SELECT 
  COUNT(*) 
FROM 
  tmall_discussion_posts 
  LEFT JOIN tmall_discussion_messages ON tmall_discussion_messages.post_id = tmall_discussion_posts.post_id 
  LEFT JOIN tmall_discussion_rating ON tmall_discussion_rating.post_id = tmall_discussion_posts.post_id 
WHERE 
  tmall_discussion_posts.thread_id = 5 
  AND tmall_discussion_posts.status = 'A'

Query time 0.00999

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.002024411,
    "nested_loop": [
      {
        "table": {
          "table_name": "tmall_discussion_posts",
          "access_type": "ref",
          "possible_keys": ["thread_id", "thread_id_2"],
          "key": "thread_id",
          "key_length": "3",
          "used_key_parts": ["thread_id"],
          "ref": ["const"],
          "loops": 1,
          "rows": 1,
          "cost": 0.002024411,
          "filtered": 100,
          "attached_condition": "tmall_discussion_posts.`status` = 'A'"
        }
      }
    ]
  }
}