Indexing boolean fields

No.

You index fields that are searched upon and have high selectivity/cardinality. A boolean field's cardinality is obliterated in nearly any table. If anything it will make your writes slower (by an oh so tiny amount).

Maybe you would make it the first field in the clustered index if every query took into account soft deletes?

What is about a deleted_at DATETIME column? There are two benefits.

1. If you need an unique column like name, you can create and soft-delete a record with the same name multiple times (if you use an unique index on the columns deleted_at AND name)
2. You can search for recently deleted records.

You query could look like this:

SELECT * FROM xyz WHERE deleted_at IS NULL

I think it would help, especially in covering indices.

How much/little is of course dependent on your data and queries.

You can have theories of all sorts about indices but final answers are given by the database engine in a database with real data. And often you are surprised by the answer (or maybe my theories are too bad ;)

Examine the query plan of your queries and determine if the queries can be improved, or if the indices can be improved.It's quite simple to alter indices and see what difference it makes