Question

I have the following table structure

CREATE TABLE `table` (   `id` int(11) NOT NULL auto_increment,   `date_expired` datetime NOT NULL,   `user_id` int(11) NOT NULL,   `foreign_id` int(11) NOT NULL,   PRIMARY KEY  (`id`),   UNIQUE KEY `date_expired` (`date_expired`,`user_id`,`foreign_id`),   KEY `user_id` (`user_id`) ) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci; 

As you’ll notice, I have duplicate indexes on user_id: date_expired & user_id. I of course want the unique index because I want to ensure the data is unique.

The reason for the duplicate indexes is because without the user_id index, my main search query takes 4 seconds. With the extra index it takes 1 second. The query is joining the table on user_id and checking date_expired.

The table only has 275 records.

• How bad is it to have a unique and normal index on the same field?
• How bad is it to have larger indexes than data when the table is purely ids?

Answer 1

I believe if you created your unique index as (user_id, date_expired, foreign_id), you’ll get the same benefit of having a normal index on user_id with just the unique index. MySQL can use the first columns of any index to pare down the number of rows in the join in the same manner as an index on user_id.

See MySQL’s index documentation for more information.

Are you referring to the id auto_increment column elsewhere in your schema to save space? Since your unique index covers all of the other columns in your table, it is in essence a primary key itself and could be dropped if you’re not.

You can check what keys your query is using by prefixing it with EXPLAIN.