diff --git a/mysql-test/r/select.result b/mysql-test/r/select.result
index cc90547722f..4c6ad1c34f0 100644
--- a/mysql-test/r/select.result
+++ b/mysql-test/r/select.result
@@ -2364,3 +2364,16 @@ EXPLAIN SELECT i FROM t1 WHERE i=1;
 table	type	possible_keys	key	key_len	ref	rows	Extra
 t1	const	PRIMARY	PRIMARY	4	const	1	Using index
 DROP TABLE t1;
+CREATE TABLE t1 ( a BLOB, INDEX (a(20)) );
+CREATE TABLE t2 ( a BLOB, INDEX (a(20)) );
+INSERT INTO t1 VALUES ('one'),('two'),('three'),('four'),('five');
+INSERT INTO t2 VALUES ('one'),('two'),('three'),('four'),('five');
+EXPLAIN SELECT * FROM t1 LEFT JOIN t2 USE INDEX (a) ON t1.a=t2.a;
+table	type	possible_keys	key	key_len	ref	rows	Extra
+t1	ALL	NULL	NULL	NULL	NULL	5	
+t2	ref	a	a	23	t1.a	5	
+EXPLAIN SELECT * FROM t1 LEFT JOIN t2 FORCE INDEX (a) ON t1.a=t2.a;
+table	type	possible_keys	key	key_len	ref	rows	Extra
+t1	ALL	NULL	NULL	NULL	NULL	5	
+t2	ref	a	a	23	t1.a	5	
+DROP TABLE t1, t2;
diff --git a/mysql-test/t/select.test b/mysql-test/t/select.test
index e6b1ffbe8d7..9be08b8390b 100644
--- a/mysql-test/t/select.test
+++ b/mysql-test/t/select.test
@@ -1909,3 +1909,18 @@ EXPLAIN SELECT i FROM t1 WHERE i=1;
 EXPLAIN SELECT i FROM t1 WHERE i=1;
 
 DROP TABLE t1;
+
+#
+# Test case for bug 7520: a wrong cost of the index for a BLOB field
+#
+
+CREATE TABLE t1 ( a BLOB, INDEX (a(20)) );
+CREATE TABLE t2 ( a BLOB, INDEX (a(20)) );
+
+INSERT INTO t1 VALUES ('one'),('two'),('three'),('four'),('five');
+INSERT INTO t2 VALUES ('one'),('two'),('three'),('four'),('five');
+
+EXPLAIN SELECT * FROM t1 LEFT JOIN t2 USE INDEX (a) ON t1.a=t2.a;
+EXPLAIN SELECT * FROM t1 LEFT JOIN t2 FORCE INDEX (a) ON t1.a=t2.a;
+
+DROP TABLE t1, t2;
diff --git a/sql/table.cc b/sql/table.cc
index 573fa11a4c4..244c7fa94f3 100644
--- a/sql/table.cc
+++ b/sql/table.cc
@@ -486,6 +486,7 @@ int openfrm(const char *name, const char *alias, uint db_stat, uint prgflag,
 	    if (!(field->flags & BINARY_FLAG))
 	      keyinfo->flags|= HA_END_SPACE_KEY;
 	  }
+          set_if_bigger(outparam->max_key_length, keyinfo->key_length);
 	  if (i == 0 && key != primary_key)
 	    field->flags |=
 	      ((keyinfo->flags & HA_NOSAME) &&