// derivativeInterval returns the time interval for the one (and only) derivative func
func derivativeInterval(stmt *influxql.SelectStatement) (time.Duration, error) {
if len(stmt.FunctionCalls()[0].Args) == 2 {
return stmt.FunctionCalls()[0].Args[1].(*influxql.DurationLiteral).Val, nil
}
interval, err := stmt.GroupByInterval()
if err != nil {
return 0, err
}
if interval > 0 {
return interval, nil
}
return time.Second, nil
}
