Alvaro Herrera <alvherre@xxxxxxxxxxxxxxxxx> writes: > Ow Mun Heng wrote: >> If it were this simple a change, I'm not certain why (I believe) PG is >> checking each and every row to see if it will fit into the new column >> definition/type. > Because the code that does the ALTER TYPE is very generic, and it > doesn't (yet) have an optimization that tells it to skip the check and > the possible table rewrite in the cases where it's obviously not needed > (like this one). Awhile back I looked into teaching ALTER TYPE that it needn't rewrite if the type conversion expression parses out as just a Var with RelabelType, but it seemed that that wouldn't cover very much of the use-cases where a human thinks that it's "obvious" that no rewrite is needed. You'd really need to build in hard-wired knowledge about the behavior of specific coercion functions, which seems entirely unappealing. regards, tom lane
