MySQL: Add column to unique index or add new index? For a MySQL database, I have an InnoDB table name user_metrics with the following columns:

   id INT(11) NOT NULL AUTO_INCREMENT,
   userid INT(11) NOT NULL,
   name VARCHAR(32) NOT NULL,
   value DECIMAL(20,9) DEFAULT NULL
   valueclass INT(11) DEFAULT NULL


For this table, I currently have a primary key (on id) and a unique index on (userid,name). For one particularly heavy select query, I need an index on (userid,name,valueclass).

I could create a new index or add the valueclass column to the unique index I already have. As far as performance of the select query is concerned, it doesn't matter. There also doesn't seem to be any significant difference in disk space or memory usage. Mutations to the table would be a bit slower, but it's essentially write-once-read-many.

Are there any good reasons why I should NOT just add a column to the unique index I already have?

The select query has some joins as such:

SELECT      `name`
        ,   AVG(`valueclass`) AS `avg`
        ,   STD(`valueclass`) AS `std`
FROM        `user_metrics`
WHERE       `valueclass`    IS NOT NULL
    AND     `userid`    IN ([list of userids])
    AND     `name` IN ([list of names we want])
GROUP BY    `name`';


Basically, it tries to get some statistical information and only needs the three columns (so the entire query could be run on the index).
For one particularly heavy select query, I need an index on
  (userid,name,valueclass).


This is a good reason to add an index. It would also be good to examine your query plan. BTW, underscores between words make variables more legible. 


  Are there any good reasons why I should NOT just add a column to the
  unique index I already have?


A unique index enforces uniqueness. If you have a need for the two column uniqueness then most likely adding a third column to that will compromise the integrity of your constraint. To assist your heavy select query the best option would be to add an index to (userid,name,valueclass) without the unique constraint.