> So we have to explicitly close the underlying tables, right?

Yes, that is what I meant to say. If you want the source tables to be closed after the query then you have to do it yourself, and the easiest way of doing so without affecting other modules is to use the tables under temporary aliases because then it is known that no other module could conceivably be using those aliases.

Another positive side effect is that you can explicitly specify the manner in which those tables are opened (SHARED, NOUPDATE) whereas the SQL commands will use ambient settings like SET EXCLUSIVE for this.