = e.deptno;
Execution Plan
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 NESTED LOOPS
2 1 TABLE ACCESS (FULL) OF 'EMP'
3 1 TABLE ACCESS (BY INDEX ROWID) OF 'DEPT'
4 3 INDEX (UNIQUE SCAN) OF 'SYS_C0063851' (UNIQUE)
Each row of the emp table is returned and the corresponding dept row is found using the index.
Introduce an outer join:
select dname,ename
from dept d,emp e
where d.deptno = e.deptno(+)
Execution Plan
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 MERGE JOIN (OUTER)
2 1 SORT (JOIN)
3 2 TABLE ACCESS (FULL) OF 'DEPT'
4 1 SORT (JOIN)
5 4 TABLE ACCESS (FULL) OF 'EMP'
and you get two full table scans with the results merged to populate the dept with no emp records