ding-young opened a new pull request, #16535:
URL: https://github.com/apache/datafusion/pull/16535
## Which issue does this PR close?
Related to #16367
## Rationale for this change
Previously, the `spilled_bytes` metric reported the estimated in-memory size
of spilled `RecordBatches`, based on their array sizes. However, with the
introduction of spill file compression in the `SpillManager`, the in-memory
size no longer reflects the actual bytes written to disk.
This PR updates the `spilled_bytes` metric to instead report the total
number of bytes physically **written to disk** during spilling.
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## What changes are included in this PR?
Whenever `in_progress_file.finish()` is called, it updates the spill file
metric based on `current_disk_usage` for this spill file. Since spill files are
append-only, we only update this value once.
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## Are these changes tested?
Yes. We manually compare the spill file size and reported metric.
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## Are there any user-facing changes?
After this PR, running certain spilling aggregate queries may show
significantly lower `spilled_bytes` values compared to previous runs—even when
using the uncompressed spill option.
This is because the previous metric was based on an inaccurate estimate of
in-memory size. In particular, when a large `RecordBatch` was sliced into
smaller batches during `spill_record_batch_by_size`, the underlying buffers
were not resized due to zero-copy semantics, resulting in inflated memory
accounting.
The updated `spilled_bytes` metric now reflects the actual number of bytes
written to disk, and therefore provides a more accurate representation of spill
size.
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