Rationalize use of list_concat + list_copy combinations.

In the wake of commit 1cff1b95a, the result of list_concat no longer
shares the ListCells of the second input.  Therefore, we can replace
"list_concat(x, list_copy(y))" with just "list_concat(x, y)".

To improve call sites that were list_copy'ing the first argument,
or both arguments, invent "list_concat_copy()" which produces a new
list sharing no ListCells with either input.  (This is a bit faster
than "list_concat(list_copy(x), y)" because it makes the result list
the right size to start with.)

In call sites that were not list_copy'ing the second argument, the new
semantics mean that we are usually leaking the second List's storage,
since typically there is no remaining pointer to it.  We considered
inventing another list_copy variant that would list_free the second
input, but concluded that for most call sites it isn't worth worrying
about, given the relative compactness of the new List representation.
(Note that in cases where such leakage would happen, the old code
already leaked the second List's header; so we're only discussing
the size of the leak not whether there is one.  I did adjust two or
three places that had been troubling to free that header so that
they manually free the whole second List.)

Patch by me; thanks to David Rowley for review.

Discussion: https://postgr.es/m/11587.1550975080@sss.pgh.pa.us

24 files changed, 122 insertions, 140 deletions

diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c
index 8dc7a398706..ef953953711 100644
--- a/src/backend/commands/indexcmds.c
+++ b/src/backend/commands/indexcmds.c
@@ -516,8 +516,8 @@ DefineIndex(Oid relationId,
 	 * is part of the key columns, and anything equal to and over is part of
 	 * the INCLUDE columns.
 	 */
-	allIndexParams = list_concat(list_copy(stmt->indexParams),
-								 list_copy(stmt->indexIncludingParams));
+	allIndexParams = list_concat_copy(stmt->indexParams,
+									  stmt->indexIncludingParams);
 	numberOfAttributes = list_length(allIndexParams);
 
 	if (numberOfAttributes <= 0)
diff --git a/src/backend/executor/functions.c b/src/backend/executor/functions.c
index 64a9e584627..83337c2eed4 100644
--- a/src/backend/executor/functions.c
+++ b/src/backend/executor/functions.c
@@ -719,8 +719,7 @@ init_sql_fcache(FmgrInfo *finfo, Oid collation, bool lazyEvalOK)
 														  fcache->pinfo,
 														  NULL);
 		queryTree_list = lappend(queryTree_list, queryTree_sublist);
-		flat_query_list = list_concat(flat_query_list,
-									  list_copy(queryTree_sublist));
+		flat_query_list = list_concat(flat_query_list, queryTree_sublist);
 	}
 
 	check_sql_fn_statements(flat_query_list);
diff --git a/src/backend/nodes/list.c b/src/backend/nodes/list.c
index 9163464de24..6bf13ae0d4a 100644
--- a/src/backend/nodes/list.c
+++ b/src/backend/nodes/list.c
@@ -501,12 +501,15 @@ lcons_oid(Oid datum, List *list)
 }
 
 /*
- * Concatenate list2 to the end of list1, and return list1. list1 is
- * destructively changed, list2 is not. (However, in the case of pointer
- * lists, list1 and list2 will point to the same structures.) Callers
- * should be sure to use the return value as the new pointer to the
- * concatenated list: the 'list1' input pointer may or may not be the
- * same as the returned pointer.
+ * Concatenate list2 to the end of list1, and return list1.
+ *
+ * This is equivalent to lappend'ing each element of list2, in order, to list1.
+ * list1 is destructively changed, list2 is not.  (However, in the case of
+ * pointer lists, list1 and list2 will point to the same structures.)
+ *
+ * Callers should be sure to use the return value as the new pointer to the
+ * concatenated list: the 'list1' input pointer may or may not be the same
+ * as the returned pointer.
  */
 List *
 list_concat(List *list1, const List *list2)
@@ -535,6 +538,41 @@ list_concat(List *list1, const List *list2)
 }
 
