SELECT 
  thread_id, 
  type, 
  object_type, 
  object_id 
FROM 
  cscart_discussion 
WHERE 
  thread_id = 5021

Query time 0.00700

JSON explain

{
  "query_block": {
    "select_id": 1,
    "message": "no matching row in const table"
  }
}