SELECT 
  COUNT(
    DISTINCT(
      cscart_discussion_posts.post_id
    )
  ) 
FROM 
  cscart_discussion_posts 
  INNER JOIN cscart_discussion ON cscart_discussion.thread_id = cscart_discussion_posts.thread_id 
  INNER JOIN cscart_discussion_messages ON cscart_discussion_messages.post_id = cscart_discussion_posts.post_id 
  INNER JOIN cscart_discussion_rating ON cscart_discussion_rating.post_id = cscart_discussion_posts.post_id 
  LEFT JOIN cscart_products ON cscart_products.product_id = cscart_discussion.object_id 
  LEFT JOIN cscart_products_categories ON cscart_products_categories.product_id = cscart_discussion.object_id 
  LEFT JOIN cscart_categories ON cscart_categories.category_id = cscart_products_categories.category_id 
WHERE 
  1 
  AND cscart_discussion_posts.status = 'A' 
  AND cscart_discussion.object_id IN (6783, 6783) 
  AND cscart_discussion.object_type = 'P' 
  AND cscart_discussion_posts.cp_pr_user_delete = 'N' 
  AND cscart_discussion.type != 'D' 
  AND cscart_discussion.company_id = 1 
  AND cscart_products.status = 'A' 
  AND cscart_categories.status = 'A'

Query time 0.00071

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_discussion",
          "access_type": "const",
          "possible_keys": ["PRIMARY", "object_id", "idx_company_id"],
          "key": "object_id",
          "key_length": "10",
          "used_key_parts": ["object_id", "object_type", "company_id"],
          "ref": ["const", "const", "const"],
          "rows": 1,
          "filtered": 100
        }
      },
      {
        "table": {
          "table_name": "cscart_products",
          "access_type": "const",
          "possible_keys": ["PRIMARY", "status"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["product_id"],
          "ref": ["const"],
          "rows": 1,
          "filtered": 100
        }
      },
      {
        "table": {
          "table_name": "cscart_discussion_posts",
          "access_type": "ref",
          "possible_keys": ["PRIMARY", "thread_id", "thread_id_2"],
          "key": "thread_id_2",
          "key_length": "6",
          "used_key_parts": ["thread_id", "status"],
          "ref": ["const", "const"],
          "rows": 1,
          "filtered": 100,
          "index_condition": "cscart_discussion_posts.`status` = 'A'",
          "attached_condition": "cscart_discussion_posts.cp_pr_user_delete = 'N'"
        }
      },
      {
        "table": {
          "table_name": "cscart_discussion_messages",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["post_id"],
          "ref": ["lillyshop_lillyshop.cscart_discussion_posts.post_id"],
          "rows": 1,
          "filtered": 100,
          "using_index": true
        }
      },
      {
        "table": {
          "table_name": "cscart_discussion_rating",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["post_id"],
          "ref": ["lillyshop_lillyshop.cscart_discussion_posts.post_id"],
          "rows": 1,
          "filtered": 100,
          "using_index": true
        }
      },
      {
        "table": {
          "table_name": "cscart_products_categories",
          "access_type": "ref",
          "possible_keys": ["PRIMARY", "pt"],
          "key": "pt",
          "key_length": "3",
          "used_key_parts": ["product_id"],
          "ref": ["const"],
          "rows": 1,
          "filtered": 100
        }
      },
      {
        "table": {
          "table_name": "cscart_categories",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY", "p_category_id"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["category_id"],
          "ref": ["lillyshop_lillyshop.cscart_products_categories.category_id"],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "cscart_categories.`status` = 'A'"
        }
      }
    ]
  }
}