 /*
+ * Form a new list by concatenating the elements of list1 and list2.
+ *
+ * Neither input list is modified.  (However, if they are pointer lists,
+ * the output list will point to the same structures.)
+ *
+ * This is equivalent to, but more efficient than,
+ * list_concat(list_copy(list1), list2).
+ * Note that some pre-v13 code might list_copy list2 as well, but that's
+ * pointless now.
+ */
+List *
+list_concat_copy(const List *list1, const List *list2)
+{
+	List	   *result;
+	int			new_len;
+
+	if (list1 == NIL)
+		return list_copy(list2);
+	if (list2 == NIL)
+		return list_copy(list1);
+
+	Assert(list1->type == list2->type);
+
+	new_len = list1->length + list2->length;
+	result = new_list(list1->type, new_len);
+	memcpy(result->elements, list1->elements,
+		   list1->length * sizeof(ListCell));
+	memcpy(result->elements + list1->length, list2->elements,
+		   list2->length * sizeof(ListCell));
+
+	check_list_invariants(result);
+	return result;
+}
+
+/*
  * Truncate 'list' to contain no more than 'new_size' elements. This
  * modifies the list in-place! Despite this, callers should use the
  * pointer returned by this function to refer to the newly truncated
diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index e9ee32b7f43..db3a68a51dd 100644
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -1266,7 +1266,7 @@ set_append_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
 		if (rel->part_scheme)
 			rel->partitioned_child_rels =
 				list_concat(rel->partitioned_child_rels,
-							list_copy(childrel->partitioned_child_rels));
+							childrel->partitioned_child_rels);
 
 		/*
 		 * Child is live, so add it to the live_childrels list for use below.
@@ -1347,9 +1347,8 @@ add_paths_to_append_rel(PlannerInfo *root, RelOptInfo *rel,
 				component = root->simple_rel_array[relid];
 				Assert(component->part_scheme != NULL);
 				Assert(list_length(component->partitioned_child_rels) >= 1);
-				partrels =
-					list_concat(partrels,
-								list_copy(component->partitioned_child_rels));
+				partrels = list_concat(partrels,
+									   component->partitioned_child_rels);
 			}
 
 			partitioned_rels = list_make1(partrels);
@@ -2048,8 +2047,7 @@ accumulate_append_subpath(Path *path, List **subpaths, List **special_subpaths)
 
 		if (!apath->path.parallel_aware || apath->first_partial_path == 0)
 		{
-			/* list_copy is important here to avoid sharing list substructure */
-			*subpaths = list_concat(*subpaths, list_copy(apath->subpaths));
+			*subpaths = list_concat(*subpaths, apath->subpaths);
 			return;
 		}
 		else if (special_subpaths != NULL)
@@ -2072,8 +2070,7 @@ accumulate_append_subpath(Path *path, List **subpaths, List **special_subpaths)
 	{
 		MergeAppendPath *mpath = (MergeAppendPath *) path;
 
-		/* list_copy is important here to avoid sharing list substructure */
-		*subpaths = list_concat(*subpaths, list_copy(mpath->subpaths));
+		*subpaths = list_concat(*subpaths, mpath->subpaths);
 		return;
 	}
 
diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index 3a9a994733b..bc6bc999573 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -4443,8 +4443,7 @@ get_parameterized_baserel_size(PlannerInfo *root, RelOptInfo *rel,
 	 * restriction clauses.  Note that we force the clauses to be treated as
 	 * non-join clauses during selectivity estimation.
 	 */
-	allclauses = list_concat(list_copy(param_clauses),
-							 rel->baserestrictinfo);
+	allclauses = list_concat_copy(param_clauses, rel->baserestrictinfo);
 	nrows = rel->tuples *
 		clauselist_selectivity(root,
 							   allclauses,
diff --git a/src/backend/optimizer/path/indxpath.c b/src/backend/optimizer/path/indxpath.c
index 5f339fdfde7..37b257cd0e9 100644
--- a/src/backend/optimizer/path/indxpath.c
+++ b/src/backend/optimizer/path/indxpath.c
@@ -656,7 +656,7 @@ get_join_index_paths(PlannerInfo *root, RelOptInfo *rel,
 			}
 		}
 
