SELECT 
  jmj_discussion.object_id AS product_id, 
  AVG(
    jmj_discussion_rating.rating_value
  ) AS average_rating, 
  jmj_discussion.type AS discussion_type, 
  jmj_discussion.thread_id AS discussion_thread_id 
FROM 
  jmj_discussion 
  LEFT JOIN jmj_discussion_posts ON jmj_discussion_posts.thread_id = jmj_discussion.thread_id 
  AND jmj_discussion_posts.status = "A" 
  LEFT JOIN jmj_discussion_rating ON jmj_discussion.thread_id = jmj_discussion_rating.thread_id 
  AND jmj_discussion_rating.post_id = jmj_discussion_posts.post_id 
  AND jmj_discussion_rating.rating_value != 0 
WHERE 
  jmj_discussion.object_id IN (
    1701, 1708, 1726, 1694, 1693, 1712, 1715, 
    1716, 1718, 1724, 1722, 1736, 1706, 
    1710, 1698, 1585, 1591, 1612, 1583, 
    1568, 1596, 1599, 1600, 1603, 1609, 
    1605, 1586, 1589, 1594, 1584
  ) 
  AND jmj_discussion.object_type = "P" 
GROUP BY 
  jmj_discussion.object_id

Query time 0.01149

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "273.01"
    },
    "grouping_operation": {
      "using_filesort": false,
      "nested_loop": [
        {
          "table": {
            "table_name": "jmj_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": "((`jmj_test`.`jmj_discussion`.`object_id` in (1701,1708,1726,1694,1693,1712,1715,1716,1718,1724,1722,1736,1706,1710,1698,1585,1591,1612,1583,1568,1596,1599,1600,1603,1609,1605,1586,1589,1594,1584)) and (`jmj_test`.`jmj_discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "66.01",
              "eval_cost": "6.00",
              "prefix_cost": "72.01",
              "data_read_per_join": "720"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type",
              "type"
            ]
          }
        },
        {
          "table": {
            "table_name": "jmj_discussion_posts",
            "access_type": "ref",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "key": "thread_id",
            "used_key_parts": [
              "thread_id"
            ],
            "key_length": "3",
            "ref": [
              "jmj_test.jmj_discussion.thread_id"
            ],
            "rows_examined_per_scan": 3,
            "rows_produced_per_join": 90,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "75.00",
              "eval_cost": "18.00",
              "prefix_cost": "165.01",
              "data_read_per_join": "39K"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "<if>(is_not_null_compl(jmj_discussion_posts), (`jmj_test`.`jmj_discussion_posts`.`status` = 'A'), true)"
          }
        },
        {
          "table": {
            "table_name": "jmj_discussion_rating",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "thread_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "post_id"
            ],
            "key_length": "3",
            "ref": [
              "jmj_test.jmj_discussion_posts.post_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 90,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "90.00",
              "eval_cost": "18.00",
              "prefix_cost": "273.01",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "rating_value",
              "post_id",
              "thread_id"
            ],
            "attached_condition": "<if>(is_not_null_compl(jmj_discussion_rating), ((`jmj_test`.`jmj_discussion_rating`.`thread_id` = `jmj_test`.`jmj_discussion`.`thread_id`) and (`jmj_test`.`jmj_discussion_rating`.`rating_value` <> 0)), true)"
          }
        }
      ]
    }
  }
}