As interesting as this is, I can see why original SQL "designers" didn't go down this route and instead have a separate DDL. SQL and particularly postgres' doc

As interesting as this is, I can see why original SQL "designers" didn't go down this route and instead have a separate DDL. SQL and particularly postgres' documentation go to great lengths to be very specific about what micro-behaviour you should expect when performing SELECTs, INSERTs, UPDATEs and DELETEs (Of major note the concurrency behaviour). Once you start operating on the schema all of these promises will go out the window. So these "queries" would mostly behave as you'd expect, but not totally. At best you'd have to fill the documentation with "oh except if you're operating on the schema..."s. At worst it could be horribly confusing to users.