Question

I’ve noticed that in PHPMyAdmin I can individually index columns or I can use checkboxes to select fields and then click index and they’re indexed in a different way. Does this mean that if for a given table I have 2 columns of that table that define each row as unique (instead of just a simple single column id`) I should index those together to increase performance?

Answer 1

A multiple-column index can be considered a sorted array containing values that are created by concatenating the values of the indexed columns.

MySQL uses multiple-column indexes in such a way that queries are fast when you specify a known quantity for the first column of the index in a WHERE clause, even if you do not specify values for the other columns.

If you have two columns named last_name and first_name and you create an index INDEX name (last_name,first_name), The index can be used for queries that specify values in a known range for last_name, or for both last_name and first_name.

Source: http://dev.mysql.com/doc/refman/5.0/en/multiple-column-indexes.html

So, it may not be helpful in your particular case. Becuase if you want to query on the later columns (for example: SELECT * FROM test WHERE first_name='Michael' or SELECT * FROM test WHERE last_name='Widenius' OR first_name='Michael), the index will not be used and the queries will be slower.