Improve the heuristic for ordering child paths of a parallel append.

Commit ab7271677 introduced code that attempts to order the child
scans of a Parallel Append node in a way that will minimize execution
time, based on total cost and startup cost.  However, it failed to
think hard about what to do when estimated costs are exactly equal;
a case that's particularly likely to occur when comparing on startup
cost.  In such a case the ordering of the child paths would be left
to the whims of qsort, an algorithm that isn't even stable.

We can improve matters by applying the rule used elsewhere in the
planner: if total costs are equal, sort on startup cost, and
vice versa.  When both cost estimates are exactly equal, rather
than letting qsort do something unpredictable, sort based on the
child paths' relids, which should typically result in sorting in
inheritance order.  (The latter provision requires inventing a
qsort-style comparator for bitmapsets, but maybe we'll have use
for that for other reasons in future.)

This results in a few plan changes in the select_parallel test,
but those all look more reasonable than before, when the actual
underlying cost numbers are taken into account.

Discussion: https://postgr.es/m/4944.1515446989@sss.pgh.pa.us

1 files changed, 45 insertions, 1 deletions

diff --git a/src/backend/nodes/bitmapset.c b/src/backend/nodes/bitmapset.c
index 733fe3cf2a0..edcd19a4fd7 100644
--- a/src/backend/nodes/bitmapset.c
+++ b/src/backend/nodes/bitmapset.c
@@ -173,6 +173,50 @@ bms_equal(const Bitmapset *a, const Bitmapset *b)
 }
 
 /*
+ * bms_compare - qsort-style comparator for bitmapsets
+ *
+ * This guarantees to report values as equal iff bms_equal would say they are
+ * equal.  Otherwise, the highest-numbered bit that is set in one value but
+ * not the other determines the result.  (This rule means that, for example,
+ * {6} is greater than {5}, which seems plausible.)
+ */
+int
+bms_compare(const Bitmapset *a, const Bitmapset *b)
+{
+	int			shortlen;
+	int			i;
+
+	/* Handle cases where either input is NULL */
+	if (a == NULL)
+		return bms_is_empty(b) ? 0 : -1;
+	else if (b == NULL)
+		return bms_is_empty(a) ? 0 : +1;
+	/* Handle cases where one input is longer than the other */
+	shortlen = Min(a->nwords, b->nwords);
+	for (i = shortlen; i < a->nwords; i++)
+	{
+		if (a->words[i] != 0)
+			return +1;
+	}
+	for (i = shortlen; i < b->nwords; i++)
+	{
+		if (b->words[i] != 0)
+			return -1;
+	}
+	/* Process words in common */
+	i = shortlen;
+	while (--i >= 0)
+	{
+		bitmapword	aw = a->words[i];
+		bitmapword	bw = b->words[i];
+
+		if (aw != bw)
+			return (aw > bw) ? +1 : -1;
+	}
+	return 0;
+}
+
+/*
  * bms_make_singleton - build a bitmapset containing a single member
  */
 Bitmapset *
@@ -838,7 +882,7 @@ bms_add_range(Bitmapset *a, int lower, int upper)
 	if (lwordnum == uwordnum)
 	{
 		a->words[lwordnum] |= ~(bitmapword) (((bitmapword) 1 << lbitnum) - 1)
-							  & (~(bitmapword) 0) >> ushiftbits;
+			& (~(bitmapword) 0) >> ushiftbits;
 	}
 	else
 	{