Question

As far as indexing goes, is it proper to index all fields that will be searched (within a WHERE) clause to speed up SELECTS? For example my database contains a profiles table which stores user information such as name, intrestCode,zip, description, and email. The profile record is identified by a PRIMARY id column which uniquely corresponds with the userid. I made zip and intrestCode an index since profiles will be searched by zip and possibly intrestCode (SELECT `blah`,`blah`... FROM profile WHERE zip=?, SELECT `blah`,`blah`... FROM profile WHERE zip=? && intrestCode=?). Am I doing it right?

Answer 1

Sounds basically correct. You should be aware that you can’t use two different indices on the same table in the same query, so if you ran

CREATE INDEX `zip` ON `profile` (`zip`);
CREATE INDEX `intrestCode` ON `profile` (`intrestCode`);

then the query

SELECT `blah`,`blah`... FROM profile WHERE zip=? && intrestCode=?

can only look up one table from the index. The secret here is that you can create a single index on two tables, like so:

CREATE INDEX `zip+intrestCode` ON `profile` (`zip`, `intrestCode`);

MySQL can use this for queries that use either zip alone in the WHERE clause, or use both zip and intrestCode, but not for queries that use only intrestCode in the WHERE clause.

(This is because each index covers the whole table. If MySQL were to try and look up zip and intrestCode from different tables, then it would be retrieving lots of irrelevant rows from the second index. Therefore, it only looks at one index. If you want it to use the index on both columns, you need to have one index that includes both columns.)