Improve estimate of distinct values in estimate_num_groups().

When adjusting the estimate for the number of distinct values from a
rel in a grouped query to take into account the selectivity of the
rel's restrictions, use a formula that is less likely to produce
under-estimates.

The old formula simply multiplied the number of distinct values in the
rel by the restriction selectivity, which would be correct if the
restrictions were fully correlated with the grouping expressions, but
can produce significant under-estimates in cases where they are not
well correlated.

The new formula is based on the random selection probability, and so
assumes that the restrictions are not correlated with the grouping
expressions. This is guaranteed to produce larger estimates, and of
course risks over-estimating in cases where the restrictions are
correlated, but that has less severe consequences than
under-estimating, which might lead to a HashAgg that consumes an
excessive amount of memory.

This could possibly be improved upon in the future by identifying
correlated restrictions and using a hybrid of the old and new
formulae.

Author: Tomas Vondra, with some hacking be me
Reviewed-by: Mark Dilger, Alexander Korotkov, Dean Rasheed and Tom Lane
Discussion: http://www.postgresql.org/message-id/flat/56CD0381.5060502@2ndquadrant.com

src/backend/utils/adt/selfuncs.c

@@ -3237,15 +3237,15 @@ add_unique_group_var(PlannerInfo *root, List *varinfos,
  *		restriction selectivity of the equality in the next step.
  *	4.  For Vars within a single source rel, we multiply together the numbers
  *		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.
+ *		more than one Var), and then multiply by a factor based on 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.)  The factor
+ *		we multiply by to adjust for the restriction selectivity assumes that
+ *		the restriction clauses are independent of the grouping, which may not
+ *		be a valid assumption, but it's hard to do better.
  *	5.  If there are Vars from multiple rels, we repeat step 4 for each such
  *		rel, and multiply the results together.
  * Note that rels not containing grouped Vars are ignored completely, as are
@@ -3439,9 +3439,51 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows,
 				reldistinct = clamp;
 
 			/*
-			 * Multiply by restriction selectivity.
+			 * Update the estimate based on the restriction selectivity,
+			 * guarding against division by zero when reldistinct is zero.
+			 * Also skip this if we know that we are returning all rows.
 			 */
-			reldistinct *= rel->rows / rel->tuples;
+			if (reldistinct > 0 && rel->rows < rel->tuples)
+			{
+				/*
+				 * Given a table containing N rows with n distinct values in a
+				 * uniform distribution, if we select p rows at random then
+				 * the expected number of distinct values selected is
+				 *
+				 * n * (1 - product((N-N/n-i)/(N-i), i=0..p-1))
+				 *
+				 * = n * (1 - (N-N/n)! / (N-N/n-p)! * (N-p)! / N!)
+				 *
+				 * See "Approximating block accesses in database
+				 * organizations", S. B. Yao, Communications of the ACM,
+				 * Volume 20 Issue 4, April 1977 Pages 260-261.
+				 *
+				 * Alternatively, re-arranging the terms from the factorials,
+				 * this may be written as
+				 *
+				 * n * (1 - product((N-p-i)/(N-i), i=0..N/n-1))
+				 *
+				 * This form of the formula is more efficient to compute in
+				 * the common case where p is larger than N/n.  Additionally,
+				 * as pointed out by Dell'Era, if i << N for all terms in the
+				 * product, it can be approximated by
+				 *
+				 * n * (1 - ((N-p)/N)^(N/n))
+				 *
+				 * See "Expected distinct values when selecting from a bag
+				 * without replacement", Alberto Dell'Era,
+				 * http://www.adellera.it/investigations/distinct_balls/.
+				 *
+				 * The condition i << N is equivalent to n >> 1, so this is a
+				 * good approximation when the number of distinct values in
+				 * the table is large.  It turns out that this formula also
+				 * works well even when n is small.
+				 */
+				reldistinct *=
+					(1 - pow((rel->tuples - rel->rows) / rel->tuples,
+							 rel->tuples / reldistinct));
+			}
+			reldistinct = clamp_row_est(reldistinct);
 
 			/*
 			 * Update estimate of total distinct groups.

src/test/regress/expected/subselect.out

@@ -808,17 +808,14 @@ explain (verbose, costs off)
 select * from int4_tbl o where (f1, f1) in
   (select f1, generate_series(1,2) / 10 g from int4_tbl i group by f1);
                            QUERY PLAN                           
-----------------------------------------------------------------------
- Hash Join
+----------------------------------------------------------------
+ Hash Semi Join
    Output: o.f1
    Hash Cond: (o.f1 = "ANY_subquery".f1)
    ->  Seq Scan on public.int4_tbl o
          Output: o.f1
    ->  Hash
          Output: "ANY_subquery".f1, "ANY_subquery".g
-         ->  HashAggregate
-               Output: "ANY_subquery".f1, "ANY_subquery".g
-               Group Key: "ANY_subquery".f1, "ANY_subquery".g
          ->  Subquery Scan on "ANY_subquery"
                Output: "ANY_subquery".f1, "ANY_subquery".g
                Filter: ("ANY_subquery".f1 = "ANY_subquery".g)
@@ -827,7 +824,7 @@ select * from int4_tbl o where (f1, f1) in
                      Group Key: i.f1
                      ->  Seq Scan on public.int4_tbl i
                            Output: i.f1
-(18 rows)
+(15 rows)
 
 select * from int4_tbl o where (f1, f1) in
   (select f1, generate_series(1,2) / 10 g from int4_tbl i group by f1);