-		/* Add restriction clauses (this is nondestructive to rclauseset) */
+		/* Add restriction clauses */
 		clauseset.indexclauses[indexcol] =
 			list_concat(clauseset.indexclauses[indexcol],
 						rclauseset->indexclauses[indexcol]);
@@ -1204,8 +1204,7 @@ build_paths_for_OR(PlannerInfo *root, RelOptInfo *rel,
 			{
 				/* Form all_clauses if not done already */
 				if (all_clauses == NIL)
-					all_clauses = list_concat(list_copy(clauses),
-											  other_clauses);
+					all_clauses = list_concat_copy(clauses, other_clauses);
 
 				if (!predicate_implied_by(index->indpred, all_clauses, false))
 					continue;	/* can't use it at all */
@@ -1270,7 +1269,7 @@ generate_bitmap_or_paths(PlannerInfo *root, RelOptInfo *rel,
 	 * We can use both the current and other clauses as context for
 	 * build_paths_for_OR; no need to remove ORs from the lists.
 	 */
-	all_clauses = list_concat(list_copy(clauses), other_clauses);
+	all_clauses = list_concat_copy(clauses, other_clauses);
 
 	foreach(lc, clauses)
 	{
@@ -1506,8 +1505,7 @@ choose_bitmap_and(PlannerInfo *root, RelOptInfo *rel, List *paths)
 		pathinfo = pathinfoarray[i];
 		paths = list_make1(pathinfo->path);
 		costsofar = bitmap_scan_cost_est(root, rel, pathinfo->path);
-		qualsofar = list_concat(list_copy(pathinfo->quals),
-								list_copy(pathinfo->preds));
+		qualsofar = list_concat_copy(pathinfo->quals, pathinfo->preds);
 		clauseidsofar = bms_copy(pathinfo->clauseids);
 
 		for (j = i + 1; j < npaths; j++)
@@ -1543,10 +1541,8 @@ choose_bitmap_and(PlannerInfo *root, RelOptInfo *rel, List *paths)
 			{
 				/* keep new path in paths, update subsidiary variables */
 				costsofar = newcost;
-				qualsofar = list_concat(qualsofar,
-										list_copy(pathinfo->quals));
-				qualsofar = list_concat(qualsofar,
-										list_copy(pathinfo->preds));
+				qualsofar = list_concat(qualsofar, pathinfo->quals);
+				qualsofar = list_concat(qualsofar, pathinfo->preds);
 				clauseidsofar = bms_add_members(clauseidsofar,
 												pathinfo->clauseids);
 			}
@@ -1849,7 +1845,7 @@ find_indexpath_quals(Path *bitmapqual, List **quals, List **preds)
 
 			*quals = lappend(*quals, iclause->rinfo->clause);
 		}
-		*preds = list_concat(*preds, list_copy(ipath->indexinfo->indpred));
+		*preds = list_concat(*preds, ipath->indexinfo->indpred);
 	}
 	else
 		elog(ERROR, "unrecognized node type: %d", nodeTag(bitmapqual));
diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index f2325694c53..0c036209f09 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -559,8 +559,8 @@ create_scan_plan(PlannerInfo *root, Path *best_path, int flags)
 	 * For paranoia's sake, don't modify the stored baserestrictinfo list.
 	 */
 	if (best_path->param_info)
-		scan_clauses = list_concat(list_copy(scan_clauses),
-								   best_path->param_info->ppi_clauses);
+		scan_clauses = list_concat_copy(scan_clauses,
+										best_path->param_info->ppi_clauses);
 
 	/*
 	 * Detect whether we have any pseudoconstant quals to deal with.  Then, if
diff --git a/src/backend/optimizer/plan/initsplan.c b/src/backend/optimizer/plan/initsplan.c
index 73da0c2d8e1..274fea076cb 100644
--- a/src/backend/optimizer/plan/initsplan.c
+++ b/src/backend/optimizer/plan/initsplan.c
@@ -973,7 +973,6 @@ deconstruct_recurse(PlannerInfo *root, Node *jtnode, bool below_outer_join,
 				*postponed_qual_list = lappend(*postponed_qual_list, pq);
 			}
 		}
-		/* list_concat is nondestructive of its second argument */
 		my_quals = list_concat(my_quals, (List *) j->quals);
 
 		/*
diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 0f918dd358d..17c5f086fbf 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -3572,10 +3572,6 @@ reorder_grouping_sets(List *groupingsets, List *sortclause)
 			}
 		}
 
