SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    265, 267, 278, 268, 270, 271, 276, 275
  )

Query time 0.00023

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "6.06"
    },
    "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": 9,
      "rows_produced_per_join": 9,
      "filtered": "100.00",
      "index_condition": "(`primapearl_r_new`.`cscart_categories`.`parent_id` in (265,267,278,268,270,271,276,275))",
      "cost_info": {
        "read_cost": "5.16",
        "eval_cost": "0.90",
        "prefix_cost": "6.06",
        "data_read_per_join": "39K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ]
    }
  }
}