Ruchirtripathi commented on code in PR #25099:
URL: https://github.com/apache/datafusion/pull/25099#discussion_r3971401718
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datafusion/expr-common/src/casts.rs:
##########
@@ -120,10 +120,34 @@ fn is_lossy_temporal_cast(from_type: &DataType, to_type:
&DataType) -> bool {
if is_date_type(from_type) && is_date_type(to_type) {
return false;
}
+ if let (DataType::Timestamp(_, from_tz), DataType::Timestamp(_, to_tz)) =
+ (from_type, to_type)
+ && from_tz.is_some() != to_tz.is_some()
+ {
+ let tz = from_tz.as_ref().or(to_tz.as_ref()).unwrap().as_ref();
+ if !is_zero_offset_timezone(tz) {
+ return true;
+ }
+ }
(is_date_type(from_type) && to_type.is_temporal())
|| (is_date_type(to_type) && from_type.is_temporal())
}
+/// Returns true if the timezone is known to have a fixed zero offset from UTC.
+///
+/// This is used to determine if a cast between a timezone-aware and
timezone-naive
+/// timestamp is lossy. If the timezone is strictly UTC-equivalent, the cast is
+/// a lossless re-labeling of the integer value.
+fn is_zero_offset_timezone(tz: &str) -> bool {
+ match tz {
+ // Standard UTC identifiers
+ "UTC" | "Etc/UTC" | "GMT" | "Etc/GMT" | "Greenwich" | "Z" => true,
+ // Common fixed offset zero strings parsed by Arrow
+ "+00:00" | "-00:00" | "+0:00" | "-0:00" => true,
+ _ => false,
Review Comment:
The arrow::array::timezone::Tz enum only allows fetching the offset
dynamically for a specific NaiveDateTime via offset_from_utc_datetime().
However, we cannot simply instantiate an arbitrary date (like the Unix Epoch)
and check if the offset is zero because geographic timezones like Europe/London
evaluate to an offset of 0 during winter time. This would create false
positives during query planning.
Because we need to evaluate this statically during optimization (where we
don't want to instantiate values just to check offsets), explicitly
whitelisting the known permanent zero-offset strings is currently the safest
and most robust approach.
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