SELECT 
  COUNT(*) 
FROM 
  cscart_discussion_posts 
  LEFT JOIN cscart_discussion_messages ON cscart_discussion_messages.post_id = cscart_discussion_posts.post_id 
  LEFT JOIN cscart_discussion_rating ON cscart_discussion_rating.post_id = cscart_discussion_posts.post_id 
WHERE 
  cscart_discussion_posts.thread_id = 1464 
  AND cscart_discussion_posts.status = 'A'

Query time 0.00005

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_discussion_posts",
          "access_type": "ALL",
          "possible_keys": ["thread_id", "thread_id_2"],
          "rows": 4,
          "filtered": 100,
          "attached_condition": "cscart_discussion_posts.thread_id = 1464 and cscart_discussion_posts.`status` = 'A'"
        }
      }
    ]
  }
}