When dealing with multiple grouping columns coming from the same table,

clamp the estimated number of groups to table row count over 10, instead
of table row count; this reflects a heuristic that people probably won't
group over a near-unique set of columns, and the knowledge that we don't
currently have any way to estimate the correlation of the columns better
than guessing.  This change creates a trivial plan change in one of the
regression tests.

diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index c556f4bc6ec2f22fae3df09296a8521ca3bbb2ce..7b020ff4895141b3f6f0268382799cfbff9a6cfc 100644 (file)
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -15,7 +15,7 @@
  *
  *
  * IDENTIFICATION
- *   $PostgreSQL: pgsql/src/backend/utils/adt/selfuncs.c,v 1.169 2004/12/31 22:01:22 pgsql Exp $
+ *   $PostgreSQL: pgsql/src/backend/utils/adt/selfuncs.c,v 1.170 2005/01/28 20:34:25 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1937,10 +1937,13 @@ add_unique_group_var(Query *root, List *varinfos,
  *     if we considered ones of the same rel, we'd be double-counting the
  *     restriction selectivity of the equality in the next step.
  * 3.  For Vars within a single source rel, we multiply together the numbers
- *     of values, clamp to the number of rows in the rel, and then multiply
- *     by the selectivity of the restriction clauses for that rel.  The
- *     initial product is probably too high (it's the worst case) but since
- *     we can clamp to the rel's rows it won't be hugely bad.  Multiplying
+ *     of values, clamp to the number of rows in the rel (divided by 10 if
+ *     more than one Var), and then multiply by the selectivity of the
+ *     restriction clauses for that rel.  When there's more than one Var,
+ *     the initial product is probably too high (it's the worst case) but
+ *     clamping to a fraction of the rel's rows seems to be a helpful
+ *     heuristic for not letting the estimate get out of hand.  (The factor
+ *     of 10 is derived from pre-Postgres-7.4 practice.)  Multiplying
  *     by the restriction selectivity is effectively assuming that the
  *     restriction clauses are independent of the grouping, which is a crummy
  *     assumption, but it's hard to do better.
@@ -2040,6 +2043,7 @@ estimate_num_groups(Query *root, List *groupExprs, double input_rows)
        GroupVarInfo *varinfo1 = (GroupVarInfo *) linitial(varinfos);
        RelOptInfo *rel = varinfo1->rel;
        double      reldistinct = varinfo1->ndistinct;
+       int         relvarcount = 1;
        List       *newvarinfos = NIL;
 
        /*
@@ -2051,7 +2055,10 @@ estimate_num_groups(Query *root, List *groupExprs, double input_rows)
            GroupVarInfo *varinfo2 = (GroupVarInfo *) lfirst(l);
 
            if (varinfo2->rel == varinfo1->rel)
+           {
                reldistinct *= varinfo2->ndistinct;
+               relvarcount++;
+           }
            else
            {
                /* not time to process varinfo2 yet */
@@ -2066,10 +2073,20 @@ estimate_num_groups(Query *root, List *groupExprs, double input_rows)
        if (rel->tuples > 0)
        {
            /*
-            * Clamp to size of rel, multiply by restriction selectivity.
+            * Clamp to size of rel, or size of rel / 10 if multiple Vars.
+            * The fudge factor is because the Vars are probably correlated
+            * but we don't know by how much.
+            */
+           double      clamp = rel->tuples;
+
+           if (relvarcount > 1)
+               clamp *= 0.1;
+           if (reldistinct > clamp)
+               reldistinct = clamp;
+
+           /*
+            * Multiply by restriction selectivity.
             */
-           if (reldistinct > rel->tuples)
-               reldistinct = rel->tuples;
            reldistinct *= rel->rows / rel->tuples;
 
            /*

diff --git a/src/test/regress/expected/subselect.out b/src/test/regress/expected/subselect.out
index 07e727de482594b5d8d2e102834c29a126a3f630..56bea0359815ff4598052e9f234967b711d53858 100644 (file)
--- a/src/test/regress/expected/subselect.out
+++ b/src/test/regress/expected/subselect.out
@@ -134,11 +134,11 @@ SELECT '' AS five, f1 AS "Correlated Field"
                      WHERE f3 IS NOT NULL);
  five | Correlated Field 
 ------+------------------
-      |                2
       |                3
       |                1
-      |                2
       |                3
+      |                2
+      |                2
 (5 rows)
 
 --