3 files changed, 74 insertions, 0 deletions
diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 796f564e4f8..f484fb91c11 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -5193,6 +5193,10 @@ plan_cluster_use_sort(Oid tableOid, Oid indexOid)
 	IndexPath  *indexScanPath;
 	ListCell   *lc;
 
+	/* We can short-circuit the cost comparison if indexscans are disabled */
+	if (!enable_indexscan)
+		return true;			/* use sort */
+
 	/* Set up mostly-dummy planner state */
 	query = makeNode(Query);
 	query->commandType = CMD_SELECT;
diff --git a/src/test/regress/expected/cluster.out b/src/test/regress/expected/cluster.out
index bc26846d6a4..4c8d834d7ab 100644
--- a/src/test/regress/expected/cluster.out
+++ b/src/test/regress/expected/cluster.out
@@ -438,10 +438,46 @@ select * from clstr_temp;
 (2 rows)
 
 drop table clstr_temp;
+RESET SESSION AUTHORIZATION;
+-- Test CLUSTER with external tuplesorting
+create table clstr_4 as select * from tenk1;
+create index cluster_sort on clstr_4 (hundred, thousand, tenthous);
+-- ensure we don't use the index in CLUSTER nor the checking SELECTs
+set enable_indexscan = off;
+-- Use external sort that only ever uses quicksort to sort runs:
+set maintenance_work_mem = '1MB';
+set replacement_sort_tuples = 0;
+cluster clstr_4 using cluster_sort;
+select * from
+(select hundred, lag(hundred) over () as lhundred,
+        thousand, lag(thousand) over () as lthousand,
+        tenthous, lag(tenthous) over () as ltenthous from clstr_4) ss
+where row(hundred, thousand, tenthous) <= row(lhundred, lthousand, ltenthous);
+ hundred | lhundred | thousand | lthousand | tenthous | ltenthous 
+---------+----------+----------+-----------+----------+-----------
+(0 rows)
+
+-- Replacement selection will now be forced.  It should only produce a single
+-- run, due to the fact that input is found to be presorted:
+set replacement_sort_tuples = 150000;
+cluster clstr_4 using cluster_sort;
+select * from
+(select hundred, lag(hundred) over () as lhundred,
+        thousand, lag(thousand) over () as lthousand,
+        tenthous, lag(tenthous) over () as ltenthous from clstr_4) ss
+where row(hundred, thousand, tenthous) <= row(lhundred, lthousand, ltenthous);
+ hundred | lhundred | thousand | lthousand | tenthous | ltenthous 
+---------+----------+----------+-----------+----------+-----------
+(0 rows)
+
+reset enable_indexscan;
+reset maintenance_work_mem;
+reset replacement_sort_tuples;
 -- clean up
 \c -
 DROP TABLE clustertest;
 DROP TABLE clstr_1;
 DROP TABLE clstr_2;
 DROP TABLE clstr_3;
+DROP TABLE clstr_4;
 DROP USER clstr_user;
diff --git a/src/test/regress/sql/cluster.sql b/src/test/regress/sql/cluster.sql
index 8d536c8c363..6e619fdcc7d 100644
--- a/src/test/regress/sql/cluster.sql
+++ b/src/test/regress/sql/cluster.sql
@@ -194,10 +194,44 @@ cluster clstr_temp using clstr_temp_pkey;
 select * from clstr_temp;
 drop table clstr_temp;
 
+RESET SESSION AUTHORIZATION;
+
+-- Test CLUSTER with external tuplesorting
+
+create table clstr_4 as select * from tenk1;
+create index cluster_sort on clstr_4 (hundred, thousand, tenthous);
+-- ensure we don't use the index in CLUSTER nor the checking SELECTs
+set enable_indexscan = off;
+
+-- Use external sort that only ever uses quicksort to sort runs:
+set maintenance_work_mem = '1MB';
+set replacement_sort_tuples = 0;
+cluster clstr_4 using cluster_sort;
+select * from
+(select hundred, lag(hundred) over () as lhundred,
+        thousand, lag(thousand) over () as lthousand,
+        tenthous, lag(tenthous) over () as ltenthous from clstr_4) ss
+where row(hundred, thousand, tenthous) <= row(lhundred, lthousand, ltenthous);
+
+-- Replacement selection will now be forced.  It should only produce a single
+-- run, due to the fact that input is found to be presorted:
+set replacement_sort_tuples = 150000;
+cluster clstr_4 using cluster_sort;
+select * from
+(select hundred, lag(hundred) over () as lhundred,
+        thousand, lag(thousand) over () as lthousand,
+        tenthous, lag(tenthous) over () as ltenthous from clstr_4) ss
+where row(hundred, thousand, tenthous) <= row(lhundred, lthousand, ltenthous);
+
+reset enable_indexscan;
+reset maintenance_work_mem;
+reset replacement_sort_tuples;
+
 -- clean up
 \c -
 DROP TABLE clustertest;
 DROP TABLE clstr_1;
 DROP TABLE clstr_2;
 DROP TABLE clstr_3;
+DROP TABLE clstr_4;
 DROP USER clstr_user;