supports_index_sort_order? should return false

[yahonda]

supports_index_sort_order? defines if the database supports ascending and descending index.

Currently it returns true, however tidb does not support descending index yet then this method should return false until pingcap/tidb#2519 is implemented to TiDB.

Refer rails/rails#28773 for Rails pull request.

[hooopo]

Yes, you are right. In the current state, the sort order syntax is compatible with tidb ddl, but it is actually ineffective.

my:root@127.0.0.1:4000=> create table a (id bigint primary key, name varchar(255));
my:root@127.0.0.1:4000=> CREATE INDEX idx ON a (name desc);
CREATE INDEX
my:root@127.0.0.1:4000=> show create table a;
 Table | Create Table
-------+-------------------------------------------------------------
 a     | CREATE TABLE "a" (                                         +
       |   "id" bigint(20) NOT NULL,                                +
       |   "name" varchar(255) DEFAULT NULL,                        +
       |   PRIMARY KEY ("id") /*T![clustered_index] CLUSTERED */,   +
       |   KEY "idx" ("name")                                       +
       | ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin

the Collation column should return 'D' for TiDB.

 show keys from a;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+-----------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression | Clustered |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+-----------+
| a     |          0 | PRIMARY  |            1 | id          | A         |           0 |     NULL | NULL   |      | BTREE      |         |               | YES     | NULL       | YES       |
| a     |          1 | idx      |            1 | name        | A         |           0 |     NULL | NULL   | YES  | BTREE      |         |               | YES     | NULL       | NO        |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+-----------+

[yahonda]

Thanks for the reply. Let me create a pull request to change supports_index_sort_order?.

[yahonda]

Opened #17