Avoid using list_length() to test for empty list.

The standard way to check for list emptiness is to compare the
List pointer to NIL; our list code goes out of its way to ensure
that that is the only representation of an empty list.  (An
acceptable alternative is a plain boolean test for non-null
pointer, but explicit mention of NIL is usually preferable.)

Various places didn't get that memo and expressed the condition
with list_length(), which might not be so bad except that there
were such a variety of ways to check it exactly: equal to zero,
less than or equal to zero, less than one, yadda yadda.  In the
name of code readability, let's standardize all those spellings
as "list == NIL" or "list != NIL".  (There's probably some
microscopic efficiency gain too, though few of these look to be
at all performance-critical.)

A very small number of cases were left as-is because they seemed
more consistent with other adjacent list_length tests that way.

Peter Smith, with bikeshedding from a number of us

Discussion: https://postgr.es/m/CAHut+PtQYe+ENX5KrONMfugf0q6NHg4hR5dAhqEXEc2eefFeig@mail.gmail.com

src/backend/catalog/objectaddress.c

@@ -2186,7 +2186,7 @@ pg_get_object_address(PG_FUNCTION_ARGS)
 	else
 	{
 		name = textarray_to_strvaluelist(namearr);
-		if (list_length(name) < 1)
+		if (name == NIL)
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 					 errmsg("name list length must be at least %d", 1)));

src/backend/catalog/pg_depend.c

@@ -947,7 +947,7 @@ getIdentitySequence(Oid relid, AttrNumber attnum, bool missing_ok)
 
 	if (list_length(seqlist) > 1)
 		elog(ERROR, "more than one owned sequence found");
-	else if (list_length(seqlist) < 1)
+	else if (seqlist == NIL)
 	{
 		if (missing_ok)
 			return InvalidOid;

src/backend/commands/event_trigger.c

@@ -1143,9 +1143,9 @@ trackDroppedObjectsNeeded(void)
 	 * true if any sql_drop, table_rewrite, ddl_command_end event trigger
 	 * exists
 	 */
-	return list_length(EventCacheLookup(EVT_SQLDrop)) > 0 ||
+	return (EventCacheLookup(EVT_SQLDrop) != NIL) ||
-		list_length(EventCacheLookup(EVT_TableRewrite)) > 0 ||
+		(EventCacheLookup(EVT_TableRewrite) != NIL) ||
-		list_length(EventCacheLookup(EVT_DDLCommandEnd)) > 0;
+		(EventCacheLookup(EVT_DDLCommandEnd) != NIL);
 }
 
 /*
@@ -1616,7 +1616,7 @@ EventTriggerAlterTableEnd(void)
 	parent = currentEventTriggerState->currentCommand->parent;
 
 	/* If no subcommands, don't collect */
-	if (list_length(currentEventTriggerState->currentCommand->d.alterTable.subcmds) != 0)
+	if (currentEventTriggerState->currentCommand->d.alterTable.subcmds != NIL)
 	{
 		MemoryContext oldcxt;
 

src/backend/commands/functioncmds.c

@@ -419,7 +419,7 @@ interpret_function_parameter_list(ParseState *pstate,
 			 * Make sure no variables are referred to (this is probably dead
 			 * code now that add_missing_from is history).
 			 */
-			if (list_length(pstate->p_rtable) != 0 ||
+			if (pstate->p_rtable != NIL ||
 				contain_var_clause(def))
 				ereport(ERROR,
 						(errcode(ERRCODE_INVALID_COLUMN_REFERENCE),
@@ -1209,7 +1209,7 @@ CreateFunction(ParseState *pstate, CreateFunctionStmt *stmt)
 		returnsSet = false;
 	}
 
-	if (list_length(trftypes_list) > 0)
+	if (trftypes_list != NIL)
 	{
 		ListCell   *lc;
 		Datum	   *arr;

src/backend/commands/publicationcmds.c

@@ -848,12 +848,12 @@ CreatePublication(ParseState *pstate, CreatePublicationStmt *stmt)
 								   &schemaidlist);
 
