findepi commented on code in PR #16859:
URL: https://github.com/apache/datafusion/pull/16859#discussion_r2229264131


##########
datafusion/functions/src/datetime/date_trunc.rs:
##########
@@ -185,6 +187,21 @@ impl ScalarUDFImpl for DateTruncFunc {
         ) -> Result<ColumnarValue> {
             let parsed_tz = parse_tz(tz_opt)?;
             let array = as_primitive_array::<T>(array)?;
+
+            // fast path for fine granularities
+            if matches!(
+                granularity.as_str(),
+                "second" | "minute" | "millisecond" | "microsecond"
+            ) || (parsed_tz.is_none() && granularity.as_str() == "hour")
+            {
+                let result = general_date_trunc_array_fine_granularity(

Review Comment:
   You're right. For minute and smaller granularities the new code path is 
always fine¹
   For hour it's also fine, for zone-less data.
   
   - (¹) in the past (early XX century and earlier) named zones often had 
offset changes not multiple of minutes at list this is what 
https://www.timeanddate.com/time/zone/nepal/kathmandu shows me. Not sure 
whether chrono respects that
   <img width="831" height="259" alt="image" 
src="https://github.com/user-attachments/assets/eea59fb7-02e2-4613-9be1-e248865b2a92";
 />
   
   - for zone-less data (timestamp without time zone), day granularity can also 
be applied with just divide and mul, just like hour. Or am i blind again?



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