How does a SQL query work

Question

Hi all...
How does a SQL query basically work? Is there any pseudocode for it?
How ro calculate the cost of query?
Can we find out what would be the execution time for a query before it is executed?
Thanks in advance.
Ameya

Answer 1

Hi Avinash,
Look closely mate!!!
You have done set autotrace on... meaning you are executing the statement while i am doing set autotrace traceonly explain meaning the statement would never get executed actually.. oracle optimizer will just select the desired plan and will show the output..
You can even ESTIMATE how many rows the qeury will return before even executing the statement using the same above tech.. i will show you how:
The explain plan gives this -- as long as you are using the CBO (cost based
optimizer). Doesn't matter if you have 1 or 50 tables. The last part of the
plan is the estimate output.
consider:
SQL> set autotrace traceonly explain
SQL> select e1.*, e2.* from emp e1, emp e2;
Execution Plan
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=15 Card=196 Bytes=12544)
1 0 MERGE JOIN (CARTESIAN) (Cost=15 Card=196 Bytes=12544)
2 1 TABLE ACCESS (FULL) OF 'EMP' (Cost=1 Card=14 Bytes=448)
3 1 SORT (JOIN) (Cost=14 Card=14 Bytes=448)
4 3 TABLE ACCESS (FULL) OF 'EMP' (Cost=1 Card=14 Bytes=448)
196 = 14 * 14 -- thats right....
SQL> select emp.ename, mgr.ename from
2 emp, emp mgr
3 where emp.mgr = mgr.empno;
Execution Plan
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=5 Card=13 Bytes=208)
1 0 MERGE JOIN (Cost=5 Card=13 Bytes=208)
2 1 SORT (JOIN) (Cost=3 Card=14 Bytes=112)
3 2 TABLE ACCESS (FULL) OF 'EMP' (Cost=1 Card=14 Bytes=112)
4 1 SORT (JOIN) (Cost=3 Card=14 Bytes=112)
5 4 TABLE ACCESS (FULL) OF 'EMP' (Cost=1 Card=14 Bytes=112)
Here is estimated that about 13 rows would be returned...
SQL> select count(*) from emp;
Execution Plan
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=1 Card=1)
1 0 SORT (AGGREGATE)
2 1 INDEX (FULL SCAN) OF 'EMP_PK' (UNIQUE) (Cost=1 Card=14)
one row...
SQL> select ename, dname
2 from emp, dept where emp.deptno = dept.deptno;
Execution Plan
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=5 Card=14 Bytes=252)
1 0 NESTED LOOPS (Cost=5 Card=14 Bytes=252)
2 1 TABLE ACCESS (FULL) OF 'DEPT' (Cost=1 Card=4 Bytes=44)
3 1 TABLE ACCESS (FULL) OF 'EMP' (Cost=1 Card=14 Bytes=98)
14 rows...
1 select ename, dname
2* from emp, dept where emp.deptno = dept.deptno and emp.ename like '%A%'
SQL> /
Execution Plan
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=18)
1 0 NESTED LOOPS (Cost=2 Card=1 Bytes=18)
2 1 TABLE ACCESS (FULL) OF 'EMP' (Cost=1 Card=1 Bytes=7)
3 1 TABLE ACCESS (BY INDEX ROWID) OF 'DEPT' (Cost=1 Card=4 Bytes=44)
4 3 INDEX (RANGE SCAN) OF 'DEPT_IDX' (NON-UNIQUE)
1 rows.....
and this was all done without executing the query. See EXPLAIN plan and the
PLAN_TABLE.