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

3 files changed, 8 insertions, 8 deletions

diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index fb28e6411aa..7292d68dbcf 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/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,
diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index e37f2933eb8..aa9d4e51b9e 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/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;
diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 64632db73cd..e636596b579 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/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)