Currently, our JDBC connector provides the option `dbtable` for users to specify the to-be-loaded JDBC source table.
Normally, users do not fetch the whole JDBC table due to the poor performance/throughput of JDBC. Thus, they normally just fetch a small set of tables. For advanced users, they can pass a subquery as the option.
However, this is straightforward to end users. We should simply allow users to specify the query by a new option `query`. We will handle the complexity for them.
Users are not allowed to specify query and dbtable at the same time.