jayzhan211 commented on code in PR #14232:
URL: https://github.com/apache/datafusion/pull/14232#discussion_r1929384667
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datafusion/functions-aggregate/src/first_last.rs:
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@@ -569,6 +573,13 @@ impl LastValueAccumulator {
})
.collect::<Vec<_>>();
+ // Order by indices for cases where the values are the same, we expect
the last index
+ let indices: UInt64Array = (0..num_rows).map(|x| x as u64).collect();
+ sort_columns.push(SortColumn {
Review Comment:
~Maybe we need benchmark on this.
With additional indices columns, we are able to optimize it with select n_th
logic
Without additional indices columns, we need to iterate them all the find the
correct one, it includes the cost of comparison~
> since this will work only for single threaded queries, while, I suppose,
the primarily use-case is multithreaded execution, which messes up the original
order of records in files due to parallel reads followed by repartitioning.
I think even in this case, we still expect to return the correct number?
I agree that in 2-phase step, the **true** last value is calculated in
`merge_batch`, but if we run it in AggregateMode::Single, we would need to get
the **true** last value in `update_batch`
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