SELECT 
  q_discussion.object_id AS product_id, 
  AVG(
    q_discussion_rating.rating_value
  ) AS average_rating, 
  q_discussion.type AS discussion_type, 
  q_discussion.thread_id AS discussion_thread_id 
FROM 
  q_discussion 
  LEFT JOIN q_discussion_posts ON q_discussion_posts.thread_id = q_discussion.thread_id 
  AND q_discussion_posts.status = "A" 
  LEFT JOIN q_discussion_rating ON q_discussion.thread_id = q_discussion_rating.thread_id 
  AND q_discussion_rating.post_id = q_discussion_posts.post_id 
  AND q_discussion_rating.rating_value != 0 
WHERE 
  q_discussion.object_id IN (
    29707, 29709, 29712, 29716, 29718, 29726, 
    29727, 29728, 29732, 29733, 29735, 
    29736, 29737, 29743, 29747, 29748, 
    29754, 29755, 29758, 29761, 29762, 
    29765, 29766, 29769, 29770, 29771, 
    29775, 29780, 29781, 29787
  ) 
  AND q_discussion.object_type = "P" 
GROUP BY 
  q_discussion.object_id

Query time 0.00366

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "216.01"
    },
    "grouping_operation": {
      "using_filesort": false,
      "nested_loop": [
        {
          "table": {
            "table_name": "q_discussion",
            "access_type": "range",
            "possible_keys": [
              "object_id"
            ],
            "key": "object_id",
            "used_key_parts": [
              "object_id",
              "object_type"
            ],
            "key_length": "6",
            "rows_examined_per_scan": 30,
            "rows_produced_per_join": 30,
            "filtered": "100.00",
            "index_condition": "((`portal`.`q_discussion`.`object_id` in (29707,29709,29712,29716,29718,29726,29727,29728,29732,29733,29735,29736,29737,29743,29747,29748,29754,29755,29758,29761,29762,29765,29766,29769,29770,29771,29775,29780,29781,29787)) and (`portal`.`q_discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "66.01",
              "eval_cost": "6.00",
              "prefix_cost": "72.01",
              "data_read_per_join": "480"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type",
              "type"
            ]
          }
        },
        {
          "table": {
            "table_name": "q_discussion_posts",
            "access_type": "ref",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "key": "thread_id",
            "used_key_parts": [
              "thread_id"
            ],
            "key_length": "3",
            "ref": [
              "portal.q_discussion.thread_id"
            ],
            "rows_examined_per_scan": 2,
            "rows_produced_per_join": 60,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "60.00",
              "eval_cost": "12.00",
              "prefix_cost": "144.01",
              "data_read_per_join": "26K"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "<if>(is_not_null_compl(q_discussion_posts), (`portal`.`q_discussion_posts`.`status` = 'A'), true)"
          }
        },
        {
          "table": {
            "table_name": "q_discussion_rating",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "thread_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "post_id"
            ],
            "key_length": "3",
            "ref": [
              "portal.q_discussion_posts.post_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 60,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "60.00",
              "eval_cost": "12.00",
              "prefix_cost": "216.01",
              "data_read_per_join": "960"
            },
            "used_columns": [
              "rating_value",
              "post_id",
              "thread_id"
            ],
            "attached_condition": "<if>(is_not_null_compl(q_discussion_rating), ((`portal`.`q_discussion_rating`.`thread_id` = `portal`.`q_discussion`.`thread_id`) and (`portal`.`q_discussion_rating`.`rating_value` <> 0)), true)"
          }
        }
      ]
    }
  }
}