SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    6342, 6498, 6414, 6047, 6015, 6255, 6321, 
    6531, 6435, 6193, 6418, 6099, 6555, 
    6463, 6229, 6154, 6513, 6607, 6294, 
    6648, 6299, 6522, 6693, 6534, 6449, 
    6747, 6485, 6788, 6815, 7015, 6840, 
    7074, 6864, 7111, 6905, 6947, 6977
  )

Query time 0.00045

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "24.25"
    },
    "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": 58,
      "rows_produced_per_join": 58,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "12.65",
        "eval_cost": "11.60",
        "prefix_cost": "24.25",
        "data_read_per_join": "343K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`webmarco`.`cscart_categories`.`parent_id` in (6342,6498,6414,6047,6015,6255,6321,6531,6435,6193,6418,6099,6555,6463,6229,6154,6513,6607,6294,6648,6299,6522,6693,6534,6449,6747,6485,6788,6815,7015,6840,7074,6864,7111,6905,6947,6977))"
    }
  }
}