SELECT 
  pfv.variant_id, 
  pfv.position, 
  pfvd.variant 
FROM 
  cscart_product_feature_variants AS pfv 
  INNER JOIN cscart_product_feature_variant_descriptions AS pfvd ON pfv.variant_id = pfvd.variant_id 
  AND pfvd.lang_code = 'en' 
WHERE 
  pfv.variant_id IN (
    63646, 63647, 63669, 63670, 63671, 63672, 
    63673, 63947, 63948
  )

Query time 0.00060

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "23.42"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "pfv",
          "access_type": "range",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "variant_id"
          ],
          "key_length": "3",
          "rows_examined_per_scan": 9,
          "rows_produced_per_join": 9,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "10.82",
            "eval_cost": "1.80",
            "prefix_cost": "12.62",
            "data_read_per_join": "10K"
          },
          "used_columns": [
            "variant_id",
            "position"
          ],
          "attached_condition": "(`webmarco`.`pfv`.`variant_id` in (63646,63647,63669,63670,63671,63672,63673,63947,63948))"
        }
      },
      {
        "table": {
          "table_name": "pfvd",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "variant_id",
            "lang_code"
          ],
          "key_length": "9",
          "ref": [
            "webmarco.pfv.variant_id",
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 9,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "9.00",
            "eval_cost": "1.80",
            "prefix_cost": "23.42",
            "data_read_per_join": "40K"
          },
          "used_columns": [
            "variant_id",
            "variant",
            "lang_code"
          ]
        }
      }
    ]
  }
}