Oh found this comment: https://github.com/apache/arrow/blob/master/cpp/src/arrow/compute/kernels/scalar_cast_temporal.cc#L156
On Wed, Feb 15, 2023 at 4:23 PM Li Jin <ice.xell...@gmail.com> wrote: > Not sure if this is actually a bug or expected behavior - I filed > https://github.com/apache/arrow/issues/34210 > > On Wed, Feb 15, 2023 at 4:15 PM Li Jin <ice.xell...@gmail.com> wrote: > >> Hmm..something feels off here - I did the following experiment on Arrow >> 11 and casting timestamp-naive to int64 is much faster than casting >> timestamp-naive to timestamp-utc: >> >> In [16]: %time table.cast(schema_int) >> CPU times: user 114 µs, sys: 30 µs, total: 144 µs >> *Wall time: 231 µs* >> Out[16]: >> pyarrow.Table >> time: int64 >> ---- >> time: [[0,1,2,3,4,...,99999995,99999996,99999997,99999998,99999999]] >> >> In [17]: %time table.cast(schema_tz) >> CPU times: user 119 ms, sys: 140 ms, total: 260 ms >> *Wall time: 259 ms* >> Out[17]: >> pyarrow.Table >> time: timestamp[ns, tz=UTC] >> ---- >> time: [[1970-01-01 00:00:00.000000000,1970-01-01 >> 00:00:00.000000001,1970-01-01 00:00:00.000000002,1970-01-01 >> 00:00:00.000000003,1970-01-01 00:00:00.000000004,...,1970-01-01 >> 00:00:00.099999995,1970-01-01 00:00:00.099999996,1970-01-01 >> 00:00:00.099999997,1970-01-01 00:00:00.099999998,1970-01-01 >> 00:00:00.099999999]] >> >> In [18]: table >> Out[18]: >> pyarrow.Table >> time: timestamp[ns] >> ---- >> time: [[1970-01-01 00:00:00.000000000,1970-01-01 >> 00:00:00.000000001,1970-01-01 00:00:00.000000002,1970-01-01 >> 00:00:00.000000003,1970-01-01 00:00:00.000000004,...,1970-01-01 >> 00:00:00.099999995,1970-01-01 00:00:00.099999996,1970-01-01 >> 00:00:00.099999997,1970-01-01 00:00:00.099999998,1970-01-01 >> 00:00:00.099999999]] >> >> On Wed, Feb 15, 2023 at 2:52 PM Rok Mihevc <rok.mih...@gmail.com> wrote: >> >>> I'm not sure about (1) but I'm pretty sure for (2) doing a cast of >>> tz-aware >>> timestamp to tz-naive should be a metadata-only change. >>> >>> On Wed, Feb 15, 2023 at 4:19 PM Li Jin <ice.xell...@gmail.com> wrote: >>> >>> > Asking (2) because IIUC this is a metadata operation that could be zero >>> > copy but I am not sure if this is actually the case. >>> > >>> > On Wed, Feb 15, 2023 at 10:17 AM Li Jin <ice.xell...@gmail.com> wrote: >>> > >>> > > Hello! >>> > > >>> > > I have some questions about type casting memory usage with pyarrow >>> Table. >>> > > Let's say I have a pyarrow Table with 100 columns. >>> > > >>> > > (1) if I want to cast n columns to a different type (e.g., float to >>> int). >>> > > What is the smallest memory overhead that I can do? (memory overhead >>> of 1 >>> > > column, n columns or 100 columns?) >>> > > >>> > > (2) if I want to cast n timestamp columns from tz-native to tz-UTC. >>> What >>> > > is the smallest memory overhead that I can do? (0, 1 column, n >>> columns or >>> > > 100 columns?) >>> > > >>> > > Thanks! >>> > > Li >>> > > >>> > >>> >>
