diff --git a/src/test/modules/Makefile b/src/test/modules/Makefile
index 29de73c060..1428529b04 100644
--- a/src/test/modules/Makefile
+++ b/src/test/modules/Makefile
@@ -7,6 +7,7 @@ include $(top_builddir)/src/Makefile.global
 SUBDIRS = \
 		  brin \
 		  commit_ts \
+		  delay_execution \
 		  dummy_index_am \
 		  dummy_seclabel \
 		  snapshot_too_old \
diff --git a/src/test/modules/delay_execution/.gitignore b/src/test/modules/delay_execution/.gitignore
new file mode 100644
index 0000000000..ba2160b66c
--- /dev/null
+++ b/src/test/modules/delay_execution/.gitignore
@@ -0,0 +1,3 @@
+# Generated subdirectories
+/output_iso/
+/tmp_check_iso/
diff --git a/src/test/modules/delay_execution/Makefile b/src/test/modules/delay_execution/Makefile
new file mode 100644
index 0000000000..f270aebf3a
--- /dev/null
+++ b/src/test/modules/delay_execution/Makefile
@@ -0,0 +1,21 @@
+# src/test/modules/delay_execution/Makefile
+
+PGFILEDESC = "delay_execution - allow delay between parsing and execution"
+
+MODULE_big = delay_execution
+OBJS = \
+	$(WIN32RES) \
+	delay_execution.o
+
+ISOLATION = partition-addition
+
+ifdef USE_PGXS
+PG_CONFIG = pg_config
+PGXS := $(shell $(PG_CONFIG) --pgxs)
+include $(PGXS)
+else
+subdir = src/test/modules/delay_execution
+top_builddir = ../../../..
+include $(top_builddir)/src/Makefile.global
+include $(top_srcdir)/contrib/contrib-global.mk
+endif
diff --git a/src/test/modules/delay_execution/delay_execution.c b/src/test/modules/delay_execution/delay_execution.c
new file mode 100644
index 0000000000..03ea23d0f2
--- /dev/null
+++ b/src/test/modules/delay_execution/delay_execution.c
@@ -0,0 +1,104 @@
+/*-------------------------------------------------------------------------
+ *
+ * delay_execution.c
+ *		Test module to allow delay between parsing and execution of a query.
+ *
+ * The delay is implemented by taking and immediately releasing a specified
+ * advisory lock.  If another process has previously taken that lock, the
+ * current process will be blocked until the lock is released; otherwise,
+ * there's no effect.  This allows an isolationtester script to reliably
+ * test behaviors where some specified action happens in another backend
+ * between parsing and execution of any desired query.
+ *
+ * Copyright (c) 2020, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *	  src/test/modules/delay_execution/delay_execution.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include <limits.h>
+
+#include "optimizer/planner.h"
+#include "utils/builtins.h"
+#include "utils/guc.h"
+#include "utils/inval.h"
+
+
+PG_MODULE_MAGIC;
+
+/* GUC: advisory lock ID to use.  Zero disables the feature. */
+static int	post_planning_lock_id = 0;
+
+/* Save previous planner hook user to be a good citizen */
+static planner_hook_type prev_planner_hook = NULL;
+
+/* Module load/unload functions */
+void		_PG_init(void);
+void		_PG_fini(void);
+
+
+/* planner_hook function to provide the desired delay */
+static PlannedStmt *
+delay_execution_planner(Query *parse, const char *query_string,
+						int cursorOptions, ParamListInfo boundParams)
+{
+	PlannedStmt *result;
+
+	/* Invoke the planner, possibly via a previous hook user */
+	if (prev_planner_hook)
+		result = prev_planner_hook(parse, query_string, cursorOptions,
+								   boundParams);
+	else
+		result = standard_planner(parse, query_string, cursorOptions,
+								  boundParams);
+
+	/* If enabled, delay by taking and releasing the specified lock */
+	if (post_planning_lock_id != 0)
+	{
+		DirectFunctionCall1(pg_advisory_lock_int8,
+							Int64GetDatum((int64) post_planning_lock_id));
+		DirectFunctionCall1(pg_advisory_unlock_int8,
+							Int64GetDatum((int64) post_planning_lock_id));
+
+		/*
+		 * Ensure that we notice any pending invalidations, since the advisory
+		 * lock functions don't do this.
