Query with `CONV` expression gives incorrect result if using plan cache

[YangKeao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

It actually has two issues:

1. The result is not consistent with / without plan cache.
2. The result is always not compatible with MySQL.

1. Minimal reproduce step (Required)

create table t (v varchar(16));
insert into t values ('156');
prepare stmt7 from 'select * from t where v = conv(?, 16, 8)';
set @arg=0x6E;
execute stmt7 using @arg;
execute stmt7 using @arg;
set @arg=0x70;
execute stmt7 using @arg;

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

The result from MySQL is:

mysql> use test;
Database changed
mysql> create table t (v varchar(16));
Query OK, 0 rows affected (0.01 sec)

mysql> insert into t values ('156');
Query OK, 1 row affected (0.00 sec)

mysql> prepare stmt7 from 'select * from t where v = conv(?, 16, 8)';
Query OK, 0 rows affected (0.00 sec)
Statement prepared

mysql> set @arg=0x6E;
Query OK, 0 rows affected (0.00 sec)

mysql> execute stmt7 using @arg;
Empty set, 1 warning (0.00 sec)

mysql> execute stmt7 using @arg;
Empty set, 1 warning (0.00 sec)

mysql> set @arg=0x70;
Query OK, 0 rows affected (0.00 sec)

mysql> execute stmt7 using @arg;
Empty set, 1 warning (0.00 sec)

3. What did you see instead (Required)

The result from TiDB is:

mysql> create table t (v varchar(16));
Query OK, 0 rows affected (0.12 sec)

mysql> insert into t values ('156');
Query OK, 1 row affected (0.01 sec)

mysql> prepare stmt7 from 'select * from t where v = conv(?, 16, 8)';
Query OK, 0 rows affected (0.00 sec)

mysql> set @arg=0x6E;
Query OK, 0 rows affected (0.00 sec)

mysql> execute stmt7 using @arg;
+------+
| v    |
+------+
| 156  |
+------+
1 row in set (0.00 sec)

mysql> execute stmt7 using @arg;
+------+
| v    |
+------+
| 156  |
+------+
1 row in set (0.00 sec)

mysql> set @arg=0x70;
Query OK, 0 rows affected (0.00 sec)

mysql> execute stmt7 using @arg;
+------+
| v    |
+------+
| 156  |
+------+
1 row in set (0.00 sec)

If you execute the second statement without using plan cache, it'll give nothing:

mysql> create table t (v varchar(16));
Query OK, 0 rows affected (0.11 sec)

mysql> insert into t values ('156');
Query OK, 1 row affected (0.01 sec)

mysql> prepare stmt7 from 'select * from t where v = conv(?, 16, 8)';
Query OK, 0 rows affected (0.00 sec)

mysql> set @arg=0x70;
Query OK, 0 rows affected (0.00 sec)

mysql> execute stmt7 using @arg;
Empty set (0.01 sec)

4. What is your TiDB version? (Required)