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/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 ||
-		list_length(EventCacheLookup(EVT_TableRewrite)) > 0 ||
-		list_length(EventCacheLookup(EVT_DDLCommandEnd)) > 0;
+	return (EventCacheLookup(EVT_SQLDrop) != NIL) ||
+		(EventCacheLookup(EVT_TableRewrite) != NIL) ||
+		(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
-	 * UNLOGGED as UNLOGGED tables can't be published.
+	 * Check that the table is not part of any publication when changing to
+	 * 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),