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diff --git a/src/test/isolation/specs/subxid-overflow.spec b/src/test/isolation/specs/subxid-overflow.spec
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+# Subtransaction overflow
+#
+# This test is designed to cover some code paths which only occur when
+# one transaction has overflowed the subtransaction cache.
+
+setup
+{
+DROP TABLE IF EXISTS subxids;
+CREATE TABLE subxids (subx integer, val integer);
+
+CREATE OR REPLACE FUNCTION gen_subxids (n integer)
+ RETURNS VOID
+ LANGUAGE plpgsql
+AS $$
+BEGIN
+  IF n <= 0 THEN
+	UPDATE subxids SET val = 1 WHERE subx = 0;
+    RETURN;
+  ELSE
+    PERFORM gen_subxids(n - 1);
+    RETURN;
+  END IF;
+EXCEPTION /* generates a subxid */
+  WHEN raise_exception THEN NULL;
+END;
+$$;
+}
+
+teardown
+{
+ DROP TABLE subxids;
+ DROP FUNCTION gen_subxids(integer);
+}
+
+session s1
+# setup step for each test
+step ins	{ TRUNCATE subxids; INSERT INTO subxids VALUES (0, 0); }
+# long running transaction with overflowed subxids
+step subxov	{ BEGIN; SELECT gen_subxids(100); }
+# commit should always come last to make this long running
+step s1c	{ COMMIT; }
+
+session s2
+# move xmax forwards
+step xmax	{ BEGIN; INSERT INTO subxids VALUES (99, 0); COMMIT;}
+
+# step for test1
+step s2sel	{ SELECT val FROM subxids WHERE subx = 0; }
+
+# steps for test2
+step s2brr { BEGIN ISOLATION LEVEL REPEATABLE READ; }
+step s2brc { BEGIN ISOLATION LEVEL READ COMMITTED; }
+# look for data written by sub3
+step s2s3	{ SELECT val FROM subxids WHERE subx = 1; }
+step s2c	{ COMMIT; }
+
+# step for test3
+step s2upd	{ UPDATE subxids SET val = 1 WHERE subx = 0; }
+
+session s3
+# transaction with subxids that can commit before s1c
+step sub3	{ BEGIN; SAVEPOINT s; INSERT INTO subxids VALUES (1, 0); }
+step s3c	{ COMMIT; }
+
+# test1
+# s2sel will see subxid as still running
+# designed to test XidInMVCCSnapshot() when overflows, xid is found
+permutation ins subxov xmax s2sel s1c
+
+# test2
+# designed to test XidInMVCCSnapshot() when overflows, xid is not found
+# both SELECTs invisible
+permutation ins subxov sub3 xmax s2brr s2s3 s3c s2s3 s2c s1c
+# 2nd SELECT visible after commit
+permutation ins subxov sub3 xmax s2brc s2s3 s3c s2s3 s2c s1c
+
+# test3
+# designed to test XactLockTableWait() for overflows
+permutation ins subxov xmax s2upd s1c