From: Jeremy Chase
Date: January 30 2013 7:25pm
Subject: InnoDB interaction between secondary and primary keys.
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 Hello,

I've been working with a secondary index and would like some clarification
about how the primary columns are included. So, in the following example,
is the secondaryKey effectively the same as redundantKey?

CREATE TABLE `example` (
  `1` int(10) unsigned NOT NULL,
  `2` int(10) unsigned NOT NULL,
  `3` int(10) unsigned NOT NULL,
  `4` int(10) unsigned NOT NULL,
  PRIMARY KEY (`1`,`2`),
  KEY `secondaryKey` (`3`, `4`)
  KEY `redundantKey` (`3`, `4`, `1`, `2`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

Background: http://dev.mysql.com/doc/refman/5.5/en/innodb-index-types.html

"In InnoDB, each record in a secondary index contains the primary key
columns for the row, as well as the columns specified for the secondary
index."

Thank you!
Jeremy

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