-		/*
-		 * Safe to use list_concat (which shares cells of the second arg)
-		 * because we know that new_elems does not share cells with anything.
-		 */
 		previous = list_concat(previous, new_elems);
 
 		gs->set = list_copy(previous);
@@ -4287,8 +4283,8 @@ consider_groupingsets_paths(PlannerInfo *root,
 			 */
 			if (!rollup->hashable)
 				return;
-			else
-				sets_data = list_concat(sets_data, list_copy(rollup->gsets_data));
+
+			sets_data = list_concat(sets_data, rollup->gsets_data);
 		}
 		foreach(lc, sets_data)
 		{
@@ -4473,7 +4469,7 @@ consider_groupingsets_paths(PlannerInfo *root,
 					{
 						if (bms_is_member(i, hash_items))
 							hash_sets = list_concat(hash_sets,
-													list_copy(rollup->gsets_data));
+													rollup->gsets_data);
 						else
 							rollups = lappend(rollups, rollup);
 						++i;
@@ -5642,8 +5638,7 @@ make_pathkeys_for_window(PlannerInfo *root, WindowClause *wc,
 				 errdetail("Window ordering columns must be of sortable datatypes.")));
 
 	/* Okay, make the combined pathkeys */
-	window_sortclauses = list_concat(list_copy(wc->partitionClause),
-									 list_copy(wc->orderClause));
+	window_sortclauses = list_concat_copy(wc->partitionClause, wc->orderClause);
 	window_pathkeys = make_pathkeys_for_sortclauses(root,
 													window_sortclauses,
 													tlist);
diff --git a/src/backend/optimizer/plan/setrefs.c b/src/backend/optimizer/plan/setrefs.c
index 329ebd5f287..566ee96da8c 100644
--- a/src/backend/optimizer/plan/setrefs.c
+++ b/src/backend/optimizer/plan/setrefs.c
@@ -893,7 +893,7 @@ set_plan_refs(PlannerInfo *root, Plan *plan, int rtoffset)
 				splan->resultRelIndex = list_length(root->glob->resultRelations);
 				root->glob->resultRelations =
 					list_concat(root->glob->resultRelations,
-								list_copy(splan->resultRelations));
+								splan->resultRelations);
 
 				/*
 				 * If the main target relation is a partitioned table, also
diff --git a/src/backend/optimizer/prep/prepjointree.c b/src/backend/optimizer/prep/prepjointree.c
index ccb32530ad2..d13cb582272 100644
--- a/src/backend/optimizer/prep/prepjointree.c
+++ b/src/backend/optimizer/prep/prepjointree.c
@@ -2659,7 +2659,6 @@ reduce_outer_joins_pass2(Node *jtnode,
 		pass_nonnullable_rels = find_nonnullable_rels(f->quals);
 		pass_nonnullable_rels = bms_add_members(pass_nonnullable_rels,
 												nonnullable_rels);
-		/* NB: we rely on list_concat to not damage its second argument */
 		pass_nonnullable_vars = find_nonnullable_vars(f->quals);
 		pass_nonnullable_vars = list_concat(pass_nonnullable_vars,
 											nonnullable_vars);
diff --git a/src/backend/optimizer/prep/prepqual.c b/src/backend/optimizer/prep/prepqual.c
index e9a94976b73..ee919575c52 100644
--- a/src/backend/optimizer/prep/prepqual.c
+++ b/src/backend/optimizer/prep/prepqual.c
@@ -328,12 +328,6 @@ pull_ands(List *andlist)
 	{
 		Node	   *subexpr = (Node *) lfirst(arg);
 
-		/*
-		 * Note: we can destructively concat the subexpression's arglist
-		 * because we know the recursive invocation of pull_ands will have
-		 * built a new arglist not shared with any other expr. Otherwise we'd
-		 * need a list_copy here.
-		 */
 		if (is_andclause(subexpr))
 			out_list = list_concat(out_list,
 								   pull_ands(((BoolExpr *) subexpr)->args));
@@ -360,12 +354,6 @@ pull_ors(List *orlist)
 	{
 		Node	   *subexpr = (Node *) lfirst(arg);
 
