Postgres: Are secondary indexes included in ACID?
Are non-unique indexes / indices covered under the Consistency clause in aCid?


Yes. Any violation of that would be considered a bug in PostgreSQL.

The docs you quoted are cases where postgres might have to temporarily scan the heap instead of doing an index-only scan or otherwise do extra work to get a consistent result.

For example, both BRIN and GIN indexes accumulate batches of pending changes and then do batch updates. When the indexes are used in queries this queue is also scanned to make sure that a current, up-to-date and consistent view is seen.

If an index is currently invalid it'll be skipped by the planner and won't be used by queries.

Redshift isn't really PostgreSQL, it just happens to share the same front-end and protocol. Drawing comparisons based on Redshift will typically just create confusion.