Re: [PR] fix: unwrap_cast_in_comparison drops the timezone shift when unwrapping CAST(timestamp AS timestamptz) = literal [datafusion]

["adriangb (via GitHub)" <gi...@apache.org>]

adriangb commented on code in PR #25099:
URL: https://github.com/apache/datafusion/pull/25099#discussion_r3971249533


##########
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:
   Is there no function in arrow-rs or chrono we could use for this?



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