SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    1991, 1992, 1994, 1995, 1996, 2000, 1998, 
    1999
  )

Query time 0.00033

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": 8,
      "filtered": 100,
      "attached_condition": "cscart_categories.parent_id in (1991,1992,1994,1995,1996,2000,1998,1999)",
      "using_index": true
    }
  }
}