use <nil> to represent NULL is weird

[zz-jason]

let's say we have the following table:

drop table if exists t;
create table t(a char(20), index idx_a(a));

The following two queries result in the same explain result but they represent different DML queries:

desc select * from t where a = "<nil>";
desc select * from t where a is null;

TiDB(localhost:4000) > desc select * from t where a = "<nil>";
+-------------------+------+---------------------------------------------------------+-------+
| id                | task | operator info                                           | count |
+-------------------+------+---------------------------------------------------------+-------+
| IndexReader_9     | root | index:IndexScan_8                                       | 10.00 |
| └─IndexScan_8     | cop  | table:t, index:a, range:[<nil>,<nil>], keep order:false | 10.00 |
+-------------------+------+---------------------------------------------------------+-------+
2 rows in set (0.00 sec)

TiDB(localhost:4000) > desc select * from t where a is null;
+-------------------+------+---------------------------------------------------------+-------+
| id                | task | operator info                                           | count |
+-------------------+------+---------------------------------------------------------+-------+
| IndexReader_9     | root | index:IndexScan_8                                       | 10.00 |
| └─IndexScan_8     | cop  | table:t, index:a, range:[<nil>,<nil>], keep order:false | 10.00 |
+-------------------+------+---------------------------------------------------------+-------+
2 rows in set (0.00 sec)

Actually no mater what string we use to represent NULL, there is a chance that a is null and a = xx can result in the same range representation once the string we use to represent is equal to xx.

A way to solve this problem is to wrap strings with ", we can use the string "NULL" to represent NULL, which means we can distinguish NULL from other strings:

TiDB(localhost:4000) > desc select * from t where a = "NULL";
+-------------------+------+---------------------------------------------------------+-------+
| id                | task | operator info                                           | count |
+-------------------+------+---------------------------------------------------------+-------+
| IndexReader_9     | root | index:IndexScan_8                                       | 10.00 |
| └─IndexScan_8     | cop  | table:t, index:a, range:["NULL","NULL"], keep order:false | 10.00 |
+-------------------+------+---------------------------------------------------------+-------+
2 rows in set (0.00 sec)

TiDB(localhost:4000) > desc select * from t where a is null;
+-------------------+------+---------------------------------------------------------+-------+
| id                | task | operator info                                           | count |
+-------------------+------+---------------------------------------------------------+-------+
| IndexReader_9     | root | index:IndexScan_8                                       | 10.00 |
| └─IndexScan_8     | cop  | table:t, index:a, range:[NULL,NULL], keep order:false | 10.00 |
+-------------------+------+---------------------------------------------------------+-------+
2 rows in set (0.00 sec)