I just noticed that if you execute the same DO command over and over
within a session, it gets slower and slower. And if you keep it up
you'll notice the backend's RAM consumption bloating too. The cause
appears to be that we leak the cached plans created for any SQL
statements or expressions within the DO command --- the next iteration
won't reuse those, but rather create its own set. Probably ought to
look into releasing those when the DO block is over.
regards, tom lane