-		/*
-		 * Note: we can destructively concat the subexpression's arglist
-		 * because we know the recursive invocation of pull_ors will have
-		 * built a new arglist not shared with any other expr. Otherwise we'd
-		 * need a list_copy here.
-		 */
 		if (is_orclause(subexpr))
 			out_list = list_concat(out_list,
 								   pull_ors(((BoolExpr *) subexpr)->args));
diff --git a/src/backend/optimizer/util/clauses.c b/src/backend/optimizer/util/clauses.c
index 7ad9d9ab79e..a04b62274d2 100644
--- a/src/backend/optimizer/util/clauses.c
+++ b/src/backend/optimizer/util/clauses.c
@@ -1009,8 +1009,8 @@ max_parallel_hazard_walker(Node *node, max_parallel_hazard_context *context)
 			max_parallel_hazard_test(PROPARALLEL_RESTRICTED, context))
 			return true;
 		save_safe_param_ids = context->safe_param_ids;
-		context->safe_param_ids = list_concat(list_copy(subplan->paramIds),
-											  context->safe_param_ids);
+		context->safe_param_ids = list_concat_copy(context->safe_param_ids,
+												   subplan->paramIds);
 		if (max_parallel_hazard_walker(subplan->testexpr, context))
 			return true;		/* no need to restore safe_param_ids */
 		list_free(context->safe_param_ids);
@@ -3697,18 +3697,12 @@ simplify_or_arguments(List *args,
 		/* flatten nested ORs as per above comment */
 		if (is_orclause(arg))
 		{
-			List	   *subargs = list_copy(((BoolExpr *) arg)->args);
+			List	   *subargs = ((BoolExpr *) arg)->args;
+			List	   *oldlist = unprocessed_args;
 
-			/* overly tense code to avoid leaking unused list header */
-			if (!unprocessed_args)
-				unprocessed_args = subargs;
-			else
-			{
-				List	   *oldhdr = unprocessed_args;
-
-				unprocessed_args = list_concat(subargs, unprocessed_args);
-				pfree(oldhdr);
-			}
+			unprocessed_args = list_concat_copy(subargs, unprocessed_args);
+			/* perhaps-overly-tense code to avoid leaking old lists */
+			list_free(oldlist);
 			continue;
 		}
 
@@ -3718,14 +3712,14 @@ simplify_or_arguments(List *args,
 		/*
 		 * It is unlikely but not impossible for simplification of a non-OR
 		 * clause to produce an OR.  Recheck, but don't be too tense about it
-		 * since it's not a mainstream case. In particular we don't worry
-		 * about const-simplifying the input twice.
+		 * since it's not a mainstream case.  In particular we don't worry
+		 * about const-simplifying the input twice, nor about list leakage.
 		 */
 		if (is_orclause(arg))
 		{
-			List	   *subargs = list_copy(((BoolExpr *) arg)->args);
+			List	   *subargs = ((BoolExpr *) arg)->args;
 
-			unprocessed_args = list_concat(subargs, unprocessed_args);
+			unprocessed_args = list_concat_copy(subargs, unprocessed_args);
 			continue;
 		}
 
@@ -3799,18 +3793,12 @@ simplify_and_arguments(List *args,
 		/* flatten nested ANDs as per above comment */
 		if (is_andclause(arg))
 		{
-			List	   *subargs = list_copy(((BoolExpr *) arg)->args);
+			List	   *subargs = ((BoolExpr *) arg)->args;
+			List	   *oldlist = unprocessed_args;
 
-			/* overly tense code to avoid leaking unused list header */
-			if (!unprocessed_args)
-				unprocessed_args = subargs;
-			else
-			{
-				List	   *oldhdr = unprocessed_args;
-
-				unprocessed_args = list_concat(subargs, unprocessed_args);
-				pfree(oldhdr);
-			}
+			unprocessed_args = list_concat_copy(subargs, unprocessed_args);
+			/* perhaps-overly-tense code to avoid leaking old lists */
+			list_free(oldlist);
 			continue;
 		}
 
@@ -3820,14 +3808,14 @@ simplify_and_arguments(List *args,
 		/*
 		 * It is unlikely but not impossible for simplification of a non-AND
 		 * clause to produce an AND.  Recheck, but don't be too tense about it
-		 * since it's not a mainstream case. In particular we don't worry
-		 * about const-simplifying the input twice.
+		 * since it's not a mainstream case.  In particular we don't worry
+		 * about const-simplifying the input twice, nor about list leakage.
 		 */
 		if (is_andclause(arg))
 		{
-			List	   *subargs = list_copy(((BoolExpr *) arg)->args);
+			List	   *subargs = ((BoolExpr *) arg)->args;
 
