Is there any performance gain in indexing a boolean field?

Question

I\'m just about to write a query that includes a WHERE isok=1. As the name implies, isok is a boolean field (actually a TINYINT(1) UNSIGNED

Answer 1

Yes an index will improve performance, check the output of EXPLAIN with and without the index.

From the docs:

Indexes are used to find rows with specific column values quickly. Without an index, MySQL must begin with the first row and then read through the entire table to find the relevant rows. The larger the table, the more this costs. If the table has an index for the columns in question, MySQL can quickly determine the position to seek to in the middle of the data file without having to look at all the data.

I think it's also safe to say an index will not DECREASE performance in this case, so you have only to gain from it.