Some good advice here, but the “don’t index boolean columns” needs an “it depends” caveat, since Postgres will sometime use multiple boolean indexes to perform a bitmap index scan, which can be advantageous.
CREATE INDEX index_accounts_balance ON accounts (id) WHERE boolean_flag;
The purpose of the index often is to lower the “cost” (greater profit), by providing more values the query is accessing (filtering, ordering, selecting) within the index entries, for faster retrieval.
When users have the skills to generate query plans and review whether an index supports a query, verifying that the index is picked by the planner, and indeed lowers the cost, then they can answer this question for their own unique combination of hardware, data distribution, queries, and indexes.
As generic advice, I think more often than not the index won’t be used for Boolean columns. But it’s generic advice and it does depend.
As you suggested, users must check their own system.
My book also covers the 2 and 3 value variations (nulls allowed) for Boolean columns.
For readers wanting to build these skills themselves, here’s more info:
https://news.ycombinator.com/item?id=38407585
Good nudge that it depends!