SELECT 
  discussion.object_id AS product_id, 
  AVG(discussion_rating.rating_value) AS average_rating, 
  discussion.type AS discussion_type, 
  discussion.thread_id AS discussion_thread_id 
FROM 
  discussion 
  LEFT JOIN discussion_posts ON discussion_posts.thread_id = discussion.thread_id 
  AND discussion_posts.status = "A" 
  LEFT JOIN discussion_rating ON discussion.thread_id = discussion_rating.thread_id 
  AND discussion_rating.post_id = discussion_posts.post_id 
  AND discussion_rating.rating_value != 0 
WHERE 
  discussion.object_id IN (
    296873, 296839, 296831, 296893, 296928, 
    296832, 296841, 296919, 296881, 296861, 
    296883, 296838, 296848, 296853, 296899, 
    296846, 296833, 296858, 296886, 296894, 
    296835, 296869, 296892, 296837, 296845, 
    296895, 296864, 296917, 296851, 296862, 
    296872, 296882, 296900, 296875, 296847, 
    296876, 296906, 296860, 296868, 296857, 
    296842
  ) 
  AND discussion.object_type = "P" 
GROUP BY 
  discussion.object_id

Query time 0.00101

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "108.70"
    },
    "grouping_operation": {
      "using_filesort": false,
      "nested_loop": [
        {
          "table": {
            "table_name": "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": 41,
            "rows_produced_per_join": 41,
            "filtered": "100.00",
            "index_condition": "((`goimagine`.`discussion`.`object_id` in (296873,296839,296831,296893,296928,296832,296841,296919,296881,296861,296883,296838,296848,296853,296899,296846,296833,296858,296886,296894,296835,296869,296892,296837,296845,296895,296864,296917,296851,296862,296872,296882,296900,296875,296847,296876,296906,296860,296868,296857,296842)) and (`goimagine`.`discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "61.92",
              "eval_cost": "4.10",
              "prefix_cost": "66.02",
              "data_read_per_join": "984"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type",
              "type"
            ]
          }
        },
        {
          "table": {
            "table_name": "discussion_posts",
            "access_type": "ref",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "key": "thread_id_2",
            "used_key_parts": [
              "thread_id",
              "status"
            ],
            "key_length": "6",
            "ref": [
              "goimagine.discussion.thread_id",
              "const"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 72,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "10.26",
              "eval_cost": "7.21",
              "prefix_cost": "83.48",
              "data_read_per_join": "31K"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ]
          }
        },
        {
          "table": {
            "table_name": "discussion_rating",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "thread_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "post_id"
            ],
            "key_length": "3",
            "ref": [
              "goimagine.discussion_posts.post_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 72,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "18.02",
              "eval_cost": "7.21",
              "prefix_cost": "108.70",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "rating_value",
              "post_id",
              "thread_id"
            ],
            "attached_condition": "<if>(is_not_null_compl(discussion_rating), ((`goimagine`.`discussion_rating`.`thread_id` = `goimagine`.`discussion`.`thread_id`) and (`goimagine`.`discussion_rating`.`rating_value` <> 0)), true)"
          }
        }
      ]
    }
  }
}