Bug Report: Incorrect results for multi-table delete query with foreign keys about vitess

[GuptaManan100]

I looked at this failure, and I know what's happening.
The plan that Vitess produces for this query is this -

{
  "OperatorType": "FkCascade",
  "Inputs": [
    {
   "InputName": "Selection",
      "OperatorType": "Route",
      "Variant": "Unsharded",
      "Keyspace": {
        "Name": "uks",
        "Sharded": false
      },
      "FieldQuery": "select fk_t11.col from fk_t11, fk_t12 where 1 != 1",
      "Query": "select fk_t11.col from fk_t11, fk_t12 where fk_t11.id = 4 and fk_t11.id = fk_t12.id for update",
      "Table": "fk_t11, fk_t12"
    },
    {
    "InputName": "CascadeChild-1",
      "OperatorType": "Delete",
      "Variant": "Unsharded",
      "Keyspace": {
        "Name": "uks",
        "Sharded": false
      },
      "TargetTabletType": "PRIMARY",
      "BvName": "fkc_vals",
      "Cols": [
        0
      ],
      "Query": "delete from fk_t12 where (col) in ::fkc_vals",
      "Table": "fk_t12"
    },
    {
    "InputName": "Parent",
      "OperatorType": "Delete",
      "Variant": "Unsharded",
      "Keyspace": {
        "Name": "uks",
        "Sharded": false
      },
      "TargetTabletType": "PRIMARY",
      "Query": "delete fk_t11 from fk_t11, fk_t12 where fk_t11.id = 4 and fk_t11.id = fk_t12.id",
      "Table": "fk_t11"
    }
  ]
}

From the outset, the plan looks fine, but when we try to see the exact order of execution, the problem becomes apparent.

The order of queries Vitess runs is this -

1. select fk_t11.col from fk_t11, fk_t12 where fk_t11.id = 4 and fk_t11.id = fk_t12.id for update
2. delete from fk_t12 where (col) in ::fkc_vals
3. delete fk_t11 from fk_t11, fk_t12 where fk_t11.id = 4 and fk_t11.id = fk_t12.id

The problem is that once the 2nd query runs, (which is the cascade), the rows with id = 4 are gone from the fk_t12 table!! So now when we run the last query, there is no delete, since no row qualifies and therefore no failure!

This essentially shows up as though the delete query only ended up deleting 2 rows from fk_t12 and nothing else!