Possibly we should make ALTER COLUMN strip any implicit coercions that appear at the top level of the default expression before it adds on the implicit coercion to the new column datatype.
That seems like a kludge. When processing a column default expression, we:
(1) Accept the default's raw parsetree from the parser (2) Convert it to a cooked parsetree via transformExpr() (3) Add a coercion to the table's column type
Can't we save the cooked parsetree that we produced in #2? That would mean we can just reuse the cooked parsetree (w/o the coercion) and add a coercion to the correct column type @ ALTER TABLE time.
-Neil
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