Clarify policy on marking inherited constraints as valid.

Amit Langote and Robert Haas

doc/src/sgml/ref/alter_table.sgml

@@ -1028,11 +1028,15 @@ ALTER TABLE ALL IN TABLESPACE <replaceable class="PARAMETER">name</replaceable>
 
    <para>
     If a table has any descendant tables, it is not permitted to add,
-    rename, or change the type of a column, or rename an inherited constraint
-    in the parent table without doing
-    the same to the descendants.  That is, <command>ALTER TABLE ONLY</command>
-    will be rejected.  This ensures that the descendants always have
-    columns matching the parent.
+    rename, or change the type of a column in the parent table without doing
+    same to the descendants.  This ensures that the descendants always have
+    columns matching the parent.  Similarly, a constraint cannot be renamed
+    in the parent without also renaming it in all descendents, so that
+    constraints also match between the parent and its descendents.
+    Also, because selecting from the parent also selects from its descendents,
+    a constraint on the parent cannot be marked valid unless it is also marked
+    valid for those descendents.  In all of these cases, <command>ALTER TABLE
+    ONLY</command> will be rejected.  
    </para>
 
    <para>

src/backend/commands/tablecmds.c

@@ -6908,7 +6908,8 @@ ATExecValidateConstraint(Relation rel, char *constrName, bool recurse,
 
 				/*
 				 * If we are told not to recurse, there had better not be any
-				 * child tables; else the addition would put them out of step.
+				 * child tables, because we can't mark the constraint on the
+				 * parent valid unless it is valid for all child tables.
 				 */
 				if (!recurse)
 					ereport(ERROR,