4 files changed, 28 insertions, 17 deletions

diff --git a/src/backend/commands/analyze.c b/src/backend/commands/analyze.c
index 87993069830..9800d762fba 100644
--- a/src/backend/commands/analyze.c
+++ b/src/backend/commands/analyze.c
@@ -2044,8 +2044,11 @@ compute_minimal_stats(VacAttrStatsP stats,
 
 		if (nmultiple == 0)
 		{
-			/* If we found no repeated values, assume it's a unique column */
-			stats->stadistinct = -1.0;
+			/*
+			 * If we found no repeated non-null values, assume it's a unique
+			 * column; but be sure to discount for any nulls we found.
+			 */
+			stats->stadistinct = -1.0 * (1.0 - stats->stanullfrac);
 		}
 		else if (track_cnt < track_max && toowide_cnt == 0 &&
 				 nmultiple == track_cnt)
@@ -2390,8 +2393,11 @@ compute_scalar_stats(VacAttrStatsP stats,
 
 		if (nmultiple == 0)
 		{
-			/* If we found no repeated values, assume it's a unique column */
-			stats->stadistinct = -1.0;
+			/*
+			 * If we found no repeated non-null values, assume it's a unique
+			 * column; but be sure to discount for any nulls we found.
+			 */
+			stats->stadistinct = -1.0 * (1.0 - stats->stanullfrac);
 		}
 		else if (toowide_cnt == 0 && nmultiple == ndistinct)
 		{
@@ -2695,7 +2701,7 @@ compute_scalar_stats(VacAttrStatsP stats,
 		else
 			stats->stawidth = stats->attrtype->typlen;
 		/* Assume all too-wide values are distinct, so it's a unique column */
-		stats->stadistinct = -1.0;
+		stats->stadistinct = -1.0 * (1.0 - stats->stanullfrac);
 	}
 	else if (null_cnt > 0)
 	{
diff --git a/src/backend/tsearch/ts_typanalyze.c b/src/backend/tsearch/ts_typanalyze.c
index 308ae0893c8..5914d27062b 100644
--- a/src/backend/tsearch/ts_typanalyze.c
+++ b/src/backend/tsearch/ts_typanalyze.c
@@ -296,7 +296,7 @@ compute_tsvector_stats(VacAttrStats *stats,
 		stats->stawidth = total_width / (double) nonnull_cnt;
 
 		/* Assume it's a unique column (see notes above) */
-		stats->stadistinct = -1.0;
+		stats->stadistinct = -1.0 * (1.0 - stats->stanullfrac);
 
 		/*
 		 * Construct an array of the interesting hashtable items, that is,
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index 2c993148271..9c7ffbf6bfb 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -4383,12 +4383,14 @@ double
 get_variable_numdistinct(VariableStatData *vardata)
 {
 	double		stadistinct;
+	double		stanullfrac = 0.0;
 	double		ntuples;
 
 	/*
 	 * Determine the stadistinct value to use.  There are cases where we can
 	 * get an estimate even without a pg_statistic entry, or can get a better
-	 * value than is in pg_statistic.
+	 * value than is in pg_statistic.  Grab stanullfrac too if we can find it
+	 * (otherwise, assume no nulls, for lack of any better idea).
 	 */
 	if (HeapTupleIsValid(vardata->statsTuple))
 	{
@@ -4397,6 +4399,7 @@ get_variable_numdistinct(VariableStatData *vardata)
 
 		stats = (Form_pg_statistic) GETSTRUCT(vardata->statsTuple);
 		stadistinct = stats->stadistinct;
+		stanullfrac = stats->stanullfrac;
 	}
 	else if (vardata->vartype == BOOLOID)
 	{
@@ -4420,7 +4423,7 @@ get_variable_numdistinct(VariableStatData *vardata)
 			{
 				case ObjectIdAttributeNumber:
 				case SelfItemPointerAttributeNumber:
-					stadistinct = -1.0; /* unique */
+					stadistinct = -1.0; /* unique (and all non null) */
 					break;
 				case TableOidAttributeNumber:
 					stadistinct = 1.0;	/* only 1 value */
@@ -4442,10 +4445,11 @@ get_variable_numdistinct(VariableStatData *vardata)
 	 * If there is a unique index for the variable, assume it is unique no
 	 * matter what pg_statistic says; the statistics could be out of date, or
 	 * we might have found a partial unique index that proves the var is
-	 * unique for this query.
+	 * unique for this query.  However, we'd better still believe
+	 * the null-fraction statistic.
 	 */
 	if (vardata->isunique)
-		stadistinct = -1.0;
+		stadistinct = -1.0 * (1.0 - stanullfrac);
 
 	/*
 	 * If we had an absolute estimate, use that.
diff --git a/src/include/catalog/pg_statistic.h b/src/include/catalog/pg_statistic.h
index 044eb446c08..ddef0f49e6d 100644
--- a/src/include/catalog/pg_statistic.h
+++ b/src/include/catalog/pg_statistic.h
@@ -67,13 +67,14 @@ CATALOG(pg_statistic,2619) BKI_WITHOUT_OIDS
 	 *		> 0		actual number of distinct values
 	 *		< 0		negative of multiplier for number of rows
 	 * The special negative case allows us to cope with columns that are
-	 * unique (stadistinct = -1) or nearly so (for example, a column in
-	 * which values appear about twice on the average could be represented
-	 * by stadistinct = -0.5).  Because the number-of-rows statistic in
-	 * pg_class may be updated more frequently than pg_statistic is, it's
-	 * important to be able to describe such situations as a multiple of
-	 * the number of rows, rather than a fixed number of distinct values.
-	 * But in other cases a fixed number is correct (eg, a boolean column).
+	 * unique (stadistinct = -1) or nearly so (for example, a column in which
+	 * non-null values appear about twice on the average could be represented
+	 * by stadistinct = -0.5 if there are no nulls, or -0.4 if 20% of the
+	 * column is nulls).  Because the number-of-rows statistic in pg_class may
+	 * be updated more frequently than pg_statistic is, it's important to be
+	 * able to describe such situations as a multiple of the number of rows,
+	 * rather than a fixed number of distinct values.  But in other cases a
+	 * fixed number is correct (eg, a boolean column).
 	 * ----------------
 	 */
 	float4		stadistinct;