SELECT 
  user_id 
FROM 
  cscart_users 
WHERE 
  company_id = 120 
  AND user_type = 'V' 
  AND is_root = 'Y'

Query time 0.00037

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_users",
          "access_type": "ALL",
          "rows": 11,
          "filtered": 100,
          "attached_condition": "cscart_users.company_id = 120 and cscart_users.user_type = 'V' and cscart_users.is_root = 'Y'"
        }
      }
    ]
  }
}