bug#4816. index search for NULL in blob

myisam/mi_key.c

@@ -177,6 +177,8 @@ uint _mi_pack_key(register MI_INFO *info, uint keynr, uchar *key, uchar *old,
       if (!(*key++= (char) 1-*old++))			/* Copy null marker */
       {
 	k_length-=length;
+        if (keyseg->flag & (HA_VAR_LENGTH | HA_BLOB_PART))
+          k_length-=2;                                  /* Skip length */
 	continue;					/* Found NULL */
       }
     }

mysql-test/r/myisam.result

@@ -453,3 +453,12 @@ table	type	possible_keys	key	key_len	ref	rows	Extra
 t1	system	NULL	NULL	NULL	NULL	1	Using temporary
 t2	index	NULL	PRIMARY	4	NULL	2	Using index; Distinct
 drop table t1,t2;
+create table t1 ( a tinytext, b char(1), index idx (a(1),b) );
+insert into t1 values (null,''), (null,'');
+explain select count(*) from t1 where a is null;
+table	type	possible_keys	key	key_len	ref	rows	Extra
+t1	ref	idx	idx	4	const	1	Using where
+select count(*) from t1 where a is null;
+count(*)
+2
+drop table t1;

mysql-test/t/myisam.test

@@ -436,3 +436,13 @@ select sql_big_result distinct t1.a from t1,t2;
 explain select sql_big_result distinct t1.a from t1,t2 order by t2.a;
 explain select distinct t1.a from t1,t2 order by t2.a;
 drop table t1,t2;
+
+#
+# index search for NULL in blob. Bug #4816
+#
+create table t1 ( a tinytext, b char(1), index idx (a(1),b) );
+insert into t1 values (null,''), (null,'');
+explain select count(*) from t1 where a is null;
+select count(*) from t1 where a is null;
+drop table t1;
+