From 1cbd204f528df12abb0da9e7a06d5c922a8b2ca1 Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Mon, 19 Feb 2024 13:55:13 +0200
Subject: [PATCH] Remove offsethash

The original motivation was to eliminate the superfluous HnswPtrAccess
call from AddToVisited. The caller has to call HnswPtrAccess() anyway,
so it makes sense to pass the HnswElement rather than HnswElementPtr
to AddToVisited(). But then I realized that we can use the
pointer-variant even with shared memory, because the visited-hash is
backend-private, and the addresses where the elements are mapped to in
shared memory are stable within the backend.
---
 src/hnsw.h      | 13 -----------
 src/hnswutils.c | 58 ++++++-------------------------------------------
 2 files changed, 7 insertions(+), 64 deletions(-)

diff --git a/src/hnsw.h b/src/hnsw.h
index 470e24e..78ba815 100644
--- a/src/hnsw.h
+++ b/src/hnsw.h
@@ -445,17 +445,4 @@ typedef struct PointerHashEntry
 #define SH_DECLARE
 #include "lib/simplehash.h"
 
-typedef struct OffsetHashEntry
-{
-	Size		offset;
-	char		status;
-}			OffsetHashEntry;
-
-#define SH_PREFIX offsethash
-#define SH_ELEMENT_TYPE OffsetHashEntry
-#define SH_KEY_TYPE Size
-#define SH_SCOPE extern
-#define SH_DECLARE
-#include "lib/simplehash.h"
-
 #endif
diff --git a/src/hnswutils.c b/src/hnswutils.c
index 212214e..6536646 100644
--- a/src/hnswutils.c
+++ b/src/hnswutils.c
@@ -81,31 +81,9 @@ hash_pointer(uintptr_t ptr)
 #define SH_DEFINE
 #include "lib/simplehash.h"
 
-/* Offset hash table */
-static uint32
-hash_offset(Size offset)
-{
-#if SIZEOF_SIZE_T == 8
-	return murmurhash64((uint64) offset);
-#else
-	return murmurhash32((uint32) offset);
-#endif
-}
-
-#define SH_PREFIX		offsethash
-#define SH_ELEMENT_TYPE	OffsetHashEntry
-#define SH_KEY_TYPE		Size
-#define	SH_KEY			offset
-#define SH_HASH_KEY(tb, key)	hash_offset(key)
-#define SH_EQUAL(tb, a, b)		(a == b)
-#define	SH_SCOPE		extern
-#define SH_DEFINE
-#include "lib/simplehash.h"
-
 typedef union
 {
 	pointerhash_hash *pointers;
-	offsethash_hash *offsets;
 	tidhash_hash *tids;
 }			visited_hash;
 
@@ -658,8 +636,6 @@ InitVisited(char *base, visited_hash * v, Relation index, int ef, int m)
 {
 	if (index != NULL)
 		v->tids = tidhash_create(CurrentMemoryContext, ef * m * 2, NULL);
-	else if (base != NULL)
-		v->offsets = offsethash_create(CurrentMemoryContext, ef * m * 2, NULL);
 	else
 		v->pointers = pointerhash_create(CurrentMemoryContext, ef * m * 2, NULL);
 }
@@ -668,34 +644,21 @@ InitVisited(char *base, visited_hash * v, Relation index, int ef, int m)
  * Add to visited
  */
 static inline void
-AddToVisited(char *base, visited_hash * v, HnswCandidate * hc, Relation index, bool *found)
+AddToVisited(char *base, visited_hash * v, HnswElement element, Relation index, bool *found)
 {
 	if (index != NULL)
 	{
-		HnswElement element = HnswPtrAccess(base, hc->element);
 		ItemPointerData indextid;
 
 		ItemPointerSet(&indextid, element->blkno, element->offno);
 		tidhash_insert(v->tids, indextid, found);
 	}
-	else if (base != NULL)
-	{
-#if PG_VERSION_NUM >= 130000
-		HnswElement element = HnswPtrAccess(base, hc->element);
-
-		offsethash_insert_hash(v->offsets, HnswPtrOffset(hc->element), element->hash, found);
-#else
-		offsethash_insert(v->offsets, HnswPtrOffset(hc->element), found);
-#endif
-	}
 	else
 	{
 #if PG_VERSION_NUM >= 130000
-		HnswElement element = HnswPtrAccess(base, hc->element);
-
-		pointerhash_insert_hash(v->pointers, (uintptr_t) HnswPtrPointer(hc->element), element->hash, found);
+		pointerhash_insert_hash(v->pointers, (uintptr_t) element, element->hash, found);
 #else
-		pointerhash_insert(v->pointers, (uintptr_t) HnswPtrPointer(hc->element), found);
+		pointerhash_insert(v->pointers, (uintptr_t) element, found);
 #endif
 	}
 }
@@ -748,7 +711,7 @@ HnswSearchLayer(char *base, Datum q, List *ep, int ef, int lc, Relation index, F
 		HnswCandidate *hc = (HnswCandidate *) lfirst(lc2);
 		bool		found;
 
-		AddToVisited(base, &v, hc, index, &found);
+		AddToVisited(base, &v, HnswPtrAccess(base, hc->element), index, &found);
 
 		pairingheap_add(C, &(CreatePairingHeapNode(hc)->ph_node));
 		pairingheap_add(W, &(CreatePairingHeapNode(hc)->ph_node));
@@ -792,14 +755,14 @@ HnswSearchLayer(char *base, Datum q, List *ep, int ef, int lc, Relation index, F
 		for (int i = 0; i < neighborhood->length; i++)
 		{
 			HnswCandidate *e = &neighborhood->items[i];
+			HnswElement eElement = HnswPtrAccess(base, e->element);
 			bool		visited;
 
-			AddToVisited(base, &v, e, index, &visited);
+			AddToVisited(base, &v, eElement, index, &visited);
 
 			if (!visited)
 			{
 				float		eDistance;
-				HnswElement eElement = HnswPtrAccess(base, e->element);
 
 				f = ((HnswPairingHeapNode *) pairingheap_first(W))->inner;
 
@@ -1180,14 +1143,7 @@ RemoveElements(char *base, List *w, HnswElement skipElement)
 static void
 PrecomputeHash(char *base, HnswElement element)
 {
-	HnswElementPtr ptr;
-
-	HnswPtrStore(base, ptr, element);
-
-	if (base == NULL)
-		element->hash = hash_pointer((uintptr_t) HnswPtrPointer(ptr));
-	else
-		element->hash = hash_offset(HnswPtrOffset(ptr));
+	element->hash = hash_pointer((uintptr_t) element);
 }
 #endif