SELECT 
  cscart_discussion.object_id AS product_id, 
  AVG(
    cscart_discussion_rating.rating_value
  ) AS average_rating, 
  cscart_discussion.type AS discussion_type, 
  cscart_discussion.thread_id AS discussion_thread_id 
FROM 
  cscart_discussion 
  LEFT JOIN cscart_discussion_posts ON cscart_discussion_posts.thread_id = cscart_discussion.thread_id 
  AND cscart_discussion_posts.status = "A" 
  LEFT JOIN cscart_discussion_rating ON cscart_discussion.thread_id = cscart_discussion_rating.thread_id 
  AND cscart_discussion_rating.post_id = cscart_discussion_posts.post_id 
  AND cscart_discussion_rating.rating_value != 0 
WHERE 
  cscart_discussion.object_id IN (295, 297, 289) 
  AND cscart_discussion.object_type = "P" 
  AND cscart_discussion.company_id = 1 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00101

JSON explain

{
  "query_block": {
    "select_id": 1,
    "filesort": {
      "temporary_table": {
        "function": "buffer",
        "table": {
          "table_name": "cscart_discussion",
          "access_type": "range",
          "possible_keys": ["object_id", "company_id"],
          "key": "object_id",
          "key_length": "10",
          "used_key_parts": ["object_id", "object_type", "company_id"],
          "rows": 3,
          "filtered": 100,
          "index_condition": "((cscart_discussion.company_id = 1) and (cscart_discussion.object_id in (295,297,289)) and (cscart_discussion.object_type = 'P'))"
        },
        "block-nl-join": {
          "table": {
            "table_name": "cscart_discussion_posts",
            "access_type": "ALL",
            "possible_keys": ["thread_id", "thread_id_2"],
            "rows": 2,
            "filtered": 100
          },
          "buffer_type": "flat",
          "buffer_size": "256Kb",
          "join_type": "BNL",
          "attached_condition": "trigcond(((cscart_discussion_posts.thread_id = cscart_discussion.thread_id) and (cscart_discussion_posts.`status` = 'A')))"
        },
        "table": {
          "table_name": "cscart_discussion_rating",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY", "thread_id"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["post_id"],
          "ref": ["egkcysbp_cscartn.cscart_discussion_posts.post_id"],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "trigcond(((cscart_discussion_rating.thread_id = cscart_discussion.thread_id) and (cscart_discussion_rating.rating_value <> 0) and trigcond((cscart_discussion_posts.post_id is not null))))"
        }
      }
    }
  }
}