Avoid using lcons and list_delete_first where it's easy to do so.

Formerly, lcons was about the same speed as lappend, but with the new
List implementation, that's not so; with a long List, data movement
imposes an O(N) cost on lcons and list_delete_first, but not lappend.

Hence, invent list_delete_last with semantics parallel to
list_delete_first (but O(1) cost), and change various places to use
lappend and list_delete_last where this can be done without much
violence to the code logic.

There are quite a few places that construct result lists using lcons not
lappend.  Some have semantic rationales for that; I added comments about
it to a couple that didn't have them already.  In many such places though,
I think the coding is that way only because back in the dark ages lcons
was faster than lappend.  Hence, switch to lappend where this can be done
without causing semantic changes.

In ExecInitExprRec(), this results in aggregates and window functions that
are in the same plan node being executed in a different order than before.
Generally, the executions of such functions ought to be independent of
each other, so this shouldn't result in visibly different query results.
But if you push it, as one regression test case does, you can show that
the order is different.  The new order seems saner; it's closer to
the order of the functions in the query text.  And we never documented
or promised anything about this, anyway.

Also, in gistfinishsplit(), don't bother building a reverse-order list;
it's easy now to iterate backwards through the original list.

It'd be possible to go further towards removing uses of lcons and
list_delete_first, but it'd require more extensive logic changes,
and I'm not convinced it's worth it.  Most of the remaining uses
deal with queues that probably never get long enough to be worth
sweating over.  (Actually, I doubt that any of the changes in this
patch will have measurable performance effects either.  But better
to have good examples than bad ones in the code base.)

Patch by me, thanks to David Rowley and Daniel Gustafsson for review.

Discussion: https://postgr.es/m/21272.1563318411@sss.pgh.pa.us

Author: tglsfdc

src/backend/access/gist/gist.c

@@ -1323,8 +1323,6 @@ static void
 gistfinishsplit(GISTInsertState *state, GISTInsertStack *stack,
 				GISTSTATE *giststate, List *splitinfo, bool unlockbuf)
 {
-	ListCell   *lc;
-	List	   *reversed;
 	GISTPageSplitInfo *right;
 	GISTPageSplitInfo *left;
 	IndexTuple	tuples[2];
@@ -1339,25 +1337,17 @@ gistfinishsplit(GISTInsertState *state, GISTInsertStack *stack,
 	 * left. Finally insert the downlink for the last new page and update the
 	 * downlink for the original page as one operation.
 	 */
-
-	/* for convenience, create a copy of the list in reverse order */
-	reversed = NIL;
-	foreach(lc, splitinfo)
-	{
-		reversed = lcons(lfirst(lc), reversed);
-	}
-
 	LockBuffer(stack->parent->buffer, GIST_EXCLUSIVE);
 	gistFindCorrectParent(state->r, stack);
 
 	/*
 	 * insert downlinks for the siblings from right to left, until there are
 	 * only two siblings left.
 	 */
-	while (list_length(reversed) > 2)
+	for (int pos = list_length(splitinfo) - 1; pos > 1; pos--)
 	{
-		right = (GISTPageSplitInfo *) linitial(reversed);
-		left = (GISTPageSplitInfo *) lsecond(reversed);
+		right = (GISTPageSplitInfo *) list_nth(splitinfo, pos);
+		left = (GISTPageSplitInfo *) list_nth(splitinfo, pos - 1);
 
 		if (gistinserttuples(state, stack->parent, giststate,
 							 &right->downlink, 1,
@@ -1371,11 +1361,10 @@ gistfinishsplit(GISTInsertState *state, GISTInsertStack *stack,
 			gistFindCorrectParent(state->r, stack);
 		}
 		/* gistinserttuples() released the lock on right->buf. */
-		reversed = list_delete_first(reversed);
 	}
 
-	right = (GISTPageSplitInfo *) linitial(reversed);
-	left = (GISTPageSplitInfo *) lsecond(reversed);
+	right = (GISTPageSplitInfo *) lsecond(splitinfo);
+	left = (GISTPageSplitInfo *) linitial(splitinfo);
 
 	/*
 	 * Finally insert downlink for the remaining right page and update the

src/backend/catalog/heap.c

@@ -633,7 +633,7 @@ CheckAttributeType(const char *attname,
 					 errmsg("composite type %s cannot be made a member of itself",
 							format_type_be(atttypid))));
 
-		containing_rowtypes = lcons_oid(atttypid, containing_rowtypes);
+		containing_rowtypes = lappend_oid(containing_rowtypes, atttypid);
 
 		relation = relation_open(get_typ_typrelid(atttypid), AccessShareLock);
 
