A fix (bug #6142: SELECT DISTINCT on key field crashes server)

sql/opt_range.cc:
  A fix (bug #6142: SELECT DISTINCT on key field crashes server)
  Code clean-up.

mysql-test/r/group_min_max.result

@@ -1897,3 +1897,9 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 drop table t1;
 drop table t2;
 drop table t3;
+create table t1 (
+a varchar(30), b varchar(30), primary key(a), key(b)
+) engine=innodb;
+select distinct a from t1;
+a
+drop table t1;

mysql-test/t/group_min_max.test

@@ -577,3 +577,13 @@ explain select sum(ord(a1)) from t1 where (a1 > 'a') group by a1,a2,b;
 drop table t1;
 drop table t2;
 drop table t3;
+
+#
+# Bug #6142: a problem with the empty innodb table
+#
+
+create table t1 (
+  a varchar(30), b varchar(30), primary key(a), key(b)
+) engine=innodb;
+select distinct a from t1;
+drop table t1;

sql/opt_range.cc

@@ -7040,17 +7040,16 @@ get_constant_key_infix(KEY *index_info, SEL_ARG *index_range_tree,
 static inline uint
 get_field_keypart(KEY *index, Field *field)
 {
-  KEY_PART_INFO *part= index->key_part;
+  KEY_PART_INFO *part, *end;
   uint key_part_num= 0;
 
-  while (part != part + index->key_parts)
+  for (part= index->key_part, end= part + index->key_parts; part < end; part++)
   {
     key_part_num++;
     if (field->eq(part->field))
       return key_part_num;
-    part++;
   }
-  return key_part_num;
+  return 0;
 }