westonpace opened a new issue, #34136:
URL: https://github.com/apache/arrow/issues/34136
### Describe the enhancement requested
Every node has an "ordering" which describes what the batch index of the
batches produced by that node corresponds to.
Source nodes will generally have an ordering based on the data they are
generating. For in-memory data this will be implicit "row number" ordering
(done in this PR). For the scanner this will generally be implicit "file
number / row number" ordering (future PR). In both cases the user should
eventually be able to assert an explicit ordering if they know for a fact that
their data is ordered (future PR). If the data is partitioned the scanner
could generate an explicit ordering based on the partition columns (future PR).
Mapping nodes like filter/project/fetch will simply propagate the ordering
of their input.
Some nodes, such as order_by (future PR) and asof join (this PR) will
establish a new ordering.
Other nodes, such as the aggregate node, the hash-join node, and the union
node will destroy the ordering (this PR). Their output will have no meaningful
order. Although, in all these cases one could devise an alternative
order-preserving approach (future PR):
* An aggregate node could pretty easily establish an ordering based on the
keys or an implicit ordering if there are no keys (if you only have 1 row then
it is ordered :)
* A hash-join node could similarly choose to order by one or more of the
join keys (unclear at the moment if there would be much cost associated with
this)
* To preserve ordering, a union node would need to read from all inputs at
the same time and output in round-robin fashion. Alternatively, a union node
could choose to fully drain one input before starting on the next input. Both
approaches would have performance implications
The fetch node will now fail to validate if it is placed in a plan that does
not make sense (e.g. immediately following a hash-join node).
The sink node, if sequenced output is requested, will similarly fail to
validate if the plan does not have an established ordering.
This all sounds far more complex than it is :)
### Component(s)
C++
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