SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    3868, 3842, 3843, 3837, 3733, 3848, 1090, 
    3851, 3869, 3732, 3798, 3790, 3726, 
    3867, 3834, 1089, 3835, 3836, 2331, 
    3847, 3855, 3856, 3866, 1088, 2330, 
    3841
  )

Query time 0.00039

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_categories",
      "access_type": "range",
      "possible_keys": ["parent"],
      "key": "parent",
      "key_length": "3",
      "used_key_parts": ["parent_id"],
      "rows": 79,
      "filtered": 100,
      "index_condition": "cscart_categories.parent_id in (3868,3842,3843,3837,3733,3848,1090,3851,3869,3732,3798,3790,3726,3867,3834,1089,3835,3836,2331,3847,3855,3856,3866,1088,2330,3841)"
    }
  }
}