@@ -653,7 +653,7 @@ CheckAttributeType(const char *attname,
 
 		relation_close(relation, AccessShareLock);
 
-		containing_rowtypes = list_delete_first(containing_rowtypes);
+		containing_rowtypes = list_delete_last(containing_rowtypes);
 	}
 	else if (OidIsValid((att_typelem = get_element_type(atttypid))))
 	{

src/backend/commands/cluster.c

@@ -1566,7 +1566,7 @@ get_tables_to_cluster(MemoryContext cluster_context)
 		rvtc = (RelToCluster *) palloc(sizeof(RelToCluster));
 		rvtc->tableOid = index->indrelid;
 		rvtc->indexOid = index->indexrelid;
-		rvs = lcons(rvtc, rvs);
+		rvs = lappend(rvs, rvtc);
 
 		MemoryContextSwitchTo(old_context);
 	}

src/backend/commands/lockcmds.c

@@ -281,11 +281,11 @@ LockViewRecurse(Oid reloid, LOCKMODE lockmode, bool nowait, List *ancestor_views
 	context.nowait = nowait;
 	context.viewowner = view->rd_rel->relowner;
 	context.viewoid = reloid;
-	context.ancestor_views = lcons_oid(reloid, ancestor_views);
+	context.ancestor_views = lappend_oid(ancestor_views, reloid);
 
 	LockViewRecurse_walker((Node *) viewquery, &context);
 
-	ancestor_views = list_delete_oid(ancestor_views, reloid);
+	(void) list_delete_last(context.ancestor_views);
 
 	table_close(view, NoLock);
 }

src/backend/commands/tablecmds.c

@@ -14480,6 +14480,11 @@ register_on_commit_action(Oid relid, OnCommitAction action)
 	oc->creating_subid = GetCurrentSubTransactionId();
 	oc->deleting_subid = InvalidSubTransactionId;
 
+	/*
+	 * We use lcons() here so that ON COMMIT actions are processed in reverse
+	 * order of registration.  That might not be essential but it seems
+	 * reasonable.
+	 */
 	on_commits = lcons(oc, on_commits);
 
 	MemoryContextSwitchTo(oldcxt);

src/backend/commands/typecmds.c

@@ -2999,7 +2999,7 @@ get_rels_with_domain(Oid domainOid, LOCKMODE lockmode)
 			rtc->rel = rel;
 			rtc->natts = 0;
 			rtc->atts = (int *) palloc(sizeof(int) * RelationGetNumberOfAttributes(rel));
-			result = lcons(rtc, result);
+			result = lappend(result, rtc);
 		}
 
 		/*

src/backend/executor/execExpr.c

@@ -786,7 +786,7 @@ ExecInitExprRec(Expr *node, ExprState *state,
 				{
 					AggState   *aggstate = (AggState *) state->parent;
 
-					aggstate->aggs = lcons(astate, aggstate->aggs);
+					aggstate->aggs = lappend(aggstate->aggs, astate);
 					aggstate->numaggs++;
 				}
 				else
@@ -834,7 +834,7 @@ ExecInitExprRec(Expr *node, ExprState *state,
 					WindowAggState *winstate = (WindowAggState *) state->parent;
 					int			nfuncs;
 
-					winstate->funcs = lcons(wfstate, winstate->funcs);
+					winstate->funcs = lappend(winstate->funcs, wfstate);
 					nfuncs = ++winstate->numfuncs;
 					if (wfunc->winagg)
 						winstate->numaggs++;

src/backend/nodes/list.c

@@ -826,6 +826,30 @@ list_delete_first(List *list)
 	return list_delete_nth_cell(list, 0);
 }
 
+/*
+ * Delete the last element of the list.
+ *
+ * This is the opposite of list_delete_first(), but is noticeably cheaper
+ * with a long list, since no data need be moved.
+ */
+List *
+list_delete_last(List *list)
+{
+	check_list_invariants(list);
+
+	if (list == NIL)
+		return NIL;				/* would an error be better? */
+
+	/* list_truncate won't free list if it goes to empty, but this should */
+	if (list_length(list) <= 1)
+	{
+		list_free(list);
+		return NIL;
+	}
+
+	return list_truncate(list, list_length(list) - 1);
+}
+
 /*
  * Generate the union of two lists. This is calculated by copying
  * list1 via list_copy(), then adding to it all the members of list2

src/backend/optimizer/util/clauses.c

@@ -881,11 +881,9 @@ is_parallel_safe(PlannerInfo *root, Node *node)
 		foreach(l, proot->init_plans)
 		{
 			SubPlan    *initsubplan = (SubPlan *) lfirst(l);
-			ListCell   *l2;
 
