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 (
    938, 939, 942, 943, 944, 945, 946, 947, 
    948, 949, 951, 952, 953, 954, 956, 958, 
    960, 962, 963, 965, 967, 968, 969, 970, 
    972, 973, 974, 975, 976, 977, 978, 979, 
    980, 981, 984, 985, 986, 987, 988, 989, 
    990, 991, 992, 993, 994, 995, 996, 997
  ) 
  AND jmj_discussion.object_type = "P" 
GROUP BY 
  jmj_discussion.object_id

Query time 0.00099

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "436.81"
    },
    "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": 48,
            "rows_produced_per_join": 48,
            "filtered": "100.00",
            "index_condition": "((`jmj_test`.`jmj_discussion`.`object_id` in (938,939,942,943,944,945,946,947,948,949,951,952,953,954,956,958,960,962,963,965,967,968,969,970,972,973,974,975,976,977,978,979,980,981,984,985,986,987,988,989,990,991,992,993,994,995,996,997)) and (`jmj_test`.`jmj_discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "105.61",
              "eval_cost": "9.60",
              "prefix_cost": "115.21",
              "data_read_per_join": "1K"
            },
            "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": 144,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "120.00",
              "eval_cost": "28.80",
              "prefix_cost": "264.01",
              "data_read_per_join": "63K"
            },
            "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": 144,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "144.00",
              "eval_cost": "28.80",
              "prefix_cost": "436.81",
              "data_read_per_join": "2K"
            },
            "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)"
          }
        }
      ]
    }
  }
}