SELECT 
  COUNT(
    DISTINCT (warehouse.warehouse_id)
  ) 
FROM 
  cscart_erp_warehouse warehouse 
WHERE 
  1 
  AND warehouse.company_id = 1 
  AND warehouse.company_id = 1

Query time 0.00028

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "warehouse",
      "access_type": "range",
      "key": "PRIMARY",
      "key_length": "8",
      "used_key_parts": ["warehouse_id", "company_id"],
      "rows": 10,
      "filtered": 100,
      "attached_condition": "warehouse.company_id = 1",
      "using_index_for_group_by": true
    }
  }
}