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 (3, 2, 5) 
  AND cscart_discussion.object_type = "P" 
  AND cscart_discussion.company_id = 1 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00048

JSON explain

{
  "query_block": {
    "select_id": 1,
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "nested_loop": [
        {
          "table": {
            "table_name": "cscart_discussion_posts",
            "access_type": "system",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "rows": 0,
            "filtered": 0,
            "const_row_not_found": true
          }
        },
        {
          "table": {
            "table_name": "cscart_discussion_rating",
            "access_type": "system",
            "possible_keys": [
              "PRIMARY",
              "thread_id"
            ],
            "rows": 0,
            "filtered": 0,
            "const_row_not_found": true
          }
        },
        {
          "table": {
            "table_name": "cscart_discussion",
            "access_type": "ref",
            "possible_keys": [
              "object_id",
              "company_id"
            ],
            "key": "company_id",
            "used_key_parts": [
              "company_id"
            ],
            "key_length": "4",
            "ref": [
              "const"
            ],
            "rows": 22,
            "filtered": 100,
            "attached_condition": "((`_testcscart`.`cscart_discussion`.`company_id` <=> 1) and ((`_testcscart`.`cscart_discussion`.`object_id` in (3,2,5)) and (`_testcscart`.`cscart_discussion`.`object_type` = 'P')))"
          }
        }
      ]
    }
  }
}