-			unprocessed_args = list_concat(subargs, unprocessed_args);
+			unprocessed_args = list_concat_copy(subargs, unprocessed_args);
 			continue;
 		}
 
@@ -4188,7 +4176,7 @@ add_function_defaults(List *args, HeapTuple func_tuple)
 		defaults = list_copy_tail(defaults, ndelete);
 
 	/* And form the combined argument list, not modifying the input list */
-	return list_concat(list_copy(args), defaults);
+	return list_concat_copy(args, defaults);
 }
 
 /*
diff --git a/src/backend/optimizer/util/orclauses.c b/src/backend/optimizer/util/orclauses.c
index 18ebc51bcac..412a3964c30 100644
--- a/src/backend/optimizer/util/orclauses.c
+++ b/src/backend/optimizer/util/orclauses.c
@@ -236,7 +236,7 @@ extract_or_clause(RestrictInfo *or_rinfo, RelOptInfo *rel)
 		subclause = (Node *) make_ands_explicit(subclauses);
 		if (is_orclause(subclause))
 			clauselist = list_concat(clauselist,
-									 list_copy(((BoolExpr *) subclause)->args));
+									 ((BoolExpr *) subclause)->args);
 		else
 			clauselist = lappend(clauselist, subclause);
 	}
diff --git a/src/backend/optimizer/util/relnode.c b/src/backend/optimizer/util/relnode.c
index 582c5dd979b..19e5a449f75 100644
--- a/src/backend/optimizer/util/relnode.c
+++ b/src/backend/optimizer/util/relnode.c
@@ -1718,43 +1718,39 @@ build_joinrel_partition_info(RelOptInfo *joinrel, RelOptInfo *outer_rel,
 	 */
 	for (cnt = 0; cnt < partnatts; cnt++)
 	{
-		List	   *outer_expr;
-		List	   *outer_null_expr;
-		List	   *inner_expr;
-		List	   *inner_null_expr;
+		/* mark these const to enforce that we copy them properly */
+		const List *outer_expr = outer_rel->partexprs[cnt];
+		const List *outer_null_expr = outer_rel->nullable_partexprs[cnt];
+		const List *inner_expr = inner_rel->partexprs[cnt];
+		const List *inner_null_expr = inner_rel->nullable_partexprs[cnt];
 		List	   *partexpr = NIL;
 		List	   *nullable_partexpr = NIL;
 
-		outer_expr = list_copy(outer_rel->partexprs[cnt]);
-		outer_null_expr = list_copy(outer_rel->nullable_partexprs[cnt]);
-		inner_expr = list_copy(inner_rel->partexprs[cnt]);
-		inner_null_expr = list_copy(inner_rel->nullable_partexprs[cnt]);
-
 		switch (jointype)
 		{
 			case JOIN_INNER:
-				partexpr = list_concat(outer_expr, inner_expr);
-				nullable_partexpr = list_concat(outer_null_expr,
-												inner_null_expr);
+				partexpr = list_concat_copy(outer_expr, inner_expr);
+				nullable_partexpr = list_concat_copy(outer_null_expr,
+													 inner_null_expr);
 				break;
 
 			case JOIN_SEMI:
 			case JOIN_ANTI:
-				partexpr = outer_expr;
-				nullable_partexpr = outer_null_expr;
+				partexpr = list_copy(outer_expr);
+				nullable_partexpr = list_copy(outer_null_expr);
 				break;
 
 			case JOIN_LEFT:
-				partexpr = outer_expr;
-				nullable_partexpr = list_concat(inner_expr,
-												outer_null_expr);
+				partexpr = list_copy(outer_expr);
+				nullable_partexpr = list_concat_copy(inner_expr,
+													 outer_null_expr);
 				nullable_partexpr = list_concat(nullable_partexpr,
 												inner_null_expr);
 				break;
 
