Fix and clarify comments on replacement selection.

These were modified by the patch to only use replacement selection for the
first run in an external sort.

src/backend/utils/sort/tuplesort.c

@@ -13,26 +13,26 @@
  * See Knuth, volume 3, for more than you want to know about the external
  * sorting algorithm.  Historically, we divided the input into sorted runs
  * using replacement selection, in the form of a priority tree implemented
- * as a heap (essentially his Algorithm 5.2.3H -- although that strategy is
+ * as a heap (essentially his Algorithm 5.2.3H), but now we only do that
- * often avoided altogether), but that can now only happen first the first
+ * for the first run, and only if the run would otherwise end up being very
- * run.  We merge the runs using polyphase merge, Knuth's Algorithm
+ * short.  We merge the runs using polyphase merge, Knuth's Algorithm
  * 5.4.2D.  The logical "tapes" used by Algorithm D are implemented by
  * logtape.c, which avoids space wastage by recycling disk space as soon
  * as each block is read from its "tape".
  *
- * We never form the initial runs using Knuth's recommended replacement
+ * We do not use Knuth's recommended data structure (Algorithm 5.4.1R) for
- * selection data structure (Algorithm 5.4.1R), because it uses a fixed
+ * the replacement selection, because it uses a fixed number of records
- * number of records in memory at all times.  Since we are dealing with
+ * in memory at all times.  Since we are dealing with tuples that may vary
- * tuples that may vary considerably in size, we want to be able to vary
+ * considerably in size, we want to be able to vary the number of records
- * the number of records kept in memory to ensure full utilization of the
+ * kept in memory to ensure full utilization of the allowed sort memory
- * allowed sort memory space.  So, we keep the tuples in a variable-size
+ * space.  So, we keep the tuples in a variable-size heap, with the next
- * heap, with the next record to go out at the top of the heap.  Like
+ * record to go out at the top of the heap.  Like Algorithm 5.4.1R, each
- * Algorithm 5.4.1R, each record is stored with the run number that it
+ * record is stored with the run number that it must go into, and we use
- * must go into, and we use (run number, key) as the ordering key for the
+ * (run number, key) as the ordering key for the heap.  When the run number
- * heap.  When the run number at the top of the heap changes, we know that
+ * at the top of the heap changes, we know that no more records of the prior
- * no more records of the prior run are left in the heap.  Note that there
+ * run are left in the heap.  Note that there are in practice only ever two
- * are in practice only ever two distinct run numbers, due to the greatly
+ * distinct run numbers, because since PostgreSQL 9.6, we only use
- * reduced use of replacement selection in PostgreSQL 9.6.
+ * replacement selection to form the first run.
  *
  * In PostgreSQL 9.6, a heap (based on Knuth's Algorithm H, with some small
  * customizations) is only used with the aim of producing just one run,