Fix type coercion in cascading non-literal updates

[GuptaManan100]

Description

This PR fixes the issue described in #14523.

I looked at the plan and followed along with what Vitess should have done. The first thing we do is run a SELECT query and that works as intended -

mysql [localhost:8032] {msandbox} (foreign_key_rfc) > select col, col <=> col * (col - col), col * (col - col) from fk_t15 for update;
+------+---------------------------+-------------------+
| col  | col <=> col * (col - col) | col * (col - col) |
+------+---------------------------+-------------------+
| -0   |                         1 |                -0 |
| -5   |                         0 |                -0 |
| 0    |                         1 |                 0 |
| 5    |                         0 |                 0 |
+------+---------------------------+-------------------+
4 rows in set (0.00 sec)

This gives us the correct value of -0 as we expect it to. However, the value is a float-type value.
Next, we end up executing the query -

update /*+ SET_VAR(foreign_key_checks=OFF) */ fk_t16 set col = -0 where col in (('-5')) 

This query is intended to cascade the update on the child. Notice that we are setting col to -0 and not '-0' because the value we received from the select was a float-type value.

This query is where the problem is. When we run this query on MySQL, col is actually set to 0 and not -0.

The proposed fix is to first typecast the value we receive from the SELECT query into the type of the column, and then send it down to MySQL.

Post the changes, the query looks like

update /*+ SET_VAR(foreign_key_checks=OFF) */ fk_t16 set col = '-0' where col in (('-5')) 

and this resolves the issue.

Related Issue(s)

• Fixes Bug Report: Vitess managed foreign key cascades incorrect value #14523

Checklist

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