+		 */
+		AcceptInvalidationMessages();
+	}
+
+	return result;
+}
+
+/* Module load function */
+void
+_PG_init(void)
+{
+	/* Set up the GUC to control which lock is used */
+	DefineCustomIntVariable("delay_execution.post_planning_lock_id",
+							"Sets the advisory lock ID to be locked/unlocked after planning.",
+							"Zero disables the delay.",
+							&post_planning_lock_id,
+							0,
+							0, INT_MAX,
+							PGC_USERSET,
+							0,
+							NULL,
+							NULL,
+							NULL);
+
+	/* Install our hook */
+	prev_planner_hook = planner_hook;
+	planner_hook = delay_execution_planner;
+}
+
+/* Module unload function (pro forma, not used currently) */
+void
+_PG_fini(void)
+{
+	planner_hook = prev_planner_hook;
+}
diff --git a/src/test/modules/delay_execution/expected/partition-addition.out b/src/test/modules/delay_execution/expected/partition-addition.out
new file mode 100644
index 0000000000..7c91090eef
--- /dev/null
+++ b/src/test/modules/delay_execution/expected/partition-addition.out
@@ -0,0 +1,21 @@
+Parsed test spec with 2 sessions
+
+starting permutation: s2lock s1exec s2addp s2unlock
+step s2lock: SELECT pg_advisory_lock(12345);
+pg_advisory_lock
+
+               
+step s1exec: LOAD 'delay_execution';
+		  SET delay_execution.post_planning_lock_id = 12345;
+		  SELECT * FROM foo WHERE a <> 1 AND a <> (SELECT 3); <waiting ...>
+step s2addp: CREATE TABLE foo2 (LIKE foo);
+		  ALTER TABLE foo ATTACH PARTITION foo2 FOR VALUES IN (2);
+		  INSERT INTO foo VALUES (2, 'ADD2');
+step s2unlock: SELECT pg_advisory_unlock(12345);
+pg_advisory_unlock
+
+t              
+step s1exec: <... completed>
+a              b              
+
+4              GHI            
diff --git a/src/test/modules/delay_execution/specs/partition-addition.spec b/src/test/modules/delay_execution/specs/partition-addition.spec
new file mode 100644
index 0000000000..2a0948247e
--- /dev/null
+++ b/src/test/modules/delay_execution/specs/partition-addition.spec
@@ -0,0 +1,38 @@
+# Test addition of a partition with less-than-exclusive locking.
+
+setup
+{
+  CREATE TABLE foo (a int, b text) PARTITION BY LIST(a);
+  CREATE TABLE foo1 PARTITION OF foo FOR VALUES IN (1);
+  CREATE TABLE foo3 PARTITION OF foo FOR VALUES IN (3);
+  CREATE TABLE foo4 PARTITION OF foo FOR VALUES IN (4);
+  INSERT INTO foo VALUES (1, 'ABC');
+  INSERT INTO foo VALUES (3, 'DEF');
+  INSERT INTO foo VALUES (4, 'GHI');
+}
+
+teardown
+{
+  DROP TABLE foo;
+}
+
+# The SELECT will be planned with just the three partitions shown above,
+# of which we expect foo1 to be pruned at planning and foo3 at execution.
+# Then we'll block, and by the time the query is actually executed,
+# partition foo2 will also exist.  We expect that not to be scanned.
+# This test is specifically designed to check ExecCreatePartitionPruneState's
+# code for matching up the partition lists in such cases.
+
+session "s1"
+step "s1exec"	{ LOAD 'delay_execution';
+		  SET delay_execution.post_planning_lock_id = 12345;
+		  SELECT * FROM foo WHERE a <> 1 AND a <> (SELECT 3); }
+
+session "s2"
+step "s2lock"	{ SELECT pg_advisory_lock(12345); }
+step "s2unlock"	{ SELECT pg_advisory_unlock(12345); }
+step "s2addp"	{ CREATE TABLE foo2 (LIKE foo);
+		  ALTER TABLE foo ATTACH PARTITION foo2 FOR VALUES IN (2);
+		  INSERT INTO foo VALUES (2, 'ADD2'); }
+
+permutation "s2lock" "s1exec" "s2addp" "s2unlock"