diff --git a/src/test/isolation/expected/toast-isolation.out b/src/test/isolation/expected/toast-isolation.out
new file mode 100644
index 0000000000..5db1e281f8
--- /dev/null
+++ b/src/test/isolation/expected/toast-isolation.out
@@ -0,0 +1,42 @@
+Parsed test spec with 2 sessions
+
+starting permutation: s1_insert s2_select s1_update s2_select s1_delete s2_select s1_commit s1_vacuum s2_select s1_reinsert s2_select s1_nullify s2_select s1_vacuum s2_select s2_commit
+step s1_insert: INSERT INTO doc VALUES (3, repeat('3',30000));
+step s2_select: SELECT id, COUNT(*), SUM(length(document)) FROM doc GROUP BY id ORDER BY id ASC;
+id             count          sum            
+
+1              1              10000          
+2              1              20000          
+step s1_update: UPDATE doc SET id = 4, document = repeat('4',40000);
+step s2_select: SELECT id, COUNT(*), SUM(length(document)) FROM doc GROUP BY id ORDER BY id ASC;
+id             count          sum            
+
+1              1              10000          
+2              1              20000          
+step s1_delete: DELETE FROM doc;
+step s2_select: SELECT id, COUNT(*), SUM(length(document)) FROM doc GROUP BY id ORDER BY id ASC;
+id             count          sum            
+
+1              1              10000          
+2              1              20000          
+step s1_commit: COMMIT;
+step s1_vacuum: VACUUM;
+step s2_select: SELECT id, COUNT(*), SUM(length(document)) FROM doc GROUP BY id ORDER BY id ASC;
+id             count          sum            
+
+step s1_reinsert: INSERT INTO doc VALUES (5, repeat('5',50000));
+step s2_select: SELECT id, COUNT(*), SUM(length(document)) FROM doc GROUP BY id ORDER BY id ASC;
+id             count          sum            
+
+5              1              50000          
+step s1_nullify: UPDATE doc SET document = NULL;
+step s2_select: SELECT id, COUNT(*), SUM(length(document)) FROM doc GROUP BY id ORDER BY id ASC;
+id             count          sum            
+
+5              1                             
+step s1_vacuum: VACUUM;
+step s2_select: SELECT id, COUNT(*), SUM(length(document)) FROM doc GROUP BY id ORDER BY id ASC;
+id             count          sum            
+
+5              1                             
+step s2_commit: COMMIT;
diff --git a/src/test/isolation/isolation_schedule b/src/test/isolation/isolation_schedule
index a2fa19230d..59528b7efd 100644
--- a/src/test/isolation/isolation_schedule
+++ b/src/test/isolation/isolation_schedule
@@ -88,3 +88,4 @@ test: plpgsql-toast
 test: truncate-conflict
 test: serializable-parallel
 test: serializable-parallel-2
+test: toast-isolation
diff --git a/src/test/isolation/specs/toast-isolation.spec b/src/test/isolation/specs/toast-isolation.spec
new file mode 100644
index 0000000000..d6a538f9eb
--- /dev/null
+++ b/src/test/isolation/specs/toast-isolation.spec
@@ -0,0 +1,41 @@
+# READ UNCOMMITTED isolation level is allowed per SQL spec to expose updates in
+# one uncommitted transaction to a reader in another transaction, though the
+# spec does not require such exposure.  PostgreSQL implements this isolation
+# level by using READ COMMITTED, which disallows such exposure.  As such, we
+# should not see the updates in session s1 when querying from session s2.
+# However, if we ever change the implementation to have a distinct READ
+# UNCOMMITTED mode, it will still be necessary that the toast table holding the
+# documents and the main table holding the IDs work together rationally.  Check
+# that here, as a defence against half-baked future implementations of READ
+# UNCOMMITTED isolation.
+
+setup
+{
+	CREATE TABLE doc (id INTEGER, document TEXT);
+	ALTER TABLE doc ALTER document SET STORAGE external;
+	INSERT INTO doc VALUES (1, repeat('1',10000));
+	INSERT INTO doc VALUES (2, repeat('2',20000));
+}
+
+teardown
+{
+	DROP TABLE doc;
+}
+
+session "s1"
+setup 				{ BEGIN TRANSACTION ISOLATION LEVEL READ UNCOMMITTED; }
+step "s1_insert"	{ INSERT INTO doc VALUES (3, repeat('3',30000)); }
+step "s1_update"	{ UPDATE doc SET id = 4, document = repeat('4',40000); }
+step "s1_delete"	{ DELETE FROM doc; }
+step "s1_commit"	{ COMMIT; }
+step "s1_vacuum"	{ VACUUM; }
+step "s1_reinsert"	{ INSERT INTO doc VALUES (5, repeat('5',50000)); }
+step "s1_nullify"	{ UPDATE doc SET document = NULL; }
+
+session "s2"
+setup 				{ BEGIN TRANSACTION ISOLATION LEVEL READ UNCOMMITTED; }
+step "s2_select"	{ SELECT id, COUNT(*), SUM(length(document)) FROM doc GROUP BY id ORDER BY id ASC; }
+step "s2_commit"	{ COMMIT; }
+
+permutation "s1_insert" "s2_select" "s1_update" "s2_select" "s1_delete" "s2_select" "s1_commit" "s1_vacuum" "s2_select" "s1_reinsert" "s2_select" "s1_nullify" "s2_select" "s1_vacuum" "s2_select" "s2_commit"
+