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

No, usually not.

You usually index fields for searching when they have high selectivity/cardinality. A boolean field's cardinality is very low in most tables. It would also make your writes fractionally slower.