Blizzara commented on code in PR #11711:
URL: https://github.com/apache/datafusion/pull/11711#discussion_r1723017470
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datafusion/expr/src/type_coercion/binary.rs:
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@@ -1034,29 +1033,61 @@ fn is_time_with_valid_unit(datatype: DataType) -> bool {
)
}
+/// Non-strict Timezone Coercion is useful in scenarios where we can guarantee
+/// a stable relationship between two timestamps of different timezones.
+///
+/// An example of this is binary comparisons (<, >, ==, etc). Arrow stores
timestamps
+/// as relative to UTC epoch, and then adds the timezone as an offset. As a
result, we can always
+/// do a binary comparison between the two times.
Review Comment:
This is only true as long as the timezone is _present_. If tz is None, then
the timestamp is not necessarily relative to UTC epoch (but an "arbitrary"
epoch).
FWIW, I think the changes you've done here make sense (*) - I'm just not
sure if the original behavior of coercing a `(None, Some(tz))` or a `(Some(tz),
None)` are correct. Or well, I'm pretty sure it's not _guaranteed to be
correct_, but maybe it's convenient enough to exist 🤷
(*): though is there need for the "strict" check to exist? I'd assume
coercing two timestamps with timezones would always be fine, given they are
always UTC-epoch based.
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