On Fri, 17 Feb 2023 at 13:23, Andres Freund <and...@anarazel.de> wrote:
> But wouldn't an even cheaper way here be to iterate over the children in
> reverse order when match_partition_order_desc? We can do that efficiently
> now. Looks like we don't have a readymade helper for it, but it'd be easy
> enough to add or open code.
That seems fair. I think open coding is a better option. I had a go
at foreach_reverse recently and decided to keep clear of it due to
behavioural differences with foreach_delete_current().
I've attached a patch for this. It seems to have similar performance
to the list_reverse()
$ psql -c "explain (analyze, timing off) select * from lp order by a
desc" postgres | grep "Planning Time"
Planning Time: 522.554 ms <- cold relcache
Planning Time: 467.776 ms
Planning Time: 466.424 ms
David
diff --git a/src/backend/optimizer/path/allpaths.c
b/src/backend/optimizer/path/allpaths.c
index ae0f9bdc8a..f4d4018960 100644
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -1703,6 +1703,9 @@ generate_orderedappend_paths(PlannerInfo *root,
RelOptInfo *rel,
ListCell *lcr;
bool match_partition_order;
bool match_partition_order_desc;
+ int end_element;
+ int first_element;
+ int direction;
/*
* Determine if this sort ordering matches any partition
pathkeys we
@@ -1723,10 +1726,31 @@ generate_orderedappend_paths(PlannerInfo *root,
RelOptInfo *rel,
(!partition_pathkeys_desc_partial &&
pathkeys_contained_in(partition_pathkeys_desc,
pathkeys)));
+ /*
+ * When the required pathkeys match the reverse of the partition
+ * order, we must build the list of paths in reverse starting
with the
+ * last matching partition first. We can get away without
making any
+ * special cases for this in the loop by just looping backward
over
+ * the child relations in this case.
+ */
+ if (match_partition_order_desc)
+ {
+ first_element = list_length(live_childrels) - 1;
+ end_element = -1;
+ direction = -1;
+ match_partition_order = true;
+ }
+ else
+ {
+ first_element = 0;
+ end_element = list_length(live_childrels);
+ direction = 1;
+ }
+
/* Select the child paths for this ordering... */
- foreach(lcr, live_childrels)
+ for (int i = first_element; i != end_element; i += direction)
{
- RelOptInfo *childrel = (RelOptInfo *) lfirst(lcr);
+ RelOptInfo *childrel = list_nth_node(RelOptInfo,
live_childrels, i);
Path *cheapest_startup,
*cheapest_total,
*cheapest_fractional = NULL;
@@ -1822,25 +1846,6 @@ generate_orderedappend_paths(PlannerInfo *root,
RelOptInfo *rel,
fractional_subpaths =
lappend(fractional_subpaths, cheapest_fractional);
}
}
- else if (match_partition_order_desc)
- {
- /*
- * As above, but we need to reverse the order
of the children,
- * because nodeAppend.c doesn't know anything
about reverse
- * ordering and will scan the children in the
order presented.
- */
- cheapest_startup =
get_singleton_append_subpath(cheapest_startup);
- cheapest_total =
get_singleton_append_subpath(cheapest_total);
-
- startup_subpaths = lcons(cheapest_startup,
startup_subpaths);
- total_subpaths = lcons(cheapest_total,
total_subpaths);
-
- if (cheapest_fractional)
- {
- cheapest_fractional =
get_singleton_append_subpath(cheapest_fractional);
- fractional_subpaths =
lcons(cheapest_fractional, fractional_subpaths);
- }
- }
else
{
/*
@@ -1859,7 +1864,7 @@ generate_orderedappend_paths(PlannerInfo *root,
RelOptInfo *rel,
}
/* ... and build the Append or MergeAppend paths */
- if (match_partition_order || match_partition_order_desc)
+ if (match_partition_order)
{
/* We only need Append */
add_path(rel, (Path *) create_append_path(root,