 		/* FOR ALL TABLES IN SCHEMA requires superuser */
-		if (list_length(schemaidlist) > 0 && !superuser())
+		if (schemaidlist != NIL && !superuser())
 			ereport(ERROR,
 					errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
 					errmsg("must be superuser to create FOR ALL TABLES IN SCHEMA publication"));
 
-		if (list_length(relations) > 0)
+		if (relations != NIL)
 		{
 			List	   *rels;
 
@@ -871,7 +871,7 @@ CreatePublication(ParseState *pstate, CreatePublicationStmt *stmt)
 			CloseTableList(rels);
 		}
 
-		if (list_length(schemaidlist) > 0)
+		if (schemaidlist != NIL)
 		{
 			/*
 			 * Schema lock is held until the publication is created to prevent

src/backend/commands/statscmds.c

@@ -339,7 +339,7 @@ CreateStatistics(CreateStatsStmt *stmt)
 	if ((list_length(stmt->exprs) == 1) && (list_length(stxexprs) == 1))
 	{
 		/* statistics kinds not specified */
-		if (list_length(stmt->stat_types) > 0)
+		if (stmt->stat_types != NIL)
 			ereport(ERROR,
 					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 					 errmsg("when building statistics on a single expression, statistics kinds may not be specified")));
@@ -391,7 +391,7 @@ CreateStatistics(CreateStatsStmt *stmt)
 	 * automatically. This allows calculating good estimates for stats that
 	 * consider per-clause estimates (e.g. functional dependencies).
 	 */
-	build_expressions = (list_length(stxexprs) > 0);
+	build_expressions = (stxexprs != NIL);
 
 	/*
 	 * Check that at least two columns were specified in the statement, or

src/backend/commands/subscriptioncmds.c

@@ -410,7 +410,7 @@ get_publications_str(List *publications, StringInfo dest, bool quote_literal)
 	ListCell   *lc;
 	bool		first = true;
 
-	Assert(list_length(publications) > 0);
+	Assert(publications != NIL);
 
 	foreach(lc, publications)
 	{

src/backend/commands/tablecmds.c

@@ -2097,7 +2097,7 @@ ExecuteTruncateGuts(List *explicit_rels,
 	 * Assemble an array of relids so we can write a single WAL record for the
 	 * whole action.
 	 */
-	if (list_length(relids_logged) > 0)
+	if (relids_logged != NIL)
 	{
 		xl_heap_truncate xlrec;
 		int			i = 0;
@@ -16264,11 +16264,11 @@ ATPrepChangePersistence(Relation rel, bool toLogged)
 	}
 
 	/*
-	 * Check that the table is not part any publication when changing to
+	 * Check that the table is not part of any publication when changing to
-	 * UNLOGGED as UNLOGGED tables can't be published.
+	 * UNLOGGED, as UNLOGGED tables can't be published.
 	 */
 	if (!toLogged &&
-		list_length(GetRelationPublications(RelationGetRelid(rel))) > 0)
+		GetRelationPublications(RelationGetRelid(rel)) != NIL)
 		ereport(ERROR,
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 				 errmsg("cannot change table \"%s\" to unlogged because it is part of a publication",

src/backend/commands/typecmds.c

@@ -3503,7 +3503,7 @@ domainAddConstraint(Oid domainOid, Oid domainNamespace, Oid baseTypeOid,
 	 * Domains don't allow variables (this is probably dead code now that
 	 * add_missing_from is history, but let's be sure).
 	 */
-	if (list_length(pstate->p_rtable) != 0 ||
+	if (pstate->p_rtable != NIL ||
 		contain_var_clause(expr))
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_COLUMN_REFERENCE),

src/backend/executor/execPartition.c

@@ -685,7 +685,7 @@ ExecInitPartitionInfo(ModifyTableState *mtstate, EState *estate,
 		 * list and searching for ancestry relationships to each index in the
 		 * ancestor table.
 		 */
-		if (list_length(rootResultRelInfo->ri_onConflictArbiterIndexes) > 0)
+		if (rootResultRelInfo->ri_onConflictArbiterIndexes != NIL)
 		{
 			List	   *childIdxs;
 

src/backend/libpq/auth.c

@@ -2915,14 +2915,14 @@ CheckRADIUSAuth(Port *port)
 	Assert(offsetof(radius_packet, vector) == 4);
 
