Re: Question on OUTER JOINS.

--- Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Ludwig Lim <lud_nowhere_man(at)yahoo(dot)com> writes:
> > 1) Is the ON clause of an OUTER JOIN always
> > evaluated first before the WHERE clause?
>
> No; the planner will do whatever it thinks is the
> most efficient way
> (assuming it can prove that the reordering it wants
> to do won't change
> the query result).

If re-ordering does change the result, is the ON
clause evaluated first and the WHERE filters out the
result of the OUTER JOIN?

> > Is there a way to rewrite the query as a view
> such
> > that one can do:
>
> I'm really not clear on what you want here. Better
> example please?
>
Sorry for not making it that clear.

Is there way of rewritting :
SELECT a.status,
employee_id
FROM permission a LEFT JOIN
( SELECT * FROM employee WHERE employee_id
=5) as b ON (a.status = b.status)
WHERE status='test'

into a query that has no subselect in the FROM clause.
I mean can the query above be rewritten into
something like:

SELECT a.status,
b.employee_id
FROM permission a LEFT JOIN employee b ON
(a.status = b.status)
WHERE a.status = 'test' and
b.employee_id = 5;

Thank you very much,

ludwig

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