SELECT 
  cscart_discussion_posts.*, 
  cscart_discussion_messages.message, 
  cscart_discussion_rating.rating_value 
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 = 26 
  AND cscart_discussion_posts.status = 'A' 
ORDER BY 
  RAND() 
LIMIT 
  5

Query time 0.00256

JSON explain

{
  "query_block": {
    "select_id": 1,
    "filesort": {
      "sort_key": "rand()",
      "temporary_table": {
        "table": {
          "table_name": "cscart_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"],
          "rows": 32,
          "filtered": 100,
          "attached_condition": "cscart_discussion_posts.`status` = 'A'"
        },
        "table": {
          "table_name": "cscart_discussion_messages",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["post_id"],
          "ref": ["orbit_cscart_orbit_cscart.cscart_discussion_posts.post_id"],
          "rows": 1,
          "filtered": 100
        },
        "table": {
          "table_name": "cscart_discussion_rating",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["post_id"],
          "ref": ["orbit_cscart_orbit_cscart.cscart_discussion_posts.post_id"],
          "rows": 1,
          "filtered": 100
        }
      }
    }
  }
}