SELECT 
  company_id 
FROM 
  cscart_orders 
WHERE 
  user_id = 0

Query time 0.00004

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_orders",
          "access_type": "ref",
          "possible_keys": ["user_id"],
          "key": "user_id",
          "key_length": "3",
          "used_key_parts": ["user_id"],
          "ref": ["const"],
          "rows": 5,
          "filtered": 100
        }
      }
    ]
  }
}