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 (
    89379, 89394, 89396, 89385, 89375, 89376, 
    89381, 89390, 89392, 89382, 89388, 
    89380, 89395, 89397, 89410, 89411, 
    89412, 89360, 89387, 89363, 89377, 
    89365, 89359, 89386, 89383, 89297, 
    89362, 89364, 89378, 89296, 89391, 
    89393, 89406, 89408, 89407, 90843, 
    89302, 89384, 89389, 89300, 89301, 
    89299, 89349, 89350, 89263, 89353, 
    89354, 89355, 89356, 89293, 89291, 
    89262, 89292, 89287, 89357, 89358, 
    89402, 89404, 89403, 89295, 89294, 
    89405, 89257, 89345, 89347, 89346, 
    89255, 89348, 89261, 89260, 89256, 
    89259, 89258
  ) 
  AND cscart_discussion.object_type = "P" 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00157

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "84.21"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": false,
      "nested_loop": [
        {
          "table": {
            "table_name": "cscart_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": 73,
            "rows_produced_per_join": 73,
            "filtered": "100.00",
            "index_condition": "((`nuie_scalesta_net`.`cscart_discussion`.`object_id` in (89379,89394,89396,89385,89375,89376,89381,89390,89392,89382,89388,89380,89395,89397,89410,89411,89412,89360,89387,89363,89377,89365,89359,89386,89383,89297,89362,89364,89378,89296,89391,89393,89406,89408,89407,90843,89302,89384,89389,89300,89301,89299,89349,89350,89263,89353,89354,89355,89356,89293,89291,89262,89292,89287,89357,89358,89402,89404,89403,89295,89294,89405,89257,89345,89347,89346,89255,89348,89261,89260,89256,89259,89258)) and (`nuie_scalesta_net`.`cscart_discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "43.81",
              "eval_cost": "7.30",
              "prefix_cost": "51.11",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type",
              "type"
            ]
          }
        },
        {
          "table": {
            "table_name": "cscart_discussion_posts",
            "access_type": "ref",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "key": "thread_id",
            "used_key_parts": [
              "thread_id"
            ],
            "key_length": "3",
            "ref": [
              "nuie_scalesta_net.cscart_discussion.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 73,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "18.25",
              "eval_cost": "7.30",
              "prefix_cost": "76.66",
              "data_read_per_join": "31K"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "<if>(is_not_null_compl(cscart_discussion_posts), (`nuie_scalesta_net`.`cscart_discussion_posts`.`status` = 'A'), true)"
          }
        },
        {
          "table": {
            "table_name": "cscart_discussion_rating",
            "access_type": "ALL",
            "possible_keys": [
              "PRIMARY",
              "thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 73,
            "filtered": "100.00",
            "using_join_buffer": "hash join",
            "cost_info": {
              "read_cost": "0.25",
              "eval_cost": "7.30",
              "prefix_cost": "84.21",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "rating_value",
              "post_id",
              "thread_id"
            ],
            "attached_condition": "<if>(is_not_null_compl(cscart_discussion_rating), ((`nuie_scalesta_net`.`cscart_discussion_rating`.`post_id` = `nuie_scalesta_net`.`cscart_discussion_posts`.`post_id`) and (`nuie_scalesta_net`.`cscart_discussion_rating`.`thread_id` = `nuie_scalesta_net`.`cscart_discussion`.`thread_id`) and (`nuie_scalesta_net`.`cscart_discussion_rating`.`rating_value` <> 0)), true)"
          }
        }
      ]
    }
  }
}