Non-concurrent indexing locks the table until it's done creating the index. If it's a large table, it locks it for a long time. This makes things trying to use

Non-concurrent indexing locks the table until it's done creating the index. If it's a large table, it locks it for a long time. This makes things trying to use that table time out. This is a non-issue on things that don't have real traffic going to them, or on small enough tables that the indexing time is trivial.