Handle NULL parameters like MySQL

[tirsen]

When you pass in a NULL parameter to a prepared statement query MySQL handles that just fine. Vitess on the other hand gives the following error:

Caused by: java.sql.SQLNonTransientException: Vitess RPC error: legacy_code: UNKNOWN_ERROR_LEGACY
message: "vtgate: http://localhost:15001/: paramsSelectEqual: hash.Map: could not parse value: "
code: UNKNOWN

[tirsen]

(Technically you should be using an IS NULL clause in this case but it wouldn't be the first time that MySQL diverges from the SQL standard. I think Vitess should conform to MySQL as much as possible to minimize porting effort.)

[sougou]

I think the vitess behavior for the use case you showed is correct. It's not able to map a NULL value to a keyspace id. Vitess allows nulls as bind vars, but they should be used with care. For example, where id = NULL is always FALSE 😭. I actually looked at doctoring equality expressions to is null, but realized the risk because values are used in so many other constructs, like IN clauses, etc. So, there's no way to uniformly extend the null handling semantics for all constructs.

[sougou]

This is now addressed via #3182