Instead of trying to force WHERE clauses into CNF or DNF normal form,

just look for common clauses that can be pulled out of ORs.  Per recent
discussion, extracting common clauses seems to be the only really useful
effect of normalization, and if we do it explicitly then we can avoid
cluttering the qual with partially-redundant duplicated expressions, which
was an unpleasant side-effect of the old approach.

src/backend/optimizer/README

@@ -200,15 +200,11 @@ planner()
  do final cleanup after planning.
 -subquery_planner()
  pull up subqueries from rangetable, if possible
- simplify constant expressions
  canonicalize qual
-     Attempt to reduce WHERE clause to either CNF or DNF canonical form.
-     CNF (top-level-AND) is preferred, since the optimizer can then use
-     any of the AND subclauses to filter tuples; but quals that are in
-     or close to DNF form will suffer exponential expansion if we try to
-     force them to CNF.  In pathological cases either transform may expand
-     the qual unreasonably; so we may have to leave it un-normalized,
-     thereby reducing the accuracy of selectivity estimates.
+     Attempt to simplify WHERE clause to the most useful form; this includes
+     flattening nested AND/ORs and detecting clauses that are duplicated in
+     different branches of an OR.
+ simplify constant expressions
  process sublinks
  convert Vars of outer query levels into Params
 --grouping_planner()