 			case JOIN_FULL:
-				nullable_partexpr = list_concat(outer_expr,
-												inner_expr);
+				nullable_partexpr = list_concat_copy(outer_expr,
+													 inner_expr);
 				nullable_partexpr = list_concat(nullable_partexpr,
 												outer_null_expr);
 				nullable_partexpr = list_concat(nullable_partexpr,
diff --git a/src/backend/optimizer/util/tlist.c b/src/backend/optimizer/util/tlist.c
index 7ccb10e4e1b..d75796ac8b9 100644
--- a/src/backend/optimizer/util/tlist.c
+++ b/src/backend/optimizer/util/tlist.c
@@ -665,9 +665,8 @@ make_tlist_from_pathtarget(PathTarget *target)
  * copy_pathtarget
  *	  Copy a PathTarget.
  *
- * The new PathTarget has its own List cells, but shares the underlying
- * target expression trees with the old one.  We duplicate the List cells
- * so that items can be added to one target without damaging the other.
+ * The new PathTarget has its own exprs List, but shares the underlying
+ * target expression trees with the old one.
  */
 PathTarget *
 copy_pathtarget(PathTarget *src)
diff --git a/src/backend/parser/parse_agg.c b/src/backend/parser/parse_agg.c
index 354030e549b..f418c615458 100644
--- a/src/backend/parser/parse_agg.c
+++ b/src/backend/parser/parse_agg.c
@@ -1649,9 +1649,8 @@ expand_groupingset_node(GroupingSet *gs)
 
 						Assert(gs_current->kind == GROUPING_SET_SIMPLE);
 
-						current_result
-							= list_concat(current_result,
-										  list_copy(gs_current->content));
+						current_result = list_concat(current_result,
+													 gs_current->content);
 
 						/* If we are done with making the current group, break */
 						if (--i == 0)
@@ -1691,11 +1690,8 @@ expand_groupingset_node(GroupingSet *gs)
 						Assert(gs_current->kind == GROUPING_SET_SIMPLE);
 
 						if (mask & i)
-						{
-							current_result
-								= list_concat(current_result,
-											  list_copy(gs_current->content));
-						}
+							current_result = list_concat(current_result,
+														 gs_current->content);
 
 						mask <<= 1;
 					}
diff --git a/src/backend/parser/parse_clause.c b/src/backend/parser/parse_clause.c
index 2a6b2ff153b..260ccd4d7fa 100644
--- a/src/backend/parser/parse_clause.c
+++ b/src/backend/parser/parse_clause.c
@@ -1214,9 +1214,6 @@ transformFromClauseItem(ParseState *pstate, Node *n,
 		 *
 		 * Notice that we don't require the merged namespace list to be
 		 * conflict-free.  See the comments for scanNameSpaceForRefname().
-		 *
-		 * NB: this coding relies on the fact that list_concat is not
-		 * destructive to its second argument.
 		 */
 		lateral_ok = (j->jointype == JOIN_INNER || j->jointype == JOIN_LEFT);
 		setNamespaceLateralState(l_namespace, true, lateral_ok);
@@ -2116,9 +2113,7 @@ flatten_grouping_sets(Node *expr, bool toplevel, bool *hasGroupingSets)
 
 					if (IsA(n1, GroupingSet) &&
 						((GroupingSet *) n1)->kind == GROUPING_SET_SETS)
-					{
 						result_set = list_concat(result_set, (List *) n2);
-					}
 					else
 						result_set = lappend(result_set, n2);
 				}
diff --git a/src/backend/partitioning/partprune.c b/src/backend/partitioning/partprune.c
index 2ee47b8c8a9..a1bd4efd0b0 100644
--- a/src/backend/partitioning/partprune.c
+++ b/src/backend/partitioning/partprune.c
@@ -642,8 +642,8 @@ gen_partprune_steps(RelOptInfo *rel, List *clauses, PartClauseTarget target,
 		if (rel->relid != 1)
 			ChangeVarNodes((Node *) partqual, 1, rel->relid, 0);
 
-		/* Use list_copy to avoid modifying the passed-in List */
-		clauses = list_concat(list_copy(clauses), partqual);
+		/* Make a copy to avoid modifying the passed-in List */
+		clauses = list_concat_copy(clauses, partqual);
 	}
 
