planner: error when access enum column by using a cached plan

[qw4990]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table tidx(e enum("a", "b", "c"), key(e));
prepare st from 'select * from t where e>?';
set @a=4;
execute st using @a;

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

No error.

3. What did you see instead (Required)

ERROR 1265 (01000): Data truncated for column '%s' at row %d

4. What is your TiDB version? (Required)

| 5.7.25-TiDB-v6.7.0-alpha-168-gad01574a71 |

[qw4990]

It's very rare to access enum columns with plan-cache, so I just give this issue a moderate severity.