suggested fix for boolean selectivity

gistfile1.diff

diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index da66f34..4e8f3f2 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -1549,11 +1549,17 @@ booltestsel(PlannerInfo *root, BoolTestType booltesttype, Node *arg,
                                        selec = 1.0 - freq_null;
                                        break;
                                case IS_TRUE:
-                               case IS_NOT_TRUE:
                                case IS_FALSE:
-                               case IS_NOT_FALSE:
                                        selec = (1.0 - freq_null) / 2.0;
                                        break;
+                               case IS_NOT_TRUE:
+                               case IS_NOT_FALSE:
+                                       /*
+                                        * include the null values plus half of the non-null ones.
+                                        * equiv. to  freq_null + (1.0 - freq_null)/2.0
+                                        */
+                                       selec = (freq_null + 1.0) / 2.0;
+                                       break;
                                default:
                                        elog(ERROR, "unrecognized booltesttype: %d",
                                                 (int) booltesttype);