SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    5596, 5678, 5603, 5651, 5877, 5784, 5797, 
    5963, 5983, 6077, 6148, 6190, 6211, 
    6274, 6347, 6401, 6413, 6507, 5848, 
    5666, 5673, 5713, 5767, 5887, 5806, 
    6031, 6000, 6096, 6163, 6242, 6226, 
    6360, 6466, 6516, 6444, 5721, 5858, 
    5897, 5835, 5950, 6036, 6011, 6256, 
    6372, 6478, 6529, 6457, 5905, 5728, 
    5868, 6109, 6132, 6287, 6491, 5738, 
    5918, 6178, 6300, 6539, 5930, 6322, 
    6337, 6553, 5941, 6391, 6572, 6588, 
    6610, 6626, 6647
  )

Query time 0.00064

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "45.89"
    },
    "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": 112,
      "rows_produced_per_join": 112,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "23.49",
        "eval_cost": "22.40",
        "prefix_cost": "45.89",
        "data_read_per_join": "664K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`webmarco`.`cscart_categories`.`parent_id` in (5596,5678,5603,5651,5877,5784,5797,5963,5983,6077,6148,6190,6211,6274,6347,6401,6413,6507,5848,5666,5673,5713,5767,5887,5806,6031,6000,6096,6163,6242,6226,6360,6466,6516,6444,5721,5858,5897,5835,5950,6036,6011,6256,6372,6478,6529,6457,5905,5728,5868,6109,6132,6287,6491,5738,5918,6178,6300,6539,5930,6322,6337,6553,5941,6391,6572,6588,6610,6626,6647))"
    }
  }
}