SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    75, 71, 78, 12, 14, 34, 37, 74, 13, 15, 73, 
    72, 35, 36, 40, 41, 42, 43, 20
  )

Query time 0.00038

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": 24,
      "filtered": 100,
      "attached_condition": "cscart_categories.parent_id in (75,71,78,12,14,34,37,74,13,15,73,72,35,36,40,41,42,43,20)",
      "using_index": true
    }
  }
}