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diff --git a/mysql-test/main/table_keyinfo-6838.test b/mysql-test/main/table_keyinfo-6838.test
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+++ b/mysql-test/main/table_keyinfo-6838.test
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+# Test case for MDEV-6838.
+# Due to incorrect key_length computation, this usecase failed with the error
+# Using too big key for internal temp table.
+
+CREATE TABLE t1 (i INT, state VARCHAR(997)) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (2,'Louisiana'),(9,'Maine');
+
+CREATE TABLE t2 (state VARCHAR(997), j INT) ENGINE=MyISAM;
+INSERT INTO t2 VALUES ('Louisiana',9),('Alaska',5);
+INSERT INTO t2 SELECT t2.* FROM t2 JOIN t2 AS t3 JOIN t2 AS t4 JOIN t2 AS t5 JOIN t2 AS t6;
+
+SET @@max_heap_table_size= 16384;
+set @@optimizer_switch='derived_merge=OFF';
+set @@optimizer_switch='extended_keys=ON';
+
+SELECT * FROM t1 AS t1_1 LEFT JOIN ( t1 AS t1_2 INNER JOIN (SELECT * FROM t2) v2 ON t1_2.i = j ) ON t1_1.state = v2.state LIMIT 1;
+
+DROP TABLE t1, t2;