To clarify: REINDEX CONCURRENTLY is a new feature. Previously there were only DROP INDEX CONCURRENTLY and CREATE INDEX CONCURRENTLY, which meant that while you could first create a new index and then drop the old one renaming still required an exclusive lock (this is also fixed in 12), as did switching over all foreign keys to the new index. REINDEX CONCURRENTLY essentially does all this work for you in a single command while also making sure no reader or writer is locked out from the table.