using explain to explain a sql which has SET_VAR in binding, SET_VAR becomes session level

[winoros]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

set @@max_execution_time=2000;
select @@max_execution_time;
explain select /*+ set_var(max_execution_time=100) */ @@max_execution_time;
select @@max_execution_time;
drop table if exists t;
create table t(a int);
create global binding for select * from t where a = 1 and sleep(0.1) using select /*+ SET_VAR(max_execution_time=500) */ * from t where a = 1 and sleep(0.1);
select * from t where a = 1 and sleep(0.1);
select @@last_plan_from_binding;
select @@max_execution_time;
explain select * from t where a = 1 and sleep(0.1);
select @@last_plan_from_binding;
select @@max_execution_time;

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

directly explain the SQL or execute it with binding, the session var is not changed after the execution is finished.

but if we explain the SQL which has a binding, and the binding has SET_VAR, we'll see that the SET_VAR changes the value of session var.

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

master branch