Post-review corrections of the fix for bug #18206.

mysql-test/r/func_group.result:
  Added another test case for bug #18206.
mysql-test/t/func_group.test:
  Added another test case for bug #18206.

mysql-test/r/func_group.result

@@ -940,3 +940,16 @@ EXPLAIN SELECT MAX(b) FROM t1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	
 DROP TABLE t1;
+CREATE TABLE t1 (id int , b varchar(512), INDEX(b(250))) COLLATE latin1_bin;
+Warnings:
+Warning	1246	Converting column 'b' from CHAR to TEXT
+INSERT INTO t1 VALUES
+(1,CONCAT(REPEAT('_', 250), "qq")), (1,CONCAT(REPEAT('_', 250), "zz")),
+(1,CONCAT(REPEAT('_', 250), "aa")), (1,CONCAT(REPEAT('_', 250), "ff"));
+SELECT MAX(b) FROM t1;
+MAX(b)
+__________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________zz
+EXPLAIN SELECT MAX(b) FROM t1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4	
+DROP TABLE t1;

mysql-test/t/func_group.test

@@ -617,4 +617,13 @@ SELECT MAX(b) FROM t1;
 EXPLAIN SELECT MAX(b) FROM t1;
 DROP TABLE t1;
 
+CREATE TABLE t1 (id int , b varchar(512), INDEX(b(250))) COLLATE latin1_bin;
+INSERT INTO t1 VALUES
+  (1,CONCAT(REPEAT('_', 250), "qq")), (1,CONCAT(REPEAT('_', 250), "zz")),
+  (1,CONCAT(REPEAT('_', 250), "aa")), (1,CONCAT(REPEAT('_', 250), "ff"));
+
+SELECT MAX(b) FROM t1;
+EXPLAIN SELECT MAX(b) FROM t1;
+DROP TABLE t1;
+
 # End of 4.1 tests

sql/opt_sum.cc

@@ -685,8 +685,10 @@ static bool find_key_for_maxmin(bool max_fl, TABLE_REF *ref,
       if (!(table->file->index_flags(idx, jdx, 0) & HA_READ_ORDER))
         return 0;
 
-        /* Check whether the index component is partial */
-      if (part->length < table->field[part->fieldnr-1]->pack_length())
+      /* Check whether the index component is partial */
+      Field *part_field= table->field[part->fieldnr-1];
+      if ((part_field->flags & BLOB_FLAG) ||
+          part->length < part_field->key_length())
         break;
 
       if (field->eq(part->field))