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01.06.2008 at 05:56AM PST, ID: 23061919
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9.4

Optimizer question on LEFT JOIN

Asked by Tayger in MySQL Server

Tags: , , ,

Hello everyone
Not a problem but just a general question on MySQLs behaviour on LEFT JOINs.
Lets assume you have a customer table and a relation table to express your relation with the customer.

There is a master table customer and a detail table called relations. The relation table looks like this:
relid     description     (both declared as NOT NULL columns)
1          Good relation
2          Not contacted yet
3          Needs special treatment
4          ....

So the relid is also in the master table, delcared as NULL column, because there are customer types which are not applicable for assigning a relation.
To get all customers the join looks like this:
SELECT *
FROM customers
LEFT JOIN relations ON customer.relid = relations.relid

That all works fine but for optimizer reason I wonder if I could improve it. So the questions now:
1. The left join wiill go through all columns in relations. Is there a way to stop the join if a join can be done?
2. Does MySQL realize that customer.relid can have NULL-values but relations.relid only NOT NULL-values and if there is a NULL-value in customer.relid any further join must not be done because it makes no sense?

Any hints are apreciated!
Thank you
Tyger

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[+][-]01.06.2008 at 06:14AM PST, ID: 20593963

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[+][-]01.06.2008 at 06:22AM PST, ID: 20593985

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[+][-]01.06.2008 at 06:29AM PST, ID: 20594002

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[+][-]01.06.2008 at 06:40AM PST, ID: 20594030

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[+][-]01.06.2008 at 08:43AM PST, ID: 20594425

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About this solution

Zone: MySQL Server
Tags: MySQL, n/a, n/a, MySQL optimize SELECT
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Solution Provided By: angelIII
Participating Experts: 2
Solution Grade: A
 
 
[+][-]01.06.2008 at 09:16AM PST, ID: 20594556

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