Proposal: A unified query intermediate representation for RediSearch and SQL

[PragmaTwice]

It is part of #2064.

RediSearch provides a special syntax for data query, while its semantics has no difference than a subset of SQL DQL.

To be comatible with RediSearch, we should support such query syntax. However, if we can also offer a SQL-like syntax simultaneously, it will greatly benefit users and minimize the learning curve. (Especially since the Redis query syntax is weird and has huge semantic gaps between different versions: v1 and v2.)

To support both syntaxes simultaneously, we can design a unified intermediate representation (called Kvrocks Query Intermediate Representation, KQIR) and transform both syntaxes into this representation when processing user input.

Hence, the frontend of KQIR will be:

• Redis query syntax (v2) and FT.SEARCH fields (for maintainance consideration, v1 will NOT be supported),
• SQL select statement (try to be compatible with MySQL)

Detailed design will be provided soon.

[jihuayu]

It seems like we don't need support JOIN because FT.SEARCH doesn't support JOIN. It is a good news for KQIR design.