presto-main/src/main/java/io/prestosql/SystemSessionProperties.java/SystemSessionProperties/getQueryPriority
class SystemSessionProperties:    public static int getQueryPriority(Session session)
    {
        Integer priority = session.getSystemProperty(QUERY_PRIORITY, Integer.class);
        checkArgument(priority > 0, "Query priority must be positive");
        return priority;
    }