Re: Specifying the type of join

On Fri, 10 Jan 1997 14:06:54 -0500, Allen Kirby <akirby_at_att.com> wrote:

>Jason Byors wrote:
>>
>> How do you specify a left to right, and a right to left join in SQL.
>> I can't seem to find it in any of the books I have looked in. Could
>> you please e-mail me as well as posting to the newsgroup.
>>
>> Thank you,
>> jb
>
>I assume you mean how to determine which table drives the other table
>in a query, or which table is accessed first. I don't believe standard
>SQL has that syntax because logically speaking, it doesn't matter which
>table is queried first - the answer is always the same. The performance
>of the query may be drastically different, but the result is the same.
>So this is really a performance tuning question. I don't think that
>Oracle has a definite way to acheive this (I know they didn't in V6)
>but there are several indirect methods you could try, like changing
>the order of the tables in the from clause or maybe an optimizer
>trick. I'm sure others know the exact tricks, but you can use the
>EXPLAIN PLAN feature to tell which table is driving the query, then
>try different things until it uses the index you want. Also, wouldn't
>the cost-based optimizer usually pick the best table?
>--
>---
>Allen Kirby AT&T ITS Production Services
>akirby_at_att.com Alpharetta, GA.

You can specify the ORDERED hint in the query to use the order of tables you desire.

Scott Moyer
dmoyer_at_gpu.com Received on Wed Jan 15 1997 - 00:00:00 CST