From ab5b8c6eb4c21dceaf24f2e16405afcc6de5d9b2 Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Thu, 21 Jan 2021 00:06:58 +0200
Subject: [PATCH v6 3/3] Optimize hash function

---
 src/backend/utils/resowner/resowner.c | 24 ++++++++++++++++++------
 src/include/common/hashfn.h           | 15 +++++++++++++++
 2 files changed, 33 insertions(+), 6 deletions(-)

diff --git a/src/backend/utils/resowner/resowner.c b/src/backend/utils/resowner/resowner.c
index 1db1e7cf14b..72e4d8a784c 100644
--- a/src/backend/utils/resowner/resowner.c
+++ b/src/backend/utils/resowner/resowner.c
@@ -163,15 +163,27 @@ static void ReleaseAuxProcessResourcesCallback(int code, Datum arg);
  *	  INTERNAL ROUTINES														 *
  *****************************************************************************/

+/*
+ * Hash function for value+kind combination.
+ */
 static inline uint32
 hash_resource_elem(Datum value, ResourceOwnerFuncs *kind)
 {
-	Datum		data[2];
-
-	data[0] = value;
-	data[1] = PointerGetDatum(kind);
-
-	return hash_bytes((unsigned char *) &data, 2 * SIZEOF_DATUM);
+	/*
+	 * Most resources types store a pointer in 'value', and pointers are
+	 * unique all on their own. But some resources store plain integers (Files
+	 * and Buffers as of this writing), so we want to incorporate the 'kind'
+	 * in the hash too, otherwise those resources will collide a lot. But
+	 * because there are only a few resource kinds like that - and only a few
+	 * resource kinds to begin with - we don't need to work too hard to mix
+	 * 'kind' into the hash. Just add it with hash_combine(), it perturbs the
+	 * result enough for our purposes.
+	 */
+#if SIZEOF_DATUM == 8
+	return hash_combine64(murmurhash64((uint64) value), (uint64) kind);
+#else
+	return hash_combine(murmurhash32((uint32) value), (uint32) kind);
+#endif
 }

 static void
diff --git a/src/include/common/hashfn.h b/src/include/common/hashfn.h
index c634cc067a1..009ffbbdd38 100644
--- a/src/include/common/hashfn.h
+++ b/src/include/common/hashfn.h
@@ -101,4 +101,19 @@ murmurhash32(uint32 data)
 	return h;
 }

+/* 64-bit variant */
+static inline uint64
+murmurhash64(uint64 data)
+{
+	uint64		h = data;
+
+	h ^= h >> 33;
+	h *= 0xff51afd7ed558ccd;
+	h ^= h >> 33;
+	h *= 0xc4ceb9fe1a85ec53;
+	h ^= h >> 33;
+
+	return h;
+}
+
 #endif							/* HASHFN_H */
-- 
2.29.2