SELECT 
  category_id, 
  parent_id 
FROM 
  netting_categories 
WHERE 
  parent_id IN(
    312, 314, 315, 316, 317, 318, 319, 394, 
    399, 320, 321, 322, 323, 324, 325, 326, 
    327, 328, 329, 330
  ) 
  AND category_id != 264

Query time 0.00058

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "netting_categories",
          "access_type": "range",
          "possible_keys": ["PRIMARY", "parent", "p_category_id"],
          "key": "parent",
          "key_length": "3",
          "used_key_parts": ["parent_id"],
          "rows": 20,
          "filtered": 100,
          "index_condition": "netting_categories.parent_id in (312,314,315,316,317,318,319,394,399,320,321,322,323,324,325,326,327,328,329,330)",
          "attached_condition": "netting_categories.category_id <> 264"
        }
      }
    ]
  }
}