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    <p>I have noticed that FORCE INDEX helps when you have multiple joins and sub-queries on VARCHAR fields where both the FK and the referenced value are not the primary key, while at the same time having where clause on a DATE field.</p> <p>Something like:</p> <pre><code>SELECT NAME, a.reference_no, i.value, p.value FROM customers AS c INNER JOIN accounts AS a ON c.id = a.customer_id INNER JOIN invoices AS i ON i.reference_no = a.reference_no INNER JOIN payments AS p ON p.invoice_no = i.invoice_no WHERE payments.date &gt;= '2011-09-01' AND DATE &lt; '2011-10-01'; </code></pre> <p>mysql will always use the PKs and FKs, where you would it to use the payment_date index on the payments table first as it is the largest one. So a <code>FORCE INDEX(payment_date)</code> on the payments table join would help a lot.</p> <p>This an example from the third-party billing database that we use at work. We had huge issues with optimisation, and FORCE INDEX did the job most of the times. Usually we found the slow quires with mysqladmin, tested them with FORCE INDEX and send them to the vendors to rewrite them in the source code of the app.</p> <p>Here are the four tables to get a better grip on the example:</p> <pre><code>CREATE TABLE `customers` ( `id` int(11) NOT NULL AUTO_INCREMENT, `name` varchar(100) NOT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=latin1; CREATE TABLE `accounts` ( `id` int(11) NOT NULL AUTO_INCREMENT, `customer_id` int(11) NOT NULL, `reference_no` varchar(10) NOT NULL, PRIMARY KEY (`id`), UNIQUE KEY `reference_no_uniq` (`reference_no`), KEY `FK_accounts` (`customer_id`), CONSTRAINT `FK_accounts` FOREIGN KEY (`customer_id`) REFERENCES `customers` (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=latin1; CREATE TABLE `invoices` ( `id` int(11) NOT NULL AUTO_INCREMENT, `reference_no` varchar(10) NOT NULL, `invoice_no` varchar(10) NOT NULL, `value` int(11) NOT NULL, PRIMARY KEY (`id`), UNIQUE KEY `invoice_no_uniq` (`invoice_no`), KEY `FK_invoices` (`reference_no`), CONSTRAINT `FK_invoices` FOREIGN KEY (`reference_no`) REFERENCES `accounts` (`reference_no`) ) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=latin1; CREATE TABLE `payments` ( `id` int(11) NOT NULL AUTO_INCREMENT, `invoice_no` varchar(10) NOT NULL, `value` int(11) NOT NULL, `date` datetime DEFAULT NULL, PRIMARY KEY (`id`), KEY `FK_payments` (`invoice_no`), KEY `payment_date` (`date`), CONSTRAINT `FK_payments` FOREIGN KEY (`invoice_no`) REFERENCES `invoices` (`invoice_no`) ) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=latin1; </code></pre>
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    1. COWhy MySQL will always use the primary key on table `customers`? Does it depend on the sequence of tables in joins?
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    2. COI have edited the post... the bigger problem that I got was not using a the date index on payments table. However I think that the the sequence of table does influence the order of the indexes being used
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    3. COYou talk the talk , but don't show us any reference anywhere to this FORCED_INDEX solution. If you're going to suggest we use it in the first query, why not list a second query with FORCED_INDEX implemented? Thanks for trying to help .
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