SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(1125, 1126, 1127) 
  AND category_id != 264

Query time 0.00020

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.006877495,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_categories",
          "access_type": "range",
          "possible_keys": ["PRIMARY", "parent", "p_category_id"],
          "key": "parent",
          "key_length": "3",
          "used_key_parts": ["parent_id"],
          "loops": 1,
          "rows": 19,
          "cost": 0.006877495,
          "filtered": 100,
          "attached_condition": "cscart_categories.parent_id in (1125,1126,1127) and cscart_categories.category_id <> 264",
          "using_index": true
        }
      }
    ]
  }
}

Result

category_id parent_id
1393 1125
1394 1125
1395 1125
1396 1125
1397 1125
1398 1125
1399 1125
1400 1126
1401 1126
1402 1126
1403 1126
1404 1126
1405 1126
1406 1126
1407 1126
1408 1127
1409 1127
1410 1127
1411 1127