A little further tweaking of the range-query selectivity logic:

to avoid undue sensitivity to roundoff error, believe that a zero
or slightly negative range estimate should represent a small
positive selectivity, rather than falling back on a generic default
estimate.

diff --git a/src/backend/optimizer/path/clausesel.c b/src/backend/optimizer/path/clausesel.c
index baa99a7..985155e 100644 (file)
--- a/src/backend/optimizer/path/clausesel.c
+++ b/src/backend/optimizer/path/clausesel.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *       $Header: /cvsroot/pgsql/src/backend/optimizer/path/clausesel.c,v 1.32 2000/03/23 00:58:36 tgl Exp $
+ *       $Header: /cvsroot/pgsql/src/backend/optimizer/path/clausesel.c,v 1.33 2000/03/23 23:35:47 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -186,20 +186,32 @@ clauselist_selectivity(Query *root,
                        /* Successfully matched a pair of range clauses */
                        Selectivity     s2 = rqlist->hibound + rqlist->lobound - 1.0;
 
-                       if (s2 > 0.0)
-                       {
-                               /* All our hard work has paid off! */
-                               s1 *= s2;
-                       }
-                       else
+                       /*
+                        * A zero or slightly negative s2 should be converted into a
+                        * small positive value; we probably are dealing with a very
+                        * tight range and got a bogus result due to roundoff errors.
+                        * However, if s2 is very negative, then we probably have
+                        * default selectivity estimates on one or both sides of the
+                        * range.  In that case, insert a not-so-wildly-optimistic
+                        * default estimate.
+                        */
+                       if (s2 <= 0.0)
                        {
-                               /* One or both is probably a default estimate,
-                                * so supply a default estimate for the selectivity
-                                * of the range query.  We rather optimistically assume
-                                * that the range is tight...
-                                */
-                               s1 *= 0.01;
+                               if (s2 < -0.01)
+                               {
+                                       /* No data available --- use a default estimate that
+                                        * is small, but not real small.
+                                        */
+                                       s2 = 0.01;
+                               }
+                               else
+                               {
+                                       /* It's just roundoff error; use a small positive value */
+                                       s2 = 1.0e-10;
+                               }
                        }
+                       /* Merge in the selectivity of the pair of clauses */
+                       s1 *= s2;
                }
                else
                {