FR: EXPLAIN on partitioned table should describe partitions

[morgo]

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

MySQL [test]> SHOW CREATE TABLE employees\G
*************************** 1. row ***************************
Table: employees
Create Table: CREATE TABLE `employees` (
`id` int(11) NOT NULL,
`fname` varchar(30) DEFAULT NULL,
`lname` varchar(30) DEFAULT NULL,
`hired` date NOT NULL DEFAULT '1970-01-01',
`separated` date NOT NULL DEFAULT '9999-12-31',
`store_id` int(11) NOT NULL,
PRIMARY KEY (`id`,`store_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin
PARTITION BY RANGE ( `store_id` ) (
PARTITION p0 VALUES LESS THAN (6),
PARTITION p1 VALUES LESS THAN (11),
PARTITION p2 VALUES LESS THAN (16)
)
1 row in set (0.00 sec)

MySQL [test]> EXPLAIN SELECT * FROM employees WHERE store_id IN (5,6);
+-------------------------+----------+------+--------------------------------------------------------------------+
| id                      | count    | task | operator info                                                      |
+-------------------------+----------+------+--------------------------------------------------------------------+
| Union_8                 | 4.00     | root |                                                                    |
| ├─TableReader_11        | 2.00     | root | data:Selection_10                                                  |
| │ └─Selection_10        | 2.00     | cop  | in(test.employees.store_id, 5, 6)                                  |
| │   └─TableScan_9       | 10000.00 | cop  | table:employees, range:[-inf,+inf], keep order:false, stats:pseudo |
| └─TableReader_14        | 2.00     | root | data:Selection_13                                                  |
|   └─Selection_13        | 2.00     | cop  | in(test.employees.store_id, 5, 6)                                  |
|     └─TableScan_12      | 10000.00 | cop  | table:employees, range:[-inf,+inf], keep order:false, stats:pseudo |
+-------------------------+----------+------+--------------------------------------------------------------------+
7 rows in set (0.00 sec)

2. What did you expect to see?

I would like to see the table:employees include the partition name. For example:
table:employees[p0]

This is helpful if I've used pd to move certain partition's regions to different geographic regions. I can confirm with EXPLAIN that the data ended up where expected.

3. What did you see instead?

All partitions are named for the base table: employees.

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

MySQL [(none)]> SELECT tidb_version()\G                                                                                                                                                                                   *************************** 1. row ***************************
tidb_version(): Release Version: v2.1.0-rc.1-13-g1255973
Git Commit Hash: 1255973764f72575d71920b95fc0861d1677c6c5
Git Branch: master
UTC Build Time: 2018-08-28 07:35:57
GoVersion: go version go1.10.2 linux/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e
Check Table Before Drop: false
1 row in set (0.00 sec)

[morgo]

Added in #7505