-			foreach(l2, initsubplan->setParam)
-				context.safe_param_ids = lcons_int(lfirst_int(l2),
-												   context.safe_param_ids);
+			context.safe_param_ids = list_concat(context.safe_param_ids,
+												 initsubplan->setParam);
 		}
 	}
 
@@ -1015,6 +1013,7 @@ max_parallel_hazard_walker(Node *node, max_parallel_hazard_context *context)
 											  context->safe_param_ids);
 		if (max_parallel_hazard_walker(subplan->testexpr, context))
 			return true;		/* no need to restore safe_param_ids */
+		list_free(context->safe_param_ids);
 		context->safe_param_ids = save_safe_param_ids;
 		/* we must also check args, but no special Param treatment there */
 		if (max_parallel_hazard_walker((Node *) subplan->args, context))
@@ -4185,8 +4184,8 @@ add_function_defaults(List *args, HeapTuple func_tuple)
 	ndelete = nargsprovided + list_length(defaults) - funcform->pronargs;
 	if (ndelete < 0)
 		elog(ERROR, "not enough default arguments");
-	while (ndelete-- > 0)
-		defaults = list_delete_first(defaults);
+	if (ndelete > 0)
+		defaults = list_copy_tail(defaults, ndelete);
 
 	/* And form the combined argument list, not modifying the input list */
 	return list_concat(list_copy(args), defaults);
@@ -4701,9 +4700,9 @@ inline_function(Oid funcid, Oid result_type, Oid result_collid,
 	 * Recursively try to simplify the modified expression.  Here we must add
 	 * the current function to the context list of active functions.
 	 */
-	context->active_fns = lcons_oid(funcid, context->active_fns);
+	context->active_fns = lappend_oid(context->active_fns, funcid);
 	newexpr = eval_const_expressions_mutator(newexpr, context);
-	context->active_fns = list_delete_first(context->active_fns);
+	context->active_fns = list_delete_last(context->active_fns);
 
 	error_context_stack = sqlerrcontext.previous;
 

src/backend/optimizer/util/plancat.c

@@ -419,6 +419,13 @@ get_relation_info(PlannerInfo *root, Oid relationObjectId, bool inhparent,
 
 			index_close(indexRelation, NoLock);
 
+			/*
+			 * We've historically used lcons() here.  It'd make more sense to
+			 * use lappend(), but that causes the planner to change behavior
+			 * in cases where two indexes seem equally attractive.  For now,
+			 * stick with lcons() --- few tables should have so many indexes
+			 * that the O(N^2) behavior of lcons() is really a problem.
+			 */
 			indexinfos = lcons(info, indexinfos);
 		}
 
@@ -1339,7 +1346,7 @@ get_relation_statistics(RelOptInfo *rel, Relation relation)
 			info->kind = STATS_EXT_NDISTINCT;
 			info->keys = bms_copy(keys);
 
-			stainfos = lcons(info, stainfos);
+			stainfos = lappend(stainfos, info);
 		}
 
 		if (statext_is_kind_built(dtup, STATS_EXT_DEPENDENCIES))
@@ -1351,7 +1358,7 @@ get_relation_statistics(RelOptInfo *rel, Relation relation)
 			info->kind = STATS_EXT_DEPENDENCIES;
 			info->keys = bms_copy(keys);
 
-			stainfos = lcons(info, stainfos);
+			stainfos = lappend(stainfos, info);
 		}
 
 		if (statext_is_kind_built(dtup, STATS_EXT_MCV))
@@ -1363,7 +1370,7 @@ get_relation_statistics(RelOptInfo *rel, Relation relation)
 			info->kind = STATS_EXT_MCV;
 			info->keys = bms_copy(keys);
 
-			stainfos = lcons(info, stainfos);
+			stainfos = lappend(stainfos, info);
 		}
 
 		ReleaseSysCache(htup);

src/backend/parser/parse_agg.c

@@ -1132,7 +1132,7 @@ parseCheckAggregates(ParseState *pstate, Query *qry)
 		if (expr == NULL)
 			continue;			/* probably cannot happen */
 
-		groupClauses = lcons(expr, groupClauses);
+		groupClauses = lappend(groupClauses, expr);
 	}
 
