westonpace commented on code in PR #34311:
URL: https://github.com/apache/arrow/pull/34311#discussion_r1117684110
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cpp/src/arrow/compute/exec/options.h:
##########
@@ -199,21 +199,32 @@ class ARROW_EXPORT ProjectNodeOptions : public
ExecNodeOptions {
std::vector<std::string> names;
};
-/// \brief Make a node which aggregates input batches, optionally grouped by
keys.
+/// \brief Make a node which aggregates input batches, optionally grouped by
keys and
+/// optionally segmented by segment-keys. Both keys and segment-keys determine
the group.
+/// However segment-keys are also used for determining grouping segments,
which should be
+/// large, and allow streaming a partial aggregation result after processing
each segment.
+/// One common use-case for segment-keys is ordered aggregation, in which the
segment-key
+/// attribute specifies a column with non-decreasing values or a
lexigographically-ordered
+/// set of such columns.
///
/// If the keys attribute is a non-empty vector, then each aggregate in
`aggregates` is
/// expected to be a HashAggregate function. If the keys attribute is an empty
vector,
/// then each aggregate is assumed to be a ScalarAggregate function.
class ARROW_EXPORT AggregateNodeOptions : public ExecNodeOptions {
public:
explicit AggregateNodeOptions(std::vector<Aggregate> aggregates,
- std::vector<FieldRef> keys = {})
- : aggregates(std::move(aggregates)), keys(std::move(keys)) {}
+ std::vector<FieldRef> keys = {},
+ std::vector<FieldRef> segment_keys = {})
+ : aggregates(std::move(aggregates)),
+ keys(std::move(keys)),
+ segment_keys(std::move(segment_keys)) {}
// aggregations which will be applied to the targetted fields
std::vector<Aggregate> aggregates;
// keys by which aggregations will be grouped
std::vector<FieldRef> keys;
+ // keys by which aggregations will be segmented
+ std::vector<FieldRef> segment_keys;
Review Comment:
Ok. So if I want to group by A, B, C and I know my data is sorted by A then
I can do segments=A and keys=B C?
In that case I agree we should explicitly disallow this to help future
users. Also, we might want to improve the description of these fields a bit
and provide some examples, but that isn't urgent for right now.
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