From a18bea24b8cd3c3c3084373f99ffe434c60b368c Mon Sep 17 00:00:00 2001
From: Andrew Kane <andrew@ankane.org>
Date: Thu, 10 Aug 2023 15:07:28 -0700
Subject: [PATCH] Improved concurrent inserts with low number of elements

---
 src/hnswinsert.c | 16 ++++++++++++++--
 src/hnswutils.c  |  2 +-
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/src/hnswinsert.c b/src/hnswinsert.c
index 250fc78..0bccfb4 100644
--- a/src/hnswinsert.c
+++ b/src/hnswinsert.c
@@ -290,6 +290,7 @@ UpdateNeighborPages(Relation index, FmgrInfo *procinfo, Oid collation, HnswEleme
 			HnswNeighborTuple ntup;
 			Size		ntupSize;
 			int			idx = -1;
+			int			startIdx;
 			OffsetNumber offno = hc->element->neighborOffno;
 
 			/* Get latest neighbors */
@@ -312,10 +313,21 @@ UpdateNeighborPages(Relation index, FmgrInfo *procinfo, Oid collation, HnswEleme
 			ntupSize = ItemIdGetLength(itemid);
 
 			/* Calculate index for update */
-			idx += (hc->element->level - lc) * m;
+			startIdx = (hc->element->level - lc) * m;
+
+			if (idx == -2)
+			{
+				/* Find free offset if still exists */
+				/* TODO Retry updating connections if not */
+				for (int i = 0; i < lm; i++)
+					if (!ItemPointerIsValid(&ntup->indextids[startIdx + i]))
+						idx = startIdx + i;
+			}
+			else
+				idx += startIdx;
 
 			/* Make robust to issues */
-			if (idx < ntup->count)
+			if (idx >= 0 && idx < ntup->count)
 			{
 				ItemPointer indextid = &ntup->indextids[idx];
 
diff --git a/src/hnswutils.c b/src/hnswutils.c
index b763647..9e4dbdc 100644
--- a/src/hnswutils.c
+++ b/src/hnswutils.c
@@ -818,7 +818,7 @@ HnswUpdateConnection(HnswElement element, HnswCandidate * hc, int m, int lc, int
 
 		/* Track update */
 		if (updateIdx != NULL)
-			*updateIdx = currentNeighbors->length - 1;
+			*updateIdx = -2;
 	}
 	else
 	{