 	/* Verify parameters */
-	if (list_length(port->hba->radiusservers) < 1)
+	if (port->hba->radiusservers == NIL)
 	{
 		ereport(LOG,
 				(errmsg("RADIUS server not specified")));
 		return STATUS_ERROR;
 	}
 
-	if (list_length(port->hba->radiussecrets) < 1)
+	if (port->hba->radiussecrets == NIL)
 	{
 		ereport(LOG,
 				(errmsg("RADIUS secret not specified")));

src/backend/libpq/hba.c

@@ -1564,7 +1564,7 @@ parse_hba_line(TokenizedAuthLine *tok_line, int elevel)
 		MANDATORY_AUTH_ARG(parsedline->radiusservers, "radiusservers", "radius");
 		MANDATORY_AUTH_ARG(parsedline->radiussecrets, "radiussecrets", "radius");
 
-		if (list_length(parsedline->radiusservers) < 1)
+		if (parsedline->radiusservers == NIL)
 		{
 			ereport(elevel,
 					(errcode(ERRCODE_CONFIG_FILE_ERROR),
@@ -1575,7 +1575,7 @@ parse_hba_line(TokenizedAuthLine *tok_line, int elevel)
 			return NULL;
 		}
 
-		if (list_length(parsedline->radiussecrets) < 1)
+		if (parsedline->radiussecrets == NIL)
 		{
 			ereport(elevel,
 					(errcode(ERRCODE_CONFIG_FILE_ERROR),

src/backend/optimizer/path/costsize.c

@@ -1957,7 +1957,7 @@ compute_cpu_sort_cost(PlannerInfo *root, List *pathkeys, int nPresortedKeys,
 	List	   *cache_varinfos = NIL;
 
 	/* fallback if pathkeys is unknown */
-	if (list_length(pathkeys) == 0)
+	if (pathkeys == NIL)
 	{
 		/*
 		 * If we'll use a bounded heap-sort keeping just K tuples in memory,

src/backend/optimizer/plan/createplan.c

@@ -2462,7 +2462,7 @@ create_groupingsets_plan(PlannerInfo *root, GroupingSetsPath *best_path)
 
 			if (rollup->is_hashed)
 				strat = AGG_HASHED;
-			else if (list_length(linitial(rollup->gsets)) == 0)
+			else if (linitial(rollup->gsets) == NIL)
 				strat = AGG_PLAIN;
 			else
 				strat = AGG_SORTED;

src/backend/optimizer/plan/planner.c

@@ -3097,7 +3097,7 @@ reorder_grouping_sets(List *groupingsets, List *sortclause)
 		GroupingSetData *gs = makeNode(GroupingSetData);
 
 		while (list_length(sortclause) > list_length(previous) &&
-			   list_length(new_elems) > 0)
+			   new_elems != NIL)
 		{
 			SortGroupClause *sc = list_nth(sortclause, list_length(previous));
 			int			ref = sc->tleSortGroupRef;
@@ -4120,7 +4120,7 @@ consider_groupingsets_paths(PlannerInfo *root,
 	/*
 	 * If we have sorted input but nothing we can do with it, bail.
 	 */
-	if (list_length(gd->rollups) == 0)
+	if (gd->rollups == NIL)
 		return;
 
 	/*
@@ -6477,7 +6477,7 @@ add_paths_to_grouping_rel(PlannerInfo *root, RelOptInfo *input_rel,
 																group_clauses,
 																orderAggPathkeys);
 
-			Assert(list_length(pathkey_orderings) > 0);
+			Assert(pathkey_orderings != NIL);
 
 			/* process all potentially interesting grouping reorderings */
 			foreach(lc2, pathkey_orderings)
@@ -6650,7 +6650,7 @@ add_paths_to_grouping_rel(PlannerInfo *root, RelOptInfo *input_rel,
 																	group_clauses,
 																	orderAggPathkeys);
 
