Incorrect Column Reference in ORDER BY Subquery Plan in vtgate

[systay]

Issue Description

Problem

When performing an ORDER BY on a subquery, the vtgate planner generates a plan that does not work.

Example

For the following query:

SELECT (SELECT col FROM user LIMIT 1) AS a 
FROM user 
JOIN user_extra 
ORDER BY a;

The planner produces a plan containing this snippet:

{
  "OperatorType": "UncorrelatedSubquery",
  "Variant": "PulloutValue",
  "PulloutVars": [
    "__sq1"
  ],
  "Inputs": [
    {
      "InputName": "SubQuery",
      "OperatorType": "Limit",
      "Count": "1",
      "Inputs": [
        {
          "OperatorType": "Route",
          "Variant": "Scatter",
          "Keyspace": {
            "Name": "user",
            "Sharded": true
          },
          "FieldQuery": "select col from `user` where 1 != 1",
          "Query": "select col from `user` limit 1",
          "Table": "`user`"
        }
      ]
    },
    {
      "InputName": "Outer",
      "OperatorType": "Route",
      "Variant": "Scatter",
      "Keyspace": {
        "Name": "user",
        "Sharded": true
      },
      "FieldQuery": "select :__sq1 as __sq1, weight_string(:__sq1) from `user` where 1 != 1",
      "OrderBy": "(0|1) ASC",
      "Query": "select :__sq1 as __sq1, weight_string(:__sq1) from `user` order by __sq1 asc",
      "Table": "`user`"
    }
  ]
}

Issue

In the query sent to the outer side of the subquery, the plan uses the column name __sq1 instead of the argument :__sq1, which would be the correct plan.

Result

This discrepancy results in a query that fails on MySQL, as no such column (__sq1) can be found.