SELECT 
  COUNT(
    DISTINCT(cscart_companies.company_id)
  ) 
FROM 
  cscart_companies 
  LEFT JOIN cscart_company_descriptions ON cscart_company_descriptions.company_id = cscart_companies.company_id 
  AND cscart_company_descriptions.lang_code = 'ja' 
  LEFT JOIN cscart_vendor_plan_descriptions ON cscart_companies.plan_id = cscart_vendor_plan_descriptions.plan_id 
  AND cscart_vendor_plan_descriptions.lang_code = 'ja' 
  LEFT JOIN cscart_discussion ON cscart_discussion.object_id = cscart_companies.company_id 
  AND cscart_discussion.object_type = 'M' 
  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 
WHERE 
  1 
  AND cscart_companies.status = 'A'

Query time 0.00065

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_companies",
      "access_type": "ALL",
      "rows": 3,
      "filtered": 100,
      "attached_condition": "cscart_companies.`status` = 'A'"
    },
    "table": {
      "table_name": "cscart_discussion",
      "access_type": "eq_ref",
      "possible_keys": ["object_id"],
      "key": "object_id",
      "key_length": "6",
      "used_key_parts": ["object_id", "object_type"],
      "ref": ["smscrm_edev.cscart_companies.company_id", "const"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "trigcond(cscart_discussion.object_id = cscart_companies.company_id and cscart_discussion.object_type = 'M')"
    },
    "table": {
      "table_name": "cscart_discussion_posts",
      "access_type": "ref",
      "possible_keys": ["thread_id", "thread_id_2"],
      "key": "thread_id",
      "key_length": "3",
      "used_key_parts": ["thread_id"],
      "ref": ["smscrm_edev.cscart_discussion.thread_id"],
      "rows": 6,
      "filtered": 100,
      "attached_condition": "trigcond(cscart_discussion_posts.`status` = 'A' and trigcond(cscart_discussion.thread_id is not null))"
    }
  }
}