MySQL indices and order

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Index order matters when your query conditions only apply to PART of the index. Consider:

  1. SELECT * FROM table WHERE first_name="john" AND last_name="doe"

  2. SELECT * FROM table WHERE first_name="john"

  3. SELECT * FROM table WHERE last_name="doe"

If your index is (first_name, last_name) queries 1 and 2 will use it, query #3 won’t.
If your index is (last_name, first_name) queries 1 and 3 will use it, query #2 won’t. Changing the condition order within WHERE clause has no effect in either case.

Details are here

Update:
In case the above is not clear – MySQL can only use an index if the columns in query conditions form a leftmost prefix of the index. Query #2 above can not use (last_name, first_name) index because it’s only based on first_name and first_name is NOT the leftmost prefix of the (last_name, first_name) index.

The order of conditions WITHIN the query does not matter; query #1 above will be able to use (last_name, first_name) index just fine because its conditions are first_name and last_name and, taken together, they DO form a leftmost prefix of (last_name, first_name) index.

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