Sandbox

SELECT 
  COUNT(*) 
FROM 
  cscart_discussion_posts 
  LEFT JOIN cscart_discussion_messages ON cscart_discussion_messages.post_id = cscart_discussion_posts.post_id 
WHERE 
  cscart_discussion_posts.thread_id = 278 
  AND cscart_discussion_messages.message <> '' 
  AND cscart_discussion_posts.status = 'A'

Query time 0.00036

JSON explain

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

Result

COUNT(*)
0