Fix comment about promising tuples.
Oversight in commit which added bottom-up index deletion.
@@ -7879,8 +7879,10 @@ bottomup_sort_and_shrink(TM_IndexDeleteOp *delstate)
/*
* We're about ready to sort block groups to determine the optimal order
* for visiting heap blocks. But before we do, round the number of
- * promising tuples for each block group up to the nearest power-of-two
- * (except for block groups where npromisingtids is already 0).
+ * promising tuples for each block group up to the next power-of-two,
+ * unless it is very low (less than 4), in which case we round up to 4.
+ * npromisingtids is far too noisy to trust when choosing between a pair
+ * of block groups that both have very low values.
*
* This scheme divides heap blocks/block groups into buckets. Each bucket
* contains blocks that have _approximately_ the same number of promising
