Re: [PR] Optimize `date_trunc` more [datafusion]

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

alamb commented on code in PR #18360:
URL: https://github.com/apache/datafusion/pull/18360#discussion_r2474653373


##########
datafusion/functions/src/datetime/date_trunc.rs:
##########
@@ -455,36 +456,56 @@ fn general_date_trunc_array_fine_granularity<T: ArrowTimestampType>(
     array: &PrimitiveArray<T>,
     granularity: &str,
 ) -> Result<ArrayRef> {
-    let unit = match (tu, granularity) {
-        (Second, "minute") => Some(Int64Array::new_scalar(60)),
-        (Second, "hour") => Some(Int64Array::new_scalar(3600)),
-        (Second, "day") => Some(Int64Array::new_scalar(86400)),
-
-        (Millisecond, "second") => Some(Int64Array::new_scalar(1_000)),
-        (Millisecond, "minute") => Some(Int64Array::new_scalar(60_000)),
-        (Millisecond, "hour") => Some(Int64Array::new_scalar(3_600_000)),
-        (Millisecond, "day") => Some(Int64Array::new_scalar(86_400_000)),
-
-        (Microsecond, "millisecond") => Some(Int64Array::new_scalar(1_000)),
-        (Microsecond, "second") => Some(Int64Array::new_scalar(1_000_000)),
-        (Microsecond, "minute") => Some(Int64Array::new_scalar(60_000_000)),
-        (Microsecond, "hour") => Some(Int64Array::new_scalar(3_600_000_000)),
-        (Microsecond, "day") => Some(Int64Array::new_scalar(86_400_000_000)),
-
-        (Nanosecond, "microsecond") => Some(Int64Array::new_scalar(1_000)),
-        (Nanosecond, "millisecond") => Some(Int64Array::new_scalar(1_000_000)),
-        (Nanosecond, "second") => Some(Int64Array::new_scalar(1_000_000_000)),
-        (Nanosecond, "minute") => Some(Int64Array::new_scalar(60_000_000_000)),
-        (Nanosecond, "hour") => Some(Int64Array::new_scalar(3_600_000_000_000)),
-        (Nanosecond, "day") => Some(Int64Array::new_scalar(86_400_000_000_000)),
+    let unit: Option<i64> = match (tu, granularity) {

Review Comment:
   The key idea here is to make this code faster by reusing the allocation and operating in place rather than allocating new arrays 
   
   



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