berkaysynnada commented on PR #14160:
URL: https://github.com/apache/datafusion/pull/14160#issuecomment-2603813832
> > BatchCoalescer
>
> Thanks @berkaysynnada for your feedback, if I got you right, you prefer to
call the `CoalesceBatchesExec` just AFTER the `SortMergeJoinExec` called in
physical planner like for other join types?
>
> I checked some tests in `joins.rs` and looks like the `CoalesceBatches`
called before the join
>
> ```
> let expected = {
> [
> "SymmetricHashJoinExec: mode=Partitioned, join_type=Full,
on=[(a2@1, a2@1)], filter=CAST(a1@0 AS Int64) > CAST(a1@1 AS Int64) + 3 AND
CAST(a1@0 AS Int64) < CAST(a1@1 AS Int64) + 10",
> " CoalesceBatchesExec: target_batch_size=8192",
> " RepartitionExec: partitioning=Hash([a2@1], 8),
input_partitions=8, preserve_order=true, sort_exprs=a1@0 ASC NULLS LAST",
> " RepartitionExec: partitioning=RoundRobinBatch(8),
input_partitions=1",
> // " CsvExec: file_groups={1 group: [[tempdir/left.csv]]},
projection=[a1, a2], has_header=false",
> " CoalesceBatchesExec: target_batch_size=8192",
> " RepartitionExec: partitioning=Hash([a2@1], 8),
input_partitions=8, preserve_order=true, sort_exprs=a1@0 ASC NULLS LAST",
> " RepartitionExec: partitioning=RoundRobinBatch(8),
input_partitions=1",
> // " CsvExec: file_groups={1 group:
[[tempdir/right.csv]]}, projection=[a1, a2], has_header=false"
> ]
> };
> ```
>
> perhaps I'm missing something?
CoalesceBatches' in your example exist because of hash repartitions
(CoalesceBatches rule adds a CoalesceBatchesExec on top of FilterExec,
HashJoinExec, and hash-repartition).
I've thought about this, and I believe the most optimal solution is to make
all join operators capable of performing both coalescing and splitting in a
built-in manner. This is because the output of a join can either be smaller or
larger than the target batch size. Ideally, there should be no need (or only
minimal need) for CoalesceBatchesExec.
To achieve this built-in coalescing and splitting, we can leverage existing
tools like BatchSplitter and BatchCoalescer (although there are no current
examples of BatchCoalescer being used in joins). My suggestion is to generalize
these tools so they can be utilized by any operator and applied wherever this
mechanism is needed. As this pattern becomes more common, it will be easier to
expand its usage and simplify its application.
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