-				Assert(list_length(pathkey_orderings) > 0);
+				Assert(pathkey_orderings != NIL);
 
 				/* process all potentially interesting grouping reorderings */
 				foreach(lc2, pathkey_orderings)
@@ -6994,7 +6994,7 @@ create_partial_grouping_paths(PlannerInfo *root,
 																group_clauses,
 																orderAggPathkeys);
 
-			Assert(list_length(pathkey_orderings) > 0);
+			Assert(pathkey_orderings != NIL);
 
 			/* process all potentially interesting grouping reorderings */
 			foreach(lc2, pathkey_orderings)
@@ -7145,7 +7145,7 @@ create_partial_grouping_paths(PlannerInfo *root,
 																group_clauses,
 																orderAggPathkeys);
 
-			Assert(list_length(pathkey_orderings) > 0);
+			Assert(pathkey_orderings != NIL);
 
 			/* process all potentially interesting grouping reorderings */
 			foreach(lc2, pathkey_orderings)

src/backend/partitioning/partprune.c

@@ -2383,7 +2383,7 @@ get_steps_using_prefix(GeneratePruningStepsContext *context,
 		   context->rel->part_scheme->strategy == PARTITION_STRATEGY_HASH);
 
 	/* Quick exit if there are no values to prefix with. */
-	if (list_length(prefix) == 0)
+	if (prefix == NIL)
 	{
 		PartitionPruneStep *step;
 

src/backend/replication/logical/tablesync.c

@@ -383,7 +383,7 @@ process_syncing_tables_for_apply(XLogRecPtr current_lsn)
 	 * immediate restarts.  We don't need it if there are no tables that need
 	 * syncing.
 	 */
-	if (table_states_not_ready && !last_start_times)
+	if (table_states_not_ready != NIL && !last_start_times)
 	{
 		HASHCTL		ctl;
 
@@ -397,7 +397,7 @@ process_syncing_tables_for_apply(XLogRecPtr current_lsn)
 	 * Clean up the hash table when we're done with all tables (just to
 	 * release the bit of memory).
 	 */
-	else if (!table_states_not_ready && last_start_times)
+	else if (table_states_not_ready == NIL && last_start_times)
 	{
 		hash_destroy(last_start_times);
 		last_start_times = NULL;
@@ -1498,7 +1498,7 @@ FetchTableStates(bool *started_tx)
 		 * if table_state_not_ready was empty we still need to check again to
 		 * see if there are 0 tables.
 		 */
-		has_subrels = (list_length(table_states_not_ready) > 0) ||
+		has_subrels = (table_states_not_ready != NIL) ||
 			HasSubscriptionRelations(MySubscription->oid);
 
 		table_states_valid = true;
@@ -1534,7 +1534,7 @@ AllTablesyncsReady(void)
 	 * Return false when there are no tables in subscription or not all tables
 	 * are in ready state; true otherwise.
 	 */
-	return has_subrels && list_length(table_states_not_ready) == 0;
+	return has_subrels && (table_states_not_ready == NIL);
 }
 
 /*

src/backend/replication/pgoutput/pgoutput.c

@@ -450,7 +450,7 @@ pgoutput_startup(LogicalDecodingContext *ctx, OutputPluginOptions *opt,
 					 errmsg("client sent proto_version=%d but we only support protocol %d or higher",
 							data->protocol_version, LOGICALREP_PROTO_MIN_VERSION_NUM)));
 
