SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    5490, 6800, 6399, 6923, 5534, 6526, 5581, 
    5638, 5683, 5729, 5808, 5906, 6010, 
    6110, 6247, 6659, 7085, 7223
  )

Query time 0.00041

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "9.02"
    },
    "table": {
      "table_name": "cscart_categories",
      "access_type": "range",
      "possible_keys": [
        "parent"
      ],
      "key": "parent",
      "used_key_parts": [
        "parent_id"
      ],
      "key_length": "3",
      "rows_examined_per_scan": 20,
      "rows_produced_per_join": 20,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "5.02",
        "eval_cost": "4.00",
        "prefix_cost": "9.02",
        "data_read_per_join": "118K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`webmarco`.`cscart_categories`.`parent_id` in (5490,6800,6399,6923,5534,6526,5581,5638,5683,5729,5808,5906,6010,6110,6247,6659,7085,7223))"
    }
  }
}