Function calc_expr_time_window_lower_bound 

fn calc_expr_time_window_lower_bound(
    phy_expr: &PhysicalExprRef,
    df_schema: &DFSchema,
    current: Timestamp,
) -> Result<Option<Timestamp>, Error>

Find the lower bound of time window in given expr and current timestamp.

i.e. for current="2021-07-01 00:01:01.000" and expr=date_bin(INTERVAL '5 minutes', ts) as time_window and ts_col=ts, return Some("2021-07-01 00:00:00.000") since it’s the lower bound return Some("2021-07-01 00:00:00.000") since it’s the lower bound of current time window given the current timestamp

if return None, meaning this time window have no lower bound