Update TODO list.

Author: bmomjian

doc/TODO

@@ -43,6 +43,7 @@ PARSER
 * SELECT ... UNION ... GROUP BY fails if column types disagree
 * redesign INSERT ... SELECT to have two levels of target list
 * -select * from pg_class where oid in (0,-1)
+* have INTERSECT/EXCEPT prevent duplicates unless ALL is specified
 
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