 	/*

src/backend/rewrite/rewriteHandler.c

@@ -1973,7 +1973,7 @@ fireRIRrules(Query *parsetree, List *activeRIRs)
 							(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
 							 errmsg("infinite recursion detected in rules for relation \"%s\"",
 									RelationGetRelationName(rel))));
-				activeRIRs = lcons_oid(RelationGetRelid(rel), activeRIRs);
+				activeRIRs = lappend_oid(activeRIRs, RelationGetRelid(rel));
 
 				foreach(l, locks)
 				{
@@ -1986,7 +1986,7 @@ fireRIRrules(Query *parsetree, List *activeRIRs)
 												  activeRIRs);
 				}
 
-				activeRIRs = list_delete_first(activeRIRs);
+				activeRIRs = list_delete_last(activeRIRs);
 			}
 		}
 
@@ -2059,7 +2059,7 @@ fireRIRrules(Query *parsetree, List *activeRIRs)
 							 errmsg("infinite recursion detected in policy for relation \"%s\"",
 									RelationGetRelationName(rel))));
 
-				activeRIRs = lcons_oid(RelationGetRelid(rel), activeRIRs);
+				activeRIRs = lappend_oid(activeRIRs, RelationGetRelid(rel));
 
 				/*
 				 * get_row_security_policies just passed back securityQuals
@@ -2084,7 +2084,7 @@ fireRIRrules(Query *parsetree, List *activeRIRs)
 				expression_tree_walker((Node *) withCheckOptions,
 									   fireRIRonSubLink, (void *) activeRIRs);
 
-				activeRIRs = list_delete_first(activeRIRs);
+				activeRIRs = list_delete_last(activeRIRs);
 			}
 
 			/*
@@ -3711,7 +3711,7 @@ RewriteQuery(Query *parsetree, List *rewrite_events)
 			rev = (rewrite_event *) palloc(sizeof(rewrite_event));
 			rev->relation = RelationGetRelid(rt_entry_relation);
 			rev->event = event;
-			rewrite_events = lcons(rev, rewrite_events);
+			rewrite_events = lappend(rewrite_events, rev);
 
 			foreach(n, product_queries)
 			{
@@ -3722,7 +3722,7 @@ RewriteQuery(Query *parsetree, List *rewrite_events)
 				rewritten = list_concat(rewritten, newstuff);
 			}
 
-			rewrite_events = list_delete_first(rewrite_events);
+			rewrite_events = list_delete_last(rewrite_events);
 		}
 
 		/*

src/backend/utils/adt/selfuncs.c

@@ -3201,20 +3201,20 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows,
 		 * Split the list of varinfos in two - one for the current rel, one
 		 * for remaining Vars on other rels.
 		 */
-		relvarinfos = lcons(varinfo1, relvarinfos);
+		relvarinfos = lappend(relvarinfos, varinfo1);
 		for_each_cell(l, varinfos, list_second_cell(varinfos))
 		{
 			GroupVarInfo *varinfo2 = (GroupVarInfo *) lfirst(l);
 
 			if (varinfo2->rel == varinfo1->rel)
 			{
 				/* varinfos on current rel */
-				relvarinfos = lcons(varinfo2, relvarinfos);
+				relvarinfos = lappend(relvarinfos, varinfo2);
 			}
 			else
 			{
 				/* not time to process varinfo2 yet */
-				newvarinfos = lcons(varinfo2, newvarinfos);
+				newvarinfos = lappend(newvarinfos, varinfo2);
 			}
 		}
 

src/include/nodes/pg_list.h

@@ -531,6 +531,7 @@ extern List *list_delete_ptr(List *list, void *datum);
 extern List *list_delete_int(List *list, int datum);
 extern List *list_delete_oid(List *list, Oid datum);
 extern List *list_delete_first(List *list);
+extern List *list_delete_last(List *list);
 extern List *list_delete_nth_cell(List *list, int n);
 extern List *list_delete_cell(List *list, ListCell *cell);
 

src/test/regress/expected/aggregates.out

@@ -2030,10 +2030,10 @@ NOTICE:  avg_transfn called with 3
 
 -- this should not share the state due to different input columns.
 select my_avg(one),my_sum(two) from (values(1,2),(3,4)) t(one,two);
-NOTICE:  avg_transfn called with 2
 NOTICE:  avg_transfn called with 1
-NOTICE:  avg_transfn called with 4
+NOTICE:  avg_transfn called with 2
 NOTICE:  avg_transfn called with 3
+NOTICE:  avg_transfn called with 4
  my_avg | my_sum 
 --------+--------
       2 |      6