A quick question about length of neighbors for one Element

[Zhang.Jinrui]

See this code snippet,

/*
 * Load neighbors from page
 */
static void
LoadNeighborsFromPage(HnswElement element, Relation index, Page page, int m)
{
	char	   *base = NULL;

	HnswNeighborTuple ntup = (HnswNeighborTuple) PageGetItem(page, PageGetItemId(page, element->neighborOffno));
	int			neighborCount = (element->level + 2) * m;

	Assert(HnswIsNeighborTuple(ntup));

	HnswInitNeighbors(base, element, m, NULL);

Why is the neighborCount is (level + 2) * m rather than (level + 1) * m?

[Andrew Kane]

Layer 0 can have up to 2 * m connections, as recommended in the HNSW paper.