 	/* Down into the rabbit-hole. */
@@ -1485,7 +1485,7 @@ gen_prune_steps_from_opexps(GeneratePruningStepsContext *context,
 														  pc->keyno,
 														  NULL,
 														  prefix);
-						opsteps = list_concat(opsteps, list_copy(pc_steps));
+						opsteps = list_concat(opsteps, pc_steps);
 					}
 				}
 				break;
@@ -1556,7 +1556,7 @@ gen_prune_steps_from_opexps(GeneratePruningStepsContext *context,
 												   pc->keyno,
 												   nullkeys,
 												   prefix);
-						opsteps = list_concat(opsteps, list_copy(pc_steps));
+						opsteps = list_concat(opsteps, pc_steps);
 					}
 				}
 				break;
diff --git a/src/backend/replication/syncrep.c b/src/backend/replication/syncrep.c
index 79c7c13ada0..a21f7d33471 100644
--- a/src/backend/replication/syncrep.c
+++ b/src/backend/replication/syncrep.c
@@ -865,8 +865,6 @@ SyncRepGetSyncStandbysPriority(bool *am_sync)
 	 */
 	if (list_length(result) + list_length(pending) <= SyncRepConfig->num_sync)
 	{
-		bool		needfree = (result != NIL && pending != NIL);
-
 		/*
 		 * Set *am_sync to true if this walsender is in the pending list
 		 * because all pending standbys are considered as sync.
@@ -875,8 +873,7 @@ SyncRepGetSyncStandbysPriority(bool *am_sync)
 			*am_sync = am_in_pending;
 
 		result = list_concat(result, pending);
-		if (needfree)
-			pfree(pending);
+		list_free(pending);
 		return result;
 	}
 
diff --git a/src/backend/rewrite/rewriteHandler.c b/src/backend/rewrite/rewriteHandler.c
index c96ef8595af..3e38007643e 100644
--- a/src/backend/rewrite/rewriteHandler.c
+++ b/src/backend/rewrite/rewriteHandler.c
@@ -1041,11 +1041,11 @@ process_matched_tle(TargetEntry *src_tle,
 			/* combine the two */
 			memcpy(fstore, prior_expr, sizeof(FieldStore));
 			fstore->newvals =
-				list_concat(list_copy(((FieldStore *) prior_expr)->newvals),
-							list_copy(((FieldStore *) src_expr)->newvals));
+				list_concat_copy(((FieldStore *) prior_expr)->newvals,
+								 ((FieldStore *) src_expr)->newvals);
 			fstore->fieldnums =
-				list_concat(list_copy(((FieldStore *) prior_expr)->fieldnums),
-							list_copy(((FieldStore *) src_expr)->fieldnums));
+				list_concat_copy(((FieldStore *) prior_expr)->fieldnums,
+								 ((FieldStore *) src_expr)->fieldnums);
 		}
 		else
 		{
diff --git a/src/backend/utils/adt/ruleutils.c b/src/backend/utils/adt/ruleutils.c
index 13d5d542f94..3e64390d819 100644
--- a/src/backend/utils/adt/ruleutils.c
+++ b/src/backend/utils/adt/ruleutils.c
@@ -10097,7 +10097,7 @@ get_from_clause_item(Node *jtnode, Query *query, deparse_context *context)
 							RangeTblFunction *rtfunc = (RangeTblFunction *) lfirst(lc);
 							List	   *args = ((FuncExpr *) rtfunc->funcexpr)->args;
 
-							allargs = list_concat(allargs, list_copy(args));
+							allargs = list_concat(allargs, args);
 						}
 
 						appendStringInfoString(buf, "UNNEST(");
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index 7eba59eff32..17101298fb0 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -5768,7 +5768,6 @@ add_predicate_to_index_quals(IndexOptInfo *index, List *indexQuals)
 		if (!predicate_implied_by(oneQual, indexQuals, false))
 			predExtraQuals = list_concat(predExtraQuals, oneQual);
 	}
-	/* list_concat avoids modifying the passed-in indexQuals list */
 	return list_concat(predExtraQuals, indexQuals);
 }
 
diff --git a/src/include/nodes/pg_list.h b/src/include/nodes/pg_list.h
index 146340877c3..409d840e793 100644
--- a/src/include/nodes/pg_list.h
+++ b/src/include/nodes/pg_list.h
@@ -519,6 +519,8 @@ extern List *lcons_int(int datum, List *list);
 extern List *lcons_oid(Oid datum, List *list);
 
 extern List *list_concat(List *list1, const List *list2);
+extern List *list_concat_copy(const List *list1, const List *list2);
+
 extern List *list_truncate(List *list, int new_size);
 
 extern bool list_member(const List *list, const void *datum);