Use smaller batch size for better performance

src/hnsw.h

@@ -185,6 +185,9 @@ typedef struct HnswSearchCandidate
 	float		distance;
 }			HnswSearchCandidate;
 
+#define HnswGetSearchCandidate(membername, ptr) pairingheap_container(HnswSearchCandidate, membername, ptr)
+#define HnswGetSearchCandidateConst(membername, ptr) pairingheap_const_container(HnswSearchCandidate, membername, ptr)
+
 /* HNSW index options */
 typedef struct HnswOptions
 {
@@ -360,7 +363,7 @@ typedef struct HnswScanOpaqueData
 	bool		first;
 	List	   *w;
 	visited_hash v;
-	List	   *discarded;
+	pairingheap *discarded;
 	Datum		q;
 	int			m;
 	int64		tuples;
@@ -409,7 +412,7 @@ bool		HnswCheckNorm(FmgrInfo *procinfo, Oid collation, Datum value);
 Buffer		HnswNewBuffer(Relation index, ForkNumber forkNum);
 void		HnswInitPage(Buffer buf, Page page);
 void		HnswInit(void);
-List	   *HnswSearchLayer(char *base, Datum q, List *ep, int ef, int lc, Relation index, FmgrInfo *procinfo, Oid collation, int m, bool inserting, HnswElement skipElement, visited_hash * v, List **discarded, bool initVisited);
+List	   *HnswSearchLayer(char *base, Datum q, List *ep, int ef, int lc, Relation index, FmgrInfo *procinfo, Oid collation, int m, bool inserting, HnswElement skipElement, visited_hash * v, pairingheap **discarded, bool initVisited);
 HnswElement HnswGetEntryPoint(Relation index);
 void		HnswGetMetaPageInfo(Relation index, int *m, HnswElement * entryPoint);
 void	   *HnswAlloc(HnswAllocator * allocator, Size size);

src/hnswscan.c

@@ -53,14 +53,24 @@ ResumeScanItems(IndexScanDesc scan)
 	Relation	index = scan->indexRelation;
 	FmgrInfo   *procinfo = so->procinfo;
 	Oid			collation = so->collation;
-	List	   *ep;
+	List	   *ep = NIL;
 	char	   *base = NULL;
 
-	if (list_length(so->discarded) == 0)
+	if (pairingheap_is_empty(so->discarded))
 		return NIL;
 
-	ep = so->discarded;
-	so->discarded = NIL;
+	for (int i = 0; i < hnsw_ef_search; i++)
+	{
+		HnswSearchCandidate *hc;
+
+		if (pairingheap_is_empty(so->discarded))
+			break;
+
+		hc = HnswGetSearchCandidate(w_node, pairingheap_remove_first(so->discarded));
+
+		ep = lappend(ep, hc);
+	}
+
 	return HnswSearchLayer(base, so->q, ep, hnsw_ef_search, 0, index, procinfo, collation, so->m, false, NULL, &so->v, &so->discarded, false);
 }
 
@@ -128,9 +138,11 @@ hnswrescan(IndexScanDesc scan, ScanKey keys, int nkeys, ScanKey orderbys, int no
 	HnswScanOpaque so = (HnswScanOpaque) scan->opaque;
 
 	if (!so->first)
+	{
+		pairingheap_reset(so->discarded);
 		tidhash_reset(so->v.tids);
+	}
 	so->first = true;
-	so->discarded = NIL;
 	so->tuples = 0;
 	MemoryContextReset(so->tmpCtx);
 
@@ -141,24 +153,6 @@ hnswrescan(IndexScanDesc scan, ScanKey keys, int nkeys, ScanKey orderbys, int no
 		memmove(scan->orderByData, orderbys, scan->numberOfOrderBys * sizeof(ScanKeyData));
 }
 
-/*
- * Compare search candidate distances
- */
-static int
-CompareSearchCandidateDistances(const ListCell *a, const ListCell *b)
-{
-	HnswSearchCandidate *hca = lfirst(a);
-	HnswSearchCandidate *hcb = lfirst(b);
-
-	if (hca->distance < hcb->distance)
-		return 1;
-
-	if (hca->distance > hcb->distance)
-		return -1;
-
-	return 0;
-}
-
 /*
  * Fetch the next tuple in the given scan
  */
@@ -225,7 +219,7 @@ hnswgettuple(IndexScanDesc scan, ScanDirection dir)
 
 			if (MemoryContextMemAllocated(so->tmpCtx, false) > (work_mem * 1024L))
 			{
-				if (list_length(so->discarded) == 0)
+				if (pairingheap_is_empty(so->discarded))
 				{
 					ereport(NOTICE,
 							(errmsg("hnsw iterative search exceeded work_mem after " INT64_FORMAT " tuples", so->tuples),
@@ -235,11 +229,7 @@ hnswgettuple(IndexScanDesc scan, ScanDirection dir)
 				}
 
