MySQL has something in that direction, where some operations can be expressed as expression trees by using Protobuf where a client can put some arbitrary query

MySQL has something in that direction, where some operations can be expressed as expression trees by using Protobuf where a client can put some arbitrary query language on top: https://dev.mysql.com/doc/internals/en/x-protocol-messages-messages.html https://dev.mysql.com/doc/internals/en/x-protocol-messages-m... it is still a bit limited, though (like no join ...)