Multiple and single indexes

Question

I\'m kinda ashamed of asking this since I\'ve been working with MySQL for years, but oh well.

I have a table with two fields, a and b. I will b

Answer 1

It's very improbable that mere existence of an index slow down a SELECT query: it just won't be used.

In theory the optimizer can incorrectly choose more long index on (a, b) rather than one on (a) to serve the query which searches only for a.

In practice, I've never seen it: MySQL usually does the opposite mistake, taking a shorter index when a longer one exists.

Update:

In your case, either of the following configurations will suffice for all queries:

(a, b); (b)

or

(b, a); (a)

MySQL can also use two separate indexes with index_intersect, so creating these indexes

(a); (b)

will also speed up the query with a = 1 AND b = 1, though to a lesser extent than any of the solutions above.

You may also want to read this article in my blog:

• Creating indexes

Update 2:

Seems I finally understood your question :)

ALTER TABLE ... ADD INDEX (a); ALTER TABLE ... ADD INDEX (b);

Excellent for a = 1 and b = 1, reasonably good for a = 1 AND b = 1

ALTER TABLE ... ADD INDEX (a, b);

Excellent for a = 1 AND b = 1, almost excellent for a = 1, poor for b = 1

ALTER TABLE ... ADD INDEX (a); ALTER TABLE ... ADD INDEX (b); ALTER TABLE ... ADD INDEX (a, b);

Excellent for all three queries.