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 baa99a7d28b9fc18671863c084e4f4b917384001..985155edf9201b6159461f215a1c610f5a6fa3b9 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
        {