Fix an incorrect check in get_memoize_path

Richard Guo · Richard Guo · commit 3b35f9a4c5e0 · 2025-04-16T10:55:44.000+09:00
Memoize typically marks cache entries as complete after fully scanning the inner side of a join. However, in the case of unique joins, we skip to the next outer tuple as soon as the first matching inner tuple is found, leaving no opportunity to scan the inner side to completion. To work around that, we mark cache entries as complete after fetching the first matching inner tuple in unique joins. This approach is only safe when all of the join's restriction clauses are parameterized; otherwise, there is no guarantee that reading just one tuple from the inner side is sufficient. Currently, we check for this by verifying that the number of clauses in ppi_clauses is no less than the number of the join's restriction clauses. However, this check isn't entirely reliable, as ppi_clauses includes join clauses available from all outer rels, not just the current outer rel. This means the check could pass even if a restriction clause isn't parameterized, as long as another join clause, which doesn't belong to the current join, is included in ppi_clauses. To fix this, we explicitly check whether each restriction clause of the current join is present in ppi_clauses. While we're here, remove the XXX comment from the modified code, as it's not justified; in certain cases, it's not possible to move a join clause to the inner side. This is arguably a bugfix, but no backpatch given the lack of field reports. Author: Richard Guo <guofenglinux@gmail.com> Reviewed-by: wenhui qiu <qiuwenhuifx@gmail.com> Reviewed-by: Andrei Lepikhov <lepihov@gmail.com> Discussion: https://postgr.es/m/CAMbWs4-8JPouj=wBDj4DhK-WO4+Xdx=A2jbjvvyyTBQneJ1=BQ@mail.gmail.com
diff --git a/src/backend/optimizer/path/joinpath.c b/src/backend/optimizer/path/joinpath.c
@@ -748,16 +748,22 @@ get_memoize_path(PlannerInfo *root, RelOptInfo *innerrel,
 	 *
 	 * Lateral vars needn't be considered here as they're not considered when
 	 * determining if the join is unique.
-	 *
-	 * XXX this could be enabled if the remaining join quals were made part of
-	 * the inner scan's filter instead of the join filter.  Maybe it's worth
-	 * considering doing that?
 	 */
-	if (extra->inner_unique &&
-		(inner_path->param_info == NULL ||
-		 bms_num_members(inner_path->param_info->ppi_serials) <
-		 list_length(extra->restrictlist)))
-		return NULL;
+	if (extra->inner_unique)
+	{
+		Bitmapset  *ppi_serials;
+
+		if (inner_path->param_info == NULL)
+			return NULL;
+
+		ppi_serials = inner_path->param_info->ppi_serials;
+
+		foreach_node(RestrictInfo, rinfo, extra->restrictlist)
+		{
+			if (!bms_is_member(rinfo->rinfo_serial, ppi_serials))
+				return NULL;
+		}
+	}
 
 	/*
 	 * We can't use a memoize node if there are volatile functions in the
