How to join 2 arrays in a performant way using jSONata?

Question

I would like to join 2 arrays having about 500 elements in a performant way using JSONata.

I have found a way to join 2 arrays but it is not very performant. See https://try.jsonata.org/VqzeZDAjA (The same input/output and query is copied here below).

Input :

{
  "_msgid": "a070e32c.e71ed",
  "topic": "",
  "rc": {
    "code": 0
  },
  "table1": {
    "array1": [
      {
        "country_region": "Thailand",
        "field_A": "A for Thailand"
      },
      {
        "country_region": "Japan",
        "field_A": "A for Japan"
      }
    ]
  },
  "array2": [
    {
      "country_region": "Thailand",
      "field_B": "B for Thailand"
    },
    {
      "country_region": "Japan",
      "field_B": "B for Japan"
    }
  ]
}

Expected output:

{
  "array1": [
    {
      "country_region": "Thailand",
      "field_A": "A for Thailand",
      "field_B": "B for Thailand"
    },
    {
      "country_region": "Japan",
      "field_A": "A for Japan",
      "field_B": "B for Japan"
    }
  ]
}

Working query but not very performant in case of arrays with 500 elements.

(
    $array2 := array2;
    table1 ~> | array1 | { "field_B" : 
              ($country_region := country_region;
               $array2[$country_region=$.country_region])
                .field_B}|
)

Update 2020-03-29

Here above it is claimed that the Working query (see above) is not very performant. Further analysis revealed that this not true: the actual performance of the above query is fine (similar to the performance of the approved query). The performance issue I encountered was caused by another JSONata query that has nothing to do with respect to this join !

Answer 1

Not sure if it's any more performant, but I'd use the join syntax to do this:

{
    "array1": array2@$A2.table1.array1@$A1[$A1.country_region = $A2.country_region].$merge([$A1, $A2])   
}