Avoid a few more SET DATA TYPE table rewrites.

When the new type is an unconstrained domain over the old type, we don't
need to rewrite the table.

Noah Misch and Robert Haas

doc/src/sgml/ref/alter_table.sgml

@@ -766,13 +766,14 @@ ALTER TABLE <replaceable class="PARAMETER">name</replaceable>
    <para>
     Adding a column with a non-null default or changing the type of an
     existing column will require the entire table and indexes to be rewritten.
-    As an exception, if the old type type is binary coercible to the new
-    type and the <literal>USING</> clause does not change the column contents,
-    a table rewrite is not needed, but any indexes on the affected columns 
-    must still be rebuilt.  Adding or removing a system <literal>oid</> column
-    also requires rewriting the entire table.  Table and/or index rebuilds may
-    take a significant amount of time for a large table; and will temporarily
-    require as much as double the disk space.
+    As an exception, if the <literal>USING</> clause does not change the column
+    contents and the old type is either binary coercible to the new type or
+    an unconstrained domain over the new type, a table rewrite is not needed,
+    but any indexes on the affected columns must still be rebuilt.  Adding or
+    removing a system <literal>oid</> column also requires rewriting the entire
+    table.  Table and/or index rebuilds may take a significant amount of time
+    for a large table; and will temporarily require as much as double the disk
+    space.
    </para>
 
    <para>

src/backend/commands/tablecmds.c

@@ -6632,10 +6632,14 @@ ATPrepAlterColumnType(List **wqueue,
 }
 
 /*
- * When the data type of a column is changed, a rewrite might not be require
- * if the data type is being changed to its current type, or more interestingly
- * to a type to which it is binary coercible.  But we must check carefully that
- * the USING clause isn't trying to insert some other value.
+ * When the data type of a column is changed, a rewrite might not be required
+ * if the new type is sufficiently identical to the old one, and the USING
+ * clause isn't trying to insert some other value.  It's safe to skip the
+ * rewrite if the old type is binary coercible to the new type, or if the
+ * new type is an unconstrained domain over the old type.  In the case of a
+ * constrained domain, we could get by with scanning the table and checking
+ * the constraint rather than actually rewriting it, but we don't currently
+ * try to do that.
  */
 static bool
 ATColumnChangeRequiresRewrite(Node *expr, AttrNumber varattno)
@@ -6649,6 +6653,14 @@ ATColumnChangeRequiresRewrite(Node *expr, AttrNumber varattno)
 			return false;
 		else if (IsA(expr, RelabelType))
 			expr = (Node *) ((RelabelType *) expr)->arg;
+		else if (IsA(expr, CoerceToDomain))
+		{
+			CoerceToDomain *d = (CoerceToDomain *) expr;
+
+			if (GetDomainConstraints(d->resulttype) != NIL)
+				return true;
+			expr = (Node *) d->arg;
+		}
 		else
 			return true;
 	}