I NEED TO KNOW ABOUT DRIVING TABLE...PLEASE

Question

Well, My name is Adan. I am from Panama.. my english is not
good. But I'll try. I need to know about driving tables and how
have to choose a driving table and how optimizer proccess an sql
statment.
Example:
Employees have (1,000,000 rows),
dept have (100 rows)
Category have (100 rows)...
I need retrieved all employees with category and dept name for
those employee with salary > 100000
Well, What is the driving table and where i have to put this one
in the from clause ? What about order in the where predicate...
First ?
Select E.name, C.category, D.Departement
from Employees e, dept d, category c
where
and d.cat = c.cat
and e.deptno = d.deptno
and e.salary > 100000
Last ?
Select E.name,C.category, D.Departement
from dept d, category c ,Employees e
where
and d.cat = c.cat
and e.deptno = d.deptno
and e.salary > 100000
I apreciate your help....Thanks and sorry for my english....
Bye..

Answer 1

Adam,
If you set your optimizer = choose and analyze the tables and
indexes to generate statistics, then your system can use the
Cost Based Optimizer (CBO) and, like David said, it will
automatically choose the best access path, and the order of
things will not matter.
If your optimizer is not set to choose or if there are no
statistics, then it will use the Rule Based Optimizer (RBO), in
which case, the only thing that matters is the order of the
tables in the from clause. When using RBO, the driving table
should be at the end of the from clause, in the right-most
position. The driving table is the one such that, after the
restrictions have been applied, returns the smallest number of
rows. You would have to compare the number of rows in the
employees table where salary > 100000 to the number of rows in
the dept table (100) and the number of rows in the category
table (100).
In RBO, if there are more than 100 rows in the employees table
where salary > 100000, then your from clause would be:
FROM employees e, dept d, category c
In RBO, if there are less than 100 rows in the employees table
where salary > 100000, then your from clause would be:
FROM category c, dept d, employees e
This is all that applies to the example you have given, however
there are other things that can apply in other situations. For
example, if there is an outer join to a table, the table that
has the (+) cannot be used as the driving table.
Barbara