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Avoid using list_length() to test for empty list.

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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
This commit is contained in:
Tom Lane
2022-08-17 11:12:35 -04:00
parent 4a319fce76
commit efd0c16bec
28 changed files with 47 additions and 48 deletions
Download Patch File Download Diff File Expand all files Collapse all files

2
src/backend/optimizer/path/costsize.c
View File

@@ -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,

2
src/backend/optimizer/plan/createplan.c
View File

@@ -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;

12
src/backend/optimizer/plan/planner.c
View File

@@ -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)