From dd8b4392085d27104f159bbb9df13070a28c6d8d Mon Sep 17 00:00:00 2001
From: David Rowley <dgrowley@gmail.com>
Date: Thu, 18 May 2017 20:10:31 +1200
Subject: [PATCH 2/2] Apply nullfrac during foreign key join estimations

---
 src/backend/optimizer/path/costsize.c | 33 ++++++++++++++++++++++++++++++---
 src/backend/utils/cache/lsyscache.c   | 24 ++++++++++++++++++++++++
 src/include/utils/lsyscache.h         |  1 +
 3 files changed, 55 insertions(+), 3 deletions(-)

diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index d7bdd41..8e14614 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -4317,11 +4317,14 @@ get_foreign_key_join_selectivity(PlannerInfo *root,
 	{
 		ForeignKeyOptInfo *fkinfo = (ForeignKeyOptInfo *) lfirst(lc);
 		RelOptInfo *ref_rel;
+		RangeTblEntry *con_rte;
 		bool		ref_is_outer;
 		List	   *removedlist;
 		ListCell   *cell;
 		ListCell   *prev;
 		ListCell   *next;
+		Selectivity nullfrac;
+		int			i;
 
 		/*
 		 * This FK is not relevant unless it connects a baserel on one side of
@@ -4351,7 +4354,6 @@ get_foreign_key_join_selectivity(PlannerInfo *root,
 		{
 			RestrictInfo *rinfo = (RestrictInfo *) lfirst(cell);
 			bool		remove_it = false;
-			int			i;
 
 			next = lnext(cell);
 			/* Drop this clause if it matches any column of the FK */
@@ -4450,6 +4452,30 @@ get_foreign_key_join_selectivity(PlannerInfo *root,
 		 */
 
 		ref_rel = find_base_rel(root, fkinfo->ref_relid);
+		con_rte = planner_rt_fetch(fkinfo->con_relid, root);
+
+		/*
+		 * Any rows on the referencing side of the foreign key which have
+		 * NULL values in the foreign key columns won't be joined to. Here
+		 * we'll estimate how many that's likely to be. Note that we take the
+		 * maximum nullfrac out of each column in the foreign key.  It's not
+		 * certain that this is correct, as, in the situation where a foreign
+		 * key is made up of say, two columns, one column may have 50% of rows
+		 * with a NULL value and the other column may have the other 50% of
+		 * rows with a NULL value, which would mean no rows would be joined
+		 * to, but here we'd estimate 50% or rows would be joined to, which
+		 * is of course, wrong, but it seems that finding the maximum nullfrac
+		 * column to have a higher chance of being correct. We may want to
+		 * revisit this and use the average of the sum (capped at 1.0) and
+		 * the max.
+		 */
+		nullfrac = 0.0;
+		for (i = 0; i < fkinfo->nkeys; i++)
+		{
+			float4 attnullfrac = get_attnullfrac(con_rte->relid,
+												 fkinfo->conkey[i]);
+			nullfrac = Max(nullfrac, attnullfrac);
+		}
 
 		if (!ref_is_outer && (jointype == JOIN_SEMI || jointype == JOIN_ANTI))
 		{
@@ -4466,7 +4492,8 @@ get_foreign_key_join_selectivity(PlannerInfo *root,
 			 * we'd better just handle this case properly in case it does
 			 * arise.
 			 */
-			fkselec *= ref_rel->rows / Max(ref_rel->tuples, 1.0);
+			fkselec *= ref_rel->rows * (1.0 - nullfrac) /
+				Max(ref_rel->tuples, 1.0);
 		}
 		else
 		{
@@ -4475,7 +4502,7 @@ get_foreign_key_join_selectivity(PlannerInfo *root,
 			 * guard against tuples == 0.  Note we should use the raw table
 			 * tuple count, not any estimate of its filtered or joined size.
 			 */
-			fkselec *= 1.0 / Max(ref_rel->tuples, 1.0);
+			fkselec *= (1.0 - nullfrac) / Max(ref_rel->tuples, 1.0);
 		}
 	}
 
diff --git a/src/backend/utils/cache/lsyscache.c b/src/backend/utils/cache/lsyscache.c
index 4def73d..5f98b70 100644
--- a/src/backend/utils/cache/lsyscache.c
+++ b/src/backend/utils/cache/lsyscache.c
@@ -2812,6 +2812,30 @@ type_is_collatable(Oid typid)
 /*				---------- STATISTICS CACHE ----------					 */
 
 /*
+ * get_attnullfrac
+ *
+ *	  Given the table and attribute number of a column, get the nullfrac.
+ *	  Return 0.0 if no data is available.
+ */
+float4
+get_attnullfrac(Oid relid, AttrNumber attnum)
+{
+	HeapTuple	tp;
+	float4		nullfrac = 0.0;
+
+	tp = SearchSysCache3(STATRELATTINH,
+						 ObjectIdGetDatum(relid),
+						 Int16GetDatum(attnum),
+						 BoolGetDatum(false));
+	if (HeapTupleIsValid(tp))
+	{
+		nullfrac = ((Form_pg_statistic) GETSTRUCT(tp))->stanullfrac;
+		ReleaseSysCache(tp);
+	}
+	return nullfrac;
+}
+
+/*
  * get_attavgwidth
  *
  *	  Given the table and attribute number of a column, get the average
diff --git a/src/include/utils/lsyscache.h b/src/include/utils/lsyscache.h
index 93588df..31b2cb0 100644
--- a/src/include/utils/lsyscache.h
+++ b/src/include/utils/lsyscache.h
@@ -169,6 +169,7 @@ extern bool type_is_collatable(Oid typid);
 extern Oid	getBaseType(Oid typid);
 extern Oid	getBaseTypeAndTypmod(Oid typid, int32 *typmod);
 extern int32 get_typavgwidth(Oid typid, int32 typmod);
+extern float4 get_attnullfrac(Oid relid, AttrNumber attnum);
 extern int32 get_attavgwidth(Oid relid, AttrNumber attnum);
 extern bool get_attstatsslot(AttStatsSlot *sslot, HeapTuple statstuple,
 				 int reqkind, Oid reqop, int flags);
-- 
1.9.5.msysgit.1