Scalar subqueries vs. joins

Question

Hi,
is someone able to tell me something about the impact on performance of scalar subqueries in select list?
I found that scalar subqueries are processed faster than joins but don't understand why.
E.g. first statement:
select e.ename, d.deptno,
(select dname from dept d where d.deptno=e.deptno) dname from emp e
where e.deptno =10;
Second statement:
select e.ename, d.deptno, d.dname
from emp e, dept.d
where e.deptno=d.deptno and d.deptno=10;
The optimizer calculates the first statement using a full table scan on emp, while the second statement using a nested loop join.
First statement is executed faster. I found also that the first staement is executed more for throughput, the second for answer time.
This is the behavior not only if there are thousands of lines in emp but also in real life applications.
Regards Frank

Answer 1

The relative performance of scalar subqueries and joins will largely depend on the relative sizes of the tables, and the indexes available.
Essentially, the scalar subquery works in the same fashion as this PL/SQL construct.
FOR r in SELECT ename FROM emp LOOP
   SELECT deptno, dname
   INTO v1, v2
   FROM dept
   WHERE deptno = r.deptno
   DBMS_OUTPUT.Put_Line(r.ename||' '||v1||' '||v2)
END LOOPIf dept is indexed on deptno, then this query will begin to return rows quickly, since it is only a little more complex that SELECT * FROM emp.
The join on the other hand, has to read all qualifying rows from both tables before it can begin doing the join. That is why it takes longer to return the first rows.
The disadvantage of a scalar subquery is that it will usually require much more I/O than a join. For every row in the outer table, you will require at least one I/O to get the value from the table in the subquery, and in your example, you will require two I/O's (one to read the index to get rowid, and another to read the row).
In addition, the two queries are not equivalent. Consider:
SQL> SELECT * FROM emp;
    EMP_ID ENAME          DEPTNO
         1 JOHN               10
SQL> SELECT * FROM dept;
    DEPTNO DNAME
        20 SIMS
SQL> SELECT e.ename, e.deptno,
            (SELECT dname FROM dept d WHERE d.deptno=e.deptno) dname
     FROM emp e
     WHERE e.deptno =10;
ENAME          DEPTNO DNAME
JOHN               10
SQL> SELECT e.ename, d.deptno, d.dname
     FROM emp e, dept d
     WHERE e.deptno=d.deptno and
           d.deptno=10;
no rows selected
SQL> SELECT e.ename, d.deptno, d.dname
     FROM emp e, dept d
     WHERE e.deptno=d.deptno and
           e.deptno=10;
no rows selectedTTFN
John