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SELECT 
  object_id, 
  thread_id, 
  type, 
  object_type 
FROM 
  cscart_discussion 
WHERE 
  1 
  AND object_id = 558 
  AND object_type = 'C'

Query time 0.00014

Explain
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	cscart_discussion	const	object_id	object_id	6	const,const	1

Result

object_id	thread_id	type	object_type
558	4915	D	C