Incomparable Varchars on vtgate

[GuptaManan100]

Overview of the Issue

Running the query select version from schema_migrations order by version ASC in the sharded mode returns the error ERROR 1105 (HY000): types are not comparable: VARCHAR vs VARCHAR. This query support is required for rails compatibility.

Reproduction Steps

The issue can be reproduced by having a table with varchar columns in a sharded keyspace and trying to select that column in ascending order.

For example

mysql> select * from schema_migrations;
+----------------+
| version        |
+----------------+
| 20190822013911 |
| 20190822021835 |
| 20190822031056 |
| 20190823000019 |
| 20190823171209 |
| 20190823175841 |
| 20190824013003 |
| 20190824113338 |
| 20190827011913 |
| 20190827030205 |
| 20210201161500 |
| 20210204134900 |
| 20210205144300 |
| 20210208114300 |
| 20210209113800 |
| 20210209125800 |
| 20210209150300 |
| 20210210095100 |
| 20210210115100 |
| 20210210115800 |
| 20210210152000 |
| 20210211101500 |
| 20210212131000 |
| 20210212131200 |
| 20210212153800 |
| 20210215132200 |
| 20210216101500 |
| 20210216160700 |
| 20210217173100 |
| 20210219164600 |
| 20210219184900 |
| 20210222101300 |
+----------------+
32 rows in set (0.01 sec)
mysql> select version from schema_migrations order by version ASC;
ERROR 1105 (HY000): types are not comparable: VARCHAR vs VARCHAR

[systay]

Solved by #7678