-		if (list_length(data->publication_names) < 1)
+		if (data->publication_names == NIL)
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 					 errmsg("publication_names parameter missing")));

src/backend/rewrite/rewriteDefine.c

@@ -313,7 +313,7 @@ DefineQueryRewrite(const char *rulename,
 		 *
 		 * So there cannot be INSTEAD NOTHING, ...
 		 */
-		if (list_length(action) == 0)
+		if (action == NIL)
 			ereport(ERROR,
 					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 					 errmsg("INSTEAD NOTHING rules on SELECT are not implemented"),

src/backend/statistics/mcv.c

@@ -1610,7 +1610,6 @@ mcv_get_match_bitmap(PlannerInfo *root, List *clauses,
 
 	/* The bitmap may be partially built. */
 	Assert(clauses != NIL);
-	Assert(list_length(clauses) >= 1);
 	Assert(mcvlist != NULL);
 	Assert(mcvlist->nitems > 0);
 	Assert(mcvlist->nitems <= STATS_MCVLIST_MAX_ITEMS);

src/backend/storage/lmgr/lmgr.c

@@ -913,7 +913,7 @@ WaitForLockersMultiple(List *locktags, LOCKMODE lockmode, bool progress)
 	int			done = 0;
 
 	/* Done if no locks to wait for */
-	if (list_length(locktags) == 0)
+	if (locktags == NIL)
 		return;
 
 	/* Collect the transactions we need to wait on */

src/backend/utils/adt/jsonb_gin.c

@@ -567,7 +567,7 @@ extract_jsp_path_expr(JsonPathGinContext *cxt, JsonPathGinPath path,
 	/* extract a list of nodes to be AND-ed */
 	List	   *nodes = extract_jsp_path_expr_nodes(cxt, path, jsp, scalar);
 
-	if (list_length(nodes) <= 0)
+	if (nodes == NIL)
 		/* no nodes were extracted => full scan is needed for this path */
 		return NULL;
 

src/backend/utils/adt/jsonpath_exec.c

@@ -2552,7 +2552,7 @@ JsonValueListLength(const JsonValueList *jvl)
 static bool
 JsonValueListIsEmpty(JsonValueList *jvl)
 {
-	return !jvl->singleton && list_length(jvl->list) <= 0;
+	return !jvl->singleton && (jvl->list == NIL);
 }
 
 static JsonbValue *

src/backend/utils/adt/jsonpath_gram.y

@@ -459,7 +459,7 @@ makeIndexArray(List *list)
 	ListCell   *cell;
 	int			i = 0;
 
-	Assert(list_length(list) > 0);
+	Assert(list != NIL);
 	v->value.array.nelems = list_length(list);
 
 	v->value.array.elems = palloc(sizeof(v->value.array.elems[0]) *

src/backend/utils/adt/ruleutils.c

@@ -8114,7 +8114,7 @@ get_parameter(Param *param, deparse_context *context)
 				{
 					deparse_namespace *dpns = lfirst(lc);
 
-					if (list_length(dpns->rtable_names) > 0)
+					if (dpns->rtable_names != NIL)
 					{
 						should_qualify = true;
 						break;

src/backend/utils/adt/selfuncs.c

@@ -3408,7 +3408,7 @@ estimate_num_groups_incremental(PlannerInfo *root, List *groupExprs,
 	 * for normal cases with GROUP BY or DISTINCT, but it is possible for
 	 * corner cases with set operations.)
 	 */
-	if (groupExprs == NIL || (pgset && list_length(*pgset) < 1))
+	if (groupExprs == NIL || (pgset && *pgset == NIL))
 		return 1.0;
 
 	/*

src/backend/utils/adt/tsquery.c

@@ -829,7 +829,7 @@ parse_tsquery(char *buf,
 
 	close_tsvector_parser(state.valstate);
 
-	if (list_length(state.polstr) == 0)
+	if (state.polstr == NIL)
 	{
 		ereport(NOTICE,
 				(errmsg("text-search query doesn't contain lexemes: \"%s\"",

src/test/modules/test_ddl_deparse/test_ddl_deparse.c

@@ -95,7 +95,7 @@ get_altertable_subcmdinfo(PG_FUNCTION_ARGS)
 
 	SetSingleFuncCall(fcinfo, 0);
 
-	if (list_length(cmd->d.alterTable.subcmds) == 0)
+	if (cmd->d.alterTable.subcmds == NIL)
 		elog(ERROR, "empty alter table subcommand list");
 
 	foreach(cell, cmd->d.alterTable.subcmds)