I had a thought about this: I don't think EXPLAIN is ever required to print the names of join alias variables (since the planner flattens all join alias variables to some kind of expression over their underlying columns). So we could skip assigning column names to join RTEs at all, if we know that it's EXPLAIN rather than view/rule decompilation. That might let us skip all the mess around unique-ifying JOIN USING column names, too.
regards, tom lane
