SELECT 
  COUNT(*) 
FROM 
  cscart_discussion d 
  LEFT JOIN cscart_discussion_posts dp ON d.thread_id = dp.thread_id 
WHERE 
  d.object_type = 'M' 
  AND dp.status = 'A' 
  AND d.object_id = 252

Query time 0.00013

JSON explain

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

Result

COUNT(*)
0