SELECT 
  companies.*, 
  AVG(
    vi_discussion_rating.rating_value
  ) AS average_rating, 
  CONCAT(
    companies.company_id, 
    '_', 
    IF (
      vi_discussion_rating.thread_id, 
      vi_discussion_rating.thread_id, 
      '0'
    )
  ) AS company_thread_ids 
FROM 
  vi_companies AS companies 
  LEFT JOIN vi_discussion ON vi_discussion.object_id = companies.company_id 
  AND vi_discussion.object_type = 'M' 
  LEFT JOIN vi_discussion_posts ON vi_discussion_posts.thread_id = vi_discussion.thread_id 
  AND vi_discussion_posts.status = 'A' 
  LEFT JOIN vi_discussion_rating ON vi_discussion.thread_id = vi_discussion_rating.thread_id 
  AND vi_discussion_rating.post_id = vi_discussion_posts.post_id 
WHERE 
  companies.company_id = 1

Query time 0.00015

JSON explain

{
  "query_block": {
    "select_id": 1,
    "const_condition": "1",
    "nested_loop": [
      {
        "table": {
          "table_name": "companies",
          "access_type": "system",
          "possible_keys": ["PRIMARY"],
          "rows": 1,
          "filtered": 100
        }
      },
      {
        "table": {
          "table_name": "vi_discussion",
          "access_type": "ref",
          "possible_keys": ["object_id"],
          "key": "object_id",
          "key_length": "6",
          "used_key_parts": ["object_id", "object_type"],
          "ref": ["const", "const"],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "trigcond(vi_discussion.object_type = 'M')"
        }
      },
      {
        "table": {
          "table_name": "vi_discussion_posts",
          "access_type": "ref",
          "possible_keys": ["thread_id", "thread_id_2"],
          "key": "thread_id_2",
          "key_length": "6",
          "used_key_parts": ["thread_id", "status"],
          "ref": ["devvipiriscom.vi_discussion.thread_id", "const"],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "trigcond(vi_discussion_posts.`status` = 'A' and trigcond(vi_discussion.thread_id is not null))"
        }
      },
      {
        "table": {
          "table_name": "vi_discussion_rating",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY", "thread_id"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["post_id"],
          "ref": ["devvipiriscom.vi_discussion_posts.post_id"],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "trigcond(vi_discussion_rating.thread_id = vi_discussion.thread_id and trigcond(vi_discussion_posts.post_id is not null))"
        }
      }
    ]
  }
}