Add drop expression support

schemainspect/inspected.py

@@ -52,6 +52,7 @@ def __init__(
         is_identity_always=False,
         is_generated=False,
         is_inherited=False,
+        can_drop_generated=False,
     ):
         self.name = name or ""
         self.dbtype = dbtype
@@ -66,6 +67,7 @@ def __init__(
         self.is_identity_always = is_identity_always
         self.is_generated = is_generated
         self.is_inherited = is_inherited
+        self.can_drop_generated = can_drop_generated
 
     def __eq__(self, other):
         return (
@@ -109,7 +111,7 @@ def alter_clauses(self, other):
         )
 
         if default_changed:
-            clauses.append(self.alter_default_clause)
+            clauses.append(self.alter_default_clause_or_generated(other))
 
         if notnull_added:
             clauses.append(self.alter_not_null_clause)
@@ -218,6 +220,17 @@ def alter_default_clause(self):
             alter = "alter column {} drop default".format(self.quoted_name)
         return alter
 
+    def alter_default_clause_or_generated(self, other):
+        if self.default:
+            alter = "alter column {} set default {}".format(
+                self.quoted_name, self.default
+            )
+        elif other.is_generated and not self.is_generated:
+            alter = "alter column {} drop expression".format(self.quoted_name)
+        else:
+            alter = "alter column {} drop default".format(self.quoted_name)
+        return alter
+
     def alter_identity_clause(self, other):
         if self.is_identity:
             identity_type = "always" if self.is_identity_always else "by default"

schemainspect/pg/obj.py

@@ -1418,6 +1418,7 @@ def get_enum(name, schema):
                     is_identity=c.is_identity,
                     is_identity_always=c.is_identity_always,
                     is_generated=c.is_generated,
+                    can_drop_generated=self.pg_version >= 13,
                 )
                 for c in clist
                 if c.position_number