1 files changed, 52 insertions, 18 deletions

diff --git a/src/backend/nodes/bitmapset.c b/src/backend/nodes/bitmapset.c
index 10dbbdddf58..7ba3cf635b1 100644
--- a/src/backend/nodes/bitmapset.c
+++ b/src/backend/nodes/bitmapset.c
@@ -5,10 +5,8 @@
  *
  * A bitmap set can represent any set of nonnegative integers, although
  * it is mainly intended for sets where the maximum value is not large,
- * say at most a few hundred.  By convention, a NULL pointer is always
- * accepted by all operations to represent the empty set.  (But beware
- * that this is not the only representation of the empty set.  Use
- * bms_is_empty() in preference to testing for NULL.)
+ * say at most a few hundred.  By convention, we always represent the
+ * empty set by a NULL pointer.
  *
  *
  * Copyright (c) 2003-2023, PostgreSQL Global Development Group
@@ -66,6 +64,8 @@
 #error "invalid BITS_PER_BITMAPWORD"
 #endif
 
+static bool bms_is_empty_internal(const Bitmapset *a);
+
 
 /*
  * bms_copy - make a palloc'd copy of a bitmapset
@@ -104,10 +104,10 @@ bms_equal(const Bitmapset *a, const Bitmapset *b)
 	{
 		if (b == NULL)
 			return true;
-		return bms_is_empty(b);
+		return false;
 	}
 	else if (b == NULL)
-		return bms_is_empty(a);
+		return false;
 	/* Identify shorter and longer input */
 	if (a->nwords <= b->nwords)
 	{
@@ -151,9 +151,9 @@ bms_compare(const Bitmapset *a, const Bitmapset *b)
 
 	/* Handle cases where either input is NULL */
 	if (a == NULL)
-		return bms_is_empty(b) ? 0 : -1;
+		return (b == NULL) ? 0 : -1;
 	else if (b == NULL)
-		return bms_is_empty(a) ? 0 : +1;
+		return +1;
 	/* Handle cases where one input is longer than the other */
 	shortlen = Min(a->nwords, b->nwords);
 	for (i = shortlen; i < a->nwords; i++)
@@ -282,6 +282,12 @@ bms_intersect(const Bitmapset *a, const Bitmapset *b)
 	resultlen = result->nwords;
 	for (i = 0; i < resultlen; i++)
 		result->words[i] &= other->words[i];
+	/* If we computed an empty result, we must return NULL */
+	if (bms_is_empty_internal(result))
+	{
+		pfree(result);
+		return NULL;
+	}
 	return result;
 }
 
@@ -300,12 +306,22 @@ bms_difference(const Bitmapset *a, const Bitmapset *b)
 		return NULL;
 	if (b == NULL)
 		return bms_copy(a);
+
+	/*
+	 * In Postgres' usage, an empty result is a very common case, so it's
+	 * worth optimizing for that by testing bms_nonempty_difference().  This
+	 * saves us a palloc/pfree cycle compared to checking after-the-fact.
+	 */
+	if (!bms_nonempty_difference(a, b))
+		return NULL;
+
 	/* Copy the left input */
 	result = bms_copy(a);
 	/* And remove b's bits from result */
 	shortlen = Min(a->nwords, b->nwords);
 	for (i = 0; i < shortlen; i++)
 		result->words[i] &= ~b->words[i];
+	/* Need not check for empty result, since we handled that case above */
 	return result;
 }
 
@@ -323,7 +339,7 @@ bms_is_subset(const Bitmapset *a, const Bitmapset *b)
 	if (a == NULL)
 		return true;			/* empty set is a subset of anything */
 	if (b == NULL)
-		return bms_is_empty(a);
+		return false;
 	/* Check common words */
 	shortlen = Min(a->nwords, b->nwords);
 	for (i = 0; i < shortlen; i++)
@@ -362,10 +378,10 @@ bms_subset_compare(const Bitmapset *a, const Bitmapset *b)
 	{
 		if (b == NULL)
 			return BMS_EQUAL;
-		return bms_is_empty(b) ? BMS_EQUAL : BMS_SUBSET1;
+		return BMS_SUBSET1;
 	}
 	if (b == NULL)
-		return bms_is_empty(a) ? BMS_EQUAL : BMS_SUBSET2;
+		return BMS_SUBSET2;
 	/* Check common words */
 	result = BMS_EQUAL;			/* status so far */
 	shortlen = Min(a->nwords, b->nwords);
@@ -554,7 +570,7 @@ bms_nonempty_difference(const Bitmapset *a, const Bitmapset *b)
 	if (a == NULL)
 		return false;
 	if (b == NULL)
-		return !bms_is_empty(a);
+		return true;
 	/* Check words in common */
 	shortlen = Min(a->nwords, b->nwords);
 	for (i = 0; i < shortlen; i++)
@@ -696,18 +712,18 @@ bms_membership(const Bitmapset *a)
 }
 
 /*
- * bms_is_empty - is a set empty?
+ * bms_is_empty_internal - is a set empty?
  *
- * This is even faster than bms_membership().
+ * This is now used only locally, to detect cases where a function has
+ * computed an empty set that we must now get rid of.  Hence, we can
+ * assume the input isn't NULL.
  */
-bool
-bms_is_empty(const Bitmapset *a)
+static bool
+bms_is_empty_internal(const Bitmapset *a)
 {
 	int			nwords;
 	int			wordnum;
 
-	if (a == NULL)
-		return true;
 	nwords = a->nwords;
 	for (wordnum = 0; wordnum < nwords; wordnum++)
 	{
@@ -786,6 +802,12 @@ bms_del_member(Bitmapset *a, int x)
 	bitnum = BITNUM(x);
 	if (wordnum < a->nwords)
 		a->words[wordnum] &= ~((bitmapword) 1 << bitnum);
+	/* If we computed an empty result, we must return NULL */
+	if (bms_is_empty_internal(a))
+	{
+		pfree(a);
+		return NULL;
+	}
 	return a;
 }
 
@@ -922,6 +944,12 @@ bms_int_members(Bitmapset *a, const Bitmapset *b)
 		a->words[i] &= b->words[i];
 	for (; i < a->nwords; i++)
 		a->words[i] = 0;
+	/* If we computed an empty result, we must return NULL */
+	if (bms_is_empty_internal(a))
+	{
+		pfree(a);
+		return NULL;
+	}
 	return a;
 }
 
@@ -943,6 +971,12 @@ bms_del_members(Bitmapset *a, const Bitmapset *b)
 	shortlen = Min(a->nwords, b->nwords);
 	for (i = 0; i < shortlen; i++)
 		a->words[i] &= ~b->words[i];
+	/* If we computed an empty result, we must return NULL */
+	if (bms_is_empty_internal(a))
+	{
+		pfree(a);
+		return NULL;
+	}
 	return a;
 }