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

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;
 	}
 

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,

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));

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

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);
 
 		/*

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);

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

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);

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));

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);
 }
 
 /*

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);
 	}

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,

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)