NGA-TRAN commented on PR #5982:
URL:
https://github.com/apache/arrow-datafusion/pull/5982#issuecomment-1511640259
@waitingkuo
You have great tests. I agree they look inconsistent. I think we may want to
define the behavior here and see whether we want to keep it or not:
I think this one is easy to understand: the origin is Jan 31 so any other
month will always be the last day of that month (28, 29, 30, 31 depending on
the month and year)
```SQL
❯ select date_bin('1 month', timestamp '2000-02-29T00:00:00', timestamp
'2000-01-31T00:00:00');
+----------------------------------------------------------------------------------+
| datebin(Utf8("1
month"),Utf8("2000-02-29T00:00:00"),Utf8("2000-01-31T00:00:00")) |
+----------------------------------------------------------------------------------+
| 2000-02-29T00:00:00
|
+----------------------------------------------------------------------------------+
1 row in set. Query took 0.003 seconds.
```
This is is tricky and we need to identify the behavior.
```SQL
❯ select date_bin('1 month', timestamp '2000-01-31T00:00:00', timestamp
'2000-02-29T00:00:00');
+----------------------------------------------------------------------------------+
| datebin(Utf8("1
month"),Utf8("2000-01-31T00:00:00"),Utf8("2000-02-29T00:00:00")) |
+----------------------------------------------------------------------------------+
| 2000-01-29T00:00:00
|
+----------------------------------------------------------------------------------+
1 row in set. Query took 0.003 seconds.
```
For February, 29 (or 28) is the last day of the month, but for other month
is it not the last day of the month. So chromo's
[Months](https://pola-rs.github.io/polars/chrono/struct.Months.html) that we
are using in this PR do not consider 29 as the last day of the month and just
simple add/minus 30 days which I think makes sense.
```SQL
❯ select date_bin('1 month', timestamp '2000-01-31T00:00:00', timestamp
'2000-03-29T00:00:00');
+----------------------------------------------------------------------------------+
| datebin(Utf8("1
month"),Utf8("2000-01-31T00:00:00"),Utf8("2000-03-29T00:00:00")) |
+----------------------------------------------------------------------------------+
| 2000-01-29T00:00:00
|
+----------------------------------------------------------------------------------+
1 row in set. Query took 0.005 seconds.
```
So I think this is the behavior is acceptable. What do you, @alamb and
@stuartcarnie think?
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