 				/* Return remaining tuples */
-				so->w = so->discarded;
-				so->discarded = NIL;
-
-				/* Sort in reverse order since results are removed from end */
-				list_sort(so->w, CompareSearchCandidateDistances);
+				so->w = lappend(so->w, HnswGetSearchCandidate(w_node, pairingheap_remove_first(so->discarded)));
 			}
 			else
 			{

src/hnswutils.c

@@ -619,9 +619,6 @@ HnswEntryCandidate(char *base, HnswElement entryPoint, Datum q, Relation index,
 	return hc;
 }
 
-#define HnswGetSearchCandidate(membername, ptr) pairingheap_container(HnswSearchCandidate, membername, ptr)
-#define HnswGetSearchCandidateConst(membername, ptr) pairingheap_const_container(HnswSearchCandidate, membername, ptr)
-
 /*
  * Compare candidate distances
  */
@@ -637,6 +634,21 @@ CompareNearestCandidates(const pairingheap_node *a, const pairingheap_node *b, v
 	return 0;
 }
 
+/*
+ * Compare discarded candidate distances
+ */
+static int
+CompareNearestDiscardedCandidates(const pairingheap_node *a, const pairingheap_node *b, void *arg)
+{
+	if (HnswGetSearchCandidateConst(w_node, a)->distance < HnswGetSearchCandidateConst(w_node, b)->distance)
+		return 1;
+
+	if (HnswGetSearchCandidateConst(w_node, a)->distance > HnswGetSearchCandidateConst(w_node, b)->distance)
+		return -1;
+
+	return 0;
+}
+
 /*
  * Compare candidate distances
  */
@@ -795,7 +807,7 @@ HnswLoadUnvisitedFromDisk(HnswElement element, HnswUnvisited * unvisited, int *u
  * Algorithm 2 from paper
  */
 List *
-HnswSearchLayer(char *base, Datum q, List *ep, int ef, int lc, Relation index, FmgrInfo *procinfo, Oid collation, int m, bool inserting, HnswElement skipElement, visited_hash * v, List **discarded, bool initVisited)
+HnswSearchLayer(char *base, Datum q, List *ep, int ef, int lc, Relation index, FmgrInfo *procinfo, Oid collation, int m, bool inserting, HnswElement skipElement, visited_hash * v, pairingheap **discarded, bool initVisited)
 {
 	List	   *w = NIL;
 	pairingheap *C = pairingheap_allocate(CompareNearestCandidates, NULL);
@@ -816,8 +828,13 @@ HnswSearchLayer(char *base, Datum q, List *ep, int ef, int lc, Relation index, F
 	}
 
 	if (initVisited)
+	{
 		InitVisited(base, v, index, ef, m);
 
+		if (discarded != NULL)
+			*discarded = pairingheap_allocate(CompareNearestDiscardedCandidates, NULL);
+	}
+
 	/* Create local memory for neighborhood if needed */
 	if (index == NULL)
 	{
@@ -895,8 +912,7 @@ HnswSearchLayer(char *base, Datum q, List *ep, int ef, int lc, Relation index, F
 					e = palloc(sizeof(HnswSearchCandidate));
 					HnswPtrStore(base, e->element, eElement);
 					e->distance = eDistance;
-
-					*discarded = lappend(*discarded, e);
+					pairingheap_add(*discarded, &e->w_node);
 				}
 
 				continue;
@@ -928,7 +944,7 @@ HnswSearchLayer(char *base, Datum q, List *ep, int ef, int lc, Relation index, F
 					HnswSearchCandidate *d = HnswGetSearchCandidate(w_node, pairingheap_remove_first(W));
 
 					if (discarded != NULL)
-						*discarded = lappend(*discarded, d);
+						pairingheap_add(*discarded, &d->w_node);
 				}
 			}
 		}