Simplify loop logic in nodeIncrementalSort.c.

The inner loop in switchToPresortedPrefixMode() can be implemented
as a conventional integer-counter for() loop, removing a couple of
redundant boolean state variables.  The old logic here was a remnant
of earlier development, but as things now stand there's no reason
for extra complexity.

Also, annotate the test case added by 82e0e293 to explain why it
manages to hit the corner case fixed in that commit, and add an
EXPLAIN to verify that it's creating an incremental-sort plan.

Back-patch to v13, like the previous patch.

James Coleman and Tom Lane

Discussion: https://postgr.es/m/16846-ae49f51ac379a4cb@postgresql.org

src/backend/executor/nodeIncrementalSort.c

@@ -288,9 +288,7 @@ switchToPresortedPrefixMode(PlanState *pstate)
 {
 	IncrementalSortState *node = castNode(IncrementalSortState, pstate);
 	ScanDirection dir;
-	int64		nTuples = 0;
-	bool		lastTuple = false;
-	bool		firstTuple = true;
+	int64		nTuples;
 	TupleDesc	tupDesc;
 	PlanState  *outerNode;
 	IncrementalSort *plannode = castNode(IncrementalSort, node->ss.ps.plan);
@@ -343,20 +341,16 @@ switchToPresortedPrefixMode(PlanState *pstate)
 	 * Copy as many tuples as we can (i.e., in the same prefix key group) from
 	 * the full sort state to the prefix sort state.
 	 */
-	for (;;)
+	for (nTuples = 0; nTuples < node->n_fullsort_remaining; nTuples++)
 	{
-		lastTuple = node->n_fullsort_remaining - nTuples == 1;
-
 		/*
 		 * When we encounter multiple prefix key groups inside the full sort
 		 * tuplesort we have to carry over the last read tuple into the next
 		 * batch.
 		 */
-		if (firstTuple && !TupIsNull(node->transfer_tuple))
+		if (nTuples == 0 && !TupIsNull(node->transfer_tuple))
 		{
 			tuplesort_puttupleslot(node->prefixsort_state, node->transfer_tuple);
-			nTuples++;
-
 			/* The carried over tuple is our new group pivot tuple. */
 			ExecCopySlot(node->group_pivot, node->transfer_tuple);
 		}
@@ -376,7 +370,6 @@ switchToPresortedPrefixMode(PlanState *pstate)
 			if (isCurrentGroup(node, node->group_pivot, node->transfer_tuple))
 			{
 				tuplesort_puttupleslot(node->prefixsort_state, node->transfer_tuple);
-				nTuples++;
 			}
 			else
 			{
@@ -395,27 +388,10 @@ switchToPresortedPrefixMode(PlanState *pstate)
 				 */
 				ExecClearTuple(node->group_pivot);
 
-				/*
-				 * Also make sure we take the didn't-consume-all-the-tuples
-				 * path below, even if this happened to be the last tuple of
-				 * the batch.
-				 */
-				lastTuple = false;
+				/* Break out of for-loop early */
 				break;
 			}
 		}
-
-		firstTuple = false;
-
-		/*
-		 * If we've copied all of the tuples from the full sort state into the
-		 * prefix sort state, then we don't actually know that we've yet found
-		 * the last tuple in that prefix key group until we check the next
-		 * tuple from the outer plan node, so we retain the current group
-		 * pivot tuple prefix key group comparison.
-		 */
-		if (lastTuple)
-			break;
 	}
 
 	/*
@@ -428,14 +404,15 @@ switchToPresortedPrefixMode(PlanState *pstate)
 	node->n_fullsort_remaining -= nTuples;
 	SO1_printf("Setting n_fullsort_remaining to " INT64_FORMAT "\n", node->n_fullsort_remaining);
 
-	if (lastTuple)
+	if (node->n_fullsort_remaining == 0)
 	{
 		/*
-		 * We've confirmed that all tuples remaining in the full sort batch is
-		 * in the same prefix key group and moved all of those tuples into the
-		 * presorted prefix tuplesort. Now we can save our pivot comparison
-		 * tuple and continue fetching tuples from the outer execution node to
-		 * load into the presorted prefix tuplesort.
+		 * We've found that all tuples remaining in the full sort batch are in
+		 * the same prefix key group and moved all of those tuples into the
+		 * presorted prefix tuplesort.  We don't know that we've yet found the
+		 * last tuple in the current prefix key group, so save our pivot
+		 * comparison tuple and continue fetching tuples from the outer
+		 * execution node to load into the presorted prefix tuplesort.
 		 */
 		ExecCopySlot(node->group_pivot, node->transfer_tuple);
 		SO_printf("Setting execution_status to INCSORT_LOADPREFIXSORT (switchToPresortedPrefixMode)\n");

src/test/regress/expected/incremental_sort.out

@@ -676,6 +676,21 @@ select * from (select * from t order by a) s order by a, b limit 70;
 (70 rows)
 
 -- Checks case where we hit a group boundary at the last tuple of a batch.
+-- Because the full sort state is bounded, we scan 64 tuples (the mode
+-- transition point) but only retain 5. Thus when we transition modes, all
+-- tuples in the full sort state have different prefix keys.
+explain (costs off) select * from (select * from t order by a) s order by a, b limit 5;
+           QUERY PLAN            
+---------------------------------
+ Limit
+   ->  Incremental Sort
+         Sort Key: t.a, t.b
+         Presorted Key: t.a
+         ->  Sort
+               Sort Key: t.a
+               ->  Seq Scan on t
+(7 rows)
+
 select * from (select * from t order by a) s order by a, b limit 5;
  a | b 
 ---+---

src/test/regress/sql/incremental_sort.sql

@@ -150,6 +150,10 @@ analyze t;
 explain (costs off) select * from (select * from t order by a) s order by a, b limit 70;
 select * from (select * from t order by a) s order by a, b limit 70;
 -- Checks case where we hit a group boundary at the last tuple of a batch.
+-- Because the full sort state is bounded, we scan 64 tuples (the mode
+-- transition point) but only retain 5. Thus when we transition modes, all
+-- tuples in the full sort state have different prefix keys.
+explain (costs off) select * from (select * from t order by a) s order by a, b limit 5;
 select * from (select * from t order by a) s order by a, b limit 5;
 
 -- Test rescan.