SQL Operator with real SQL

[Yicong-Huang]

A general SQL Operator which lets user to write SQL query directly, this way it is more flexible to express the need. This feature can be combined with PreSet to replace existing SQL operators.

One thing to investigate is how to obtain the schema before executing the SQL query. There are two ways:

1. DB/JDBC might have API to query for the schema with a given SQL query.
2. some SQL compiler like calcite to compile schema out from SQL query.

[zuozhiw]

Discussion 2022.05.19: low priority

[Xiao-zhen-Liu]

Discussion 2022.05.12: Low priority.