SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    2782, 2783, 2784, 2730, 3167, 2500, 2501, 
    2502, 2503, 3421, 3051, 2785, 2786, 
    2787, 2788, 2789, 2790, 2791, 2792
  )

Query time 0.00020

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": 19,
      "filtered": 100,
      "index_condition": "cscart_categories.parent_id in (2782,2783,2784,2730,3167,2500,2501,2502,2503,3421,3051,2785,2786,2787,2788,2789,2790,2791,2792)"
    }
  }
}