Fix thinko in hash cost estimation: average frequency

should be computed from total number of distinct values in whole
relation, not # distinct values we expect to have after restriction
clauses are applied.

diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index 65c211deaeeb3326d3df5cd9bc98681f6c1e5be1..06793f1d8b4133d419125e2a307df07f8aa623d3 100644 (file)
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -42,7 +42,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *       $Header: /cvsroot/pgsql/src/backend/optimizer/path/costsize.c,v 1.75 2001/06/05 05:26:04 tgl Exp $
+ *       $Header: /cvsroot/pgsql/src/backend/optimizer/path/costsize.c,v 1.76 2001/06/10 02:59:35 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -873,6 +873,9 @@ estimate_hash_bucketsize(Query *root, Var *var)
        if (ndistinct < 0.0)
                ndistinct = -ndistinct * rel->tuples;
 
+       /* Also compute avg freq of all distinct data values in raw relation */
+       avgfreq = (1.0 - stats->stanullfrac) / ndistinct;
+
        /*
         * Adjust ndistinct to account for restriction clauses.  Observe we are
         * assuming that the data distribution is affected uniformly by the
@@ -883,17 +886,6 @@ estimate_hash_bucketsize(Query *root, Var *var)
         */
        ndistinct *= rel->rows / rel->tuples;
 
-       /*
-        * Discourage use of hash join if there seem not to be very many distinct
-        * data values.  The threshold here is somewhat arbitrary, as is the
-        * fraction used to "discourage" the choice.
-        */
-       if (ndistinct < 50.0)
-       {
-               ReleaseSysCache(tuple);
-               return 0.5;
-       }
-
        /*
         * Form initial estimate of bucketsize fraction.  Here we use rel->rows,
         * ie the number of rows after applying restriction clauses, because
@@ -903,8 +895,8 @@ estimate_hash_bucketsize(Query *root, Var *var)
        estfract = (double) NTUP_PER_BUCKET / rel->rows;
 
        /*
-        * Adjust estimated bucketsize if too few distinct values to fill
-        * all the buckets.
+        * Adjust estimated bucketsize if too few distinct values (after
+        * restriction clauses) to fill all the buckets.
         */
        needdistinct = rel->rows / (double) NTUP_PER_BUCKET;
        if (ndistinct < needdistinct)
@@ -931,8 +923,6 @@ estimate_hash_bucketsize(Query *root, Var *var)
        /*
         * Adjust estimated bucketsize upward to account for skewed distribution.
         */
-       avgfreq = (1.0 - stats->stanullfrac) / ndistinct;
-
        if (avgfreq > 0.0 && mcvfreq > avgfreq)
                estfract *= mcvfreq / avgfreq;