Fix YA incremental sort bug.

switchToPresortedPrefixMode() did the wrong thing if it detected
a batch boundary just at the last tuple of a fullsort group.

The initially-reported symptom was a "retrieved too many tuples in a
bounded sort" error, but the test case added here just silently gives
the wrong answer without this patch.

I (tgl) am not really happy about committing this patch without review
from the incremental-sort authors, but they seem AWOL and we are hard
against a release deadline.  This does demonstrably make some cases
better, anyway.

Per bug #16846 from Yoran Heling.  Back-patch to v13 where incremental
sort was introduced.

Neil Chen

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

src/backend/executor/nodeIncrementalSort.c

@@ -394,6 +394,13 @@ switchToPresortedPrefixMode(PlanState *pstate)
 				 * current prefix key group.
 				 */
 				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;
 			}
 		}

src/test/regress/expected/incremental_sort.out

@@ -675,6 +675,17 @@ select * from (select * from t order by a) s order by a, b limit 70;
  9 | 70
 (70 rows)
 
+-- Checks case where we hit a group boundary at the last tuple of a batch.
+select * from (select * from t order by a) s order by a, b limit 5;
+ a | b 
+---+---
+ 1 | 1
+ 2 | 2
+ 3 | 3
+ 4 | 4
+ 9 | 5
+(5 rows)
+
 -- Test rescan.
 begin;
 -- We force the planner to choose a plan with incremental sort on the right side

src/test/regress/sql/incremental_sort.sql

@@ -149,6 +149,9 @@ insert into t(a, b) select (case when i < 5 then i else 9 end), i from generate_
 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.
+select * from (select * from t order by a) s order by a, b limit 5;
+
 -- Test rescan.
 begin;
 -- We force the planner to choose a plan with incremental sort on the right side