planner: refine the infomation when execute desc format="dot" for core.Explain.

[LittleFall]

1. What did you do?

desc select 1+1;
desc for connection 1; 
desc select 1+1;
desc format="dot" for connection 1;

2. What did you expect to see?

Output an empty graph, no error.

3. What did you see instead?

mysql> desc for connection 1;
Empty set (0.00 sec)

mysql> desc format="dot" for connection 1;
ERROR 1105 (HY000): interface conversion: *core.Explain is not core.PhysicalPlan: missing method Children

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

mysql> select tidb_version();
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version()

          |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v4.0.0-beta.2-441-g36a29da52
Git Commit Hash: 36a29da529c4abc1f59958a465469aca8726e492
Git Branch: master
UTC Build Time: 2020-05-12 11:51:52
GoVersion: go1.14.1
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)