Feature Request: better typing while planning

[systay]

Feature Description

Vitess comes with a good expression typing system in the evaluation engine, but that is not used today while planning. This means that for a large number of queries, we end up using weight_strings since the very limited typing system available for the planner can't figure out the type of most expressions.

It would be great if we could use the already existing type system during query planning.

Example

For this simple query:

select 1 from user order by user.col + user.col

the produced plan still uses weight_strings because the planner can't figure out the type of user.col + user.col.

{
  "QueryType": "SELECT",
  "Original": "select 1 from user order by user.col + user.col",
  "Instructions": {
    "OperatorType": "Route",
    "Variant": "Scatter",
    "Keyspace": {
      "Name": "user",
      "Sharded": true
    },
    "FieldQuery": "select 1, `user`.col + `user`.col, weight_string(`user`.col + `user`.col) from `user` where 1 != 1",
    "OrderBy": "(1|2) ASC",
    "Query": "select 1, `user`.col + `user`.col, weight_string(`user`.col + `user`.col) from `user` order by `user`.col + `user`.col asc",
    "ResultColumns": 1,
    "Table": "`user`"
  },
  "TablesUsed": [
    "user.user"
  ]
}