explain for connection shows incorrect empty result

[zz-jason]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

select connection_id(); -- which is 2199023255563 in my case
drop table if exists t1;
create table t1 (id int unsigned not null auto_increment primary key, t text, index(t(8)));
insert into t1 (t) values (md5('a')),(md5('b'));
select * from t1 where t='0cc175b9c0f1b6a831c399e269772661' or t='92eb5ffee6ae2fec3ad71c777531578f';

explain for connection 2199023255563; -- the id of the current conection

2. What did you expect to see? (Required)

the execution plan of the current connection, not an empty result.

3. What did you see instead (Required)

TiDB(root@127.0.0.1:test) > explain for connection 2199023255563;
Query OK, 0 rows affected (0.00 sec)

4. What is your TiDB version? (Required)

TiDB(root@127.0.0.1:test) > select tidb_version()\G
*************************** 1. row ***************************
tidb_version(): Release Version: v4.0.0-beta.2-1606-gfbaab3ecb
Edition: Community
Git Commit Hash: fbaab3ecb60ad0524c22d3fb5532ebc00bc19d91
Git Branch: master
UTC Build Time: 2020-11-21 02:52:10
GoVersion: go1.15
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false
1 row in set (0.00 sec)

[zz-jason]

I found this when reproducing #21234

[SunRunAway]

/assign @crazycs520

[lzmhhh123]

If the SQL finished. The explain for connection will return empty. In otherwise, SQL not finish, the explain for is still working.

[SunRunAway]

Could you describe this feature which is different from MySQL in the documentation? @crazycs520

[ti-srebot]

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