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 82e0e2930 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

2 files changed, 19 insertions, 0 deletions

diff --git a/src/test/regress/expected/incremental_sort.out b/src/test/regress/expected/incremental_sort.out
index d5745838440..68ca321163b 100644
--- a/src/test/regress/expected/incremental_sort.out
+++ b/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 
 ---+---
diff --git a/src/test/regress/sql/incremental_sort.sql b/src/test/regress/sql/incremental_sort.sql
index 9965fcd7776..81429164d45 100644
--- a/src/test/regress/sql/incremental_sort.sql
+++ 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.