Explaining for connection return runtime error after EXECUTE Statement

[ChenPeng2013]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

prepared-plan-cache.enabled=true;

use test;
drop table if exists t;
create table t(a int);
select connection_id();
prepare stmt from "select * from t where a=?";
set @a=1;
execute stmt using @a;
# explain for connection id;

2. What did you expect to see?

mysql> explain for connection 1;
+-------------------------+----------+-----------+---------------+--------------------------------+
| id                      | estRows  | task      | access object | operator info                  |
+-------------------------+----------+-----------+---------------+--------------------------------+
| TableReader_7           | 10.00    | root      |               | data:Selection_6               |
| └─Selection_6       | 10.00    | cop[tikv] |               | eq(test.t.a, 1)                |
|   └─TableFullScan_5 | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------+--------------------------------+

3. What did you see instead?

mysql> explain for connection 1;
ERROR 1105 (HY000): runtime error: index out of range [0] with length 0

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

Release Version: v4.0.0-beta.2-151-g78922a8ab
Git Commit Hash: 78922a8abef2f21e4d8990d37c6c67ac7b0eef42
Git Branch: release-4.0
UTC Build Time: 2020-04-07 06:22:22
GoVersion: go1.13.1
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false