less copy

arrow-select/src/concat.rs

@@ -135,13 +135,15 @@ fn concat_lists<OffsetSize: OffsetSizeTrait>(
 ) -> Result<ArrayRef, ArrowError> {
     let mut output_len = 0;
     let mut list_has_nulls = false;
+    let mut list_has_slices = false;
 
     let lists = arrays
         .iter()
         .map(|x| x.as_list::<OffsetSize>())
         .inspect(|l| {
             output_len += l.len();
             list_has_nulls |= l.null_count() != 0;
+            list_has_slices |= l.offsets()[0].as_usize() > 0;
         })
         .collect::<Vec<_>>();
 
@@ -156,19 +158,23 @@ fn concat_lists<OffsetSize: OffsetSizeTrait>(
         NullBuffer::new(nulls.finish())
     });
 
-    let values: Vec<ArrayRef> = lists
-        .iter()
-        .map(|x| {
-            // offsets may not refer to all values in the array
-            // for example a sliced list array may have offsets that start at
-            // a non-zero offset and not use all the values
-            let offsets = x.offsets();
+    // If any of the lists have slices, we need to slice the values
+    // to ensure that the offsets are correct
+    let mut sliced_values;
+    let values: Vec<&dyn Array> = if list_has_slices {
+        sliced_values = Vec::with_capacity(lists.len());
+        for l in &lists {
+            // if the first offset is non-zero, we need to slice the values so when
+            // we concatenate them below only the relevant values are included
+            let offsets = l.offsets();
             let start_offset = offsets[0].as_usize();
             let end_offset = offsets[offsets.len() - 1].as_usize();
-            x.values().slice(start_offset, end_offset - start_offset)
-        })
-        .collect::<Vec<_>>();
-    let values: Vec<&dyn Array> = values.iter().map(|a| a.as_ref()).collect();
+            sliced_values.push(l.values().slice(start_offset, end_offset - start_offset));
+        }
+        sliced_values.iter().map(|a| a.as_ref()).collect()
+    } else {
+        lists.iter().map(|x| x.values().as_ref()).collect()
+    };
 
     let concatenated_values = concat(values.as_slice())?;