[GitHub] [arrow-datafusion] alamb commented on a diff in pull request #2786: wip: Implement extract epoch

[GitBox <gi...@apache.org>]

alamb commented on code in PR #2786:
URL: https://github.com/apache/arrow-datafusion/pull/2786#discussion_r906281699


##########
datafusion/physical-expr/src/datetime_expressions.rs:
##########
@@ -356,8 +360,9 @@ pub fn date_part(args: &[ColumnarValue]) -> Result<ColumnarValue> {
         "hour" => extract_date_part!(array, temporal::hour),
         "minute" => extract_date_part!(array, temporal::minute),
         "second" => extract_date_part!(array, temporal::second),
+        "epoch" => extract_date_part!(array, extract_epoch),
         _ => Err(DataFusionError::Execution(format!(
-            "Date part '{}' not supported",
+            "Date partx '{}' not supported",

Review Comment:
   ```suggestion
               "Date part '{}' not supported",
   ```



##########
datafusion/physical-expr/src/datetime_expressions.rs:
##########
@@ -372,6 +377,40 @@ pub fn date_part(args: &[ColumnarValue]) -> Result<ColumnarValue> {
     })
 }
 
+fn extract_epoch<T>(array: &PrimitiveArray<T>) -> Result<Int32Array>
+where
+    T: ArrowTemporalType + ArrowNumericType,
+    i64: std::convert::From<T::Native>,
+{
+    let mut b = Int32Builder::new(array.len());
+    match array.data_type() {
+        &DataType::Timestamp(ref unit, _) => {
+            for i in 0..array.len() {
+                if array.is_null(i) {
+                    b.append_null()?;
+                } else {
+                    let scale = match unit {

Review Comment:
   I would recommend using the `chrono` functions for this, such as done here:
   
   https://sourcegraph.com/github.com/apache/arrow-rs/-/blob/arrow/src/compute/kernels/temporal.rs?L214-240&subtree=true
   
   



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