Add ParquetScanTask for all parquet scan tasks - #23953
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@Matt711 @TomAugspurger - For #22128, I need to get the row-group mapping for each scan task to extract |
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WalkthroughChangesThe streaming Parquet path wraps split and fused tasks in ChangesParquet scan task evaluation
Estimated code review effort: 4 (Complex) | ~45 minutes Merge Risk: ⚪ Minimal · up to This change unifies streaming Parquet scan tasks and cached metadata attachment while retaining fallback scan behavior. No current merge-blocking risk is identified. Suggested reviewers: 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
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✨ Finishing Touches🧪 Generate unit tests (beta)
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In `@python/cudf_polars/cudf_polars/streaming/io.py`:
- Around line 752-754: Update ParquetScanTask.from_scan to return None when the
scan includes a non-default skip_rows, n_rows, or row_index, before constructing
the task. Preserve the existing parquet-type and cached_parquet_info checks, and
only call the ParquetScanTask constructor for scans with no row index or row
slice.
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TomAugspurger
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We'll want to coordinate with any outstanding changes @Matt711 has.
| scans[i] = parquet_scan | ||
| converted = True | ||
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| if converted: |
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I don't love the mutation here. What's preventing us from creating these as ParquetScanTask in the first place, I think when we're creating the StreamingScan? Given that ParquetScanTask.from_scan takes just a scan, hopefully we have everything we need.
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The tricky part is that we cannot convert from SplitScan or FusedScan until we have footer metadata with the row-group indices. We could potentially attach this information "later", but that makes the contract much messier. I prefer a design where we know the ParquetScanTask (or whatever we want to call it) has row-group information available.
I'll try to think a way to avoid this mutation, but I'm not sure how to do it right now (we are already "mutating" things to attach the metadata).
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So the parquet FileMetadata is what we're missing when the StreamingScan is created? Gotcha. That does indeed seem unavoidable.
I agree that we should create some kind of placeholder ParquetScanTask lacking the information it needs to actually be usable (the row group indices).
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Yea, I'm experimenting with this idea now. Hopefully I can come up with something that feels cleaner that what I proposed here.
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Okay - I ended up falling down a bit of a rabbit hole and making ParquetScanTask the only task used for performing Parquet reads. The ParquetScanTask object still wraps a (simplified) Split/FusedScan object for now, but I may be able to avoid that as well.
I realize this is a much bigger change, but something like this may simplify our life in the future.
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Inline comments:
In `@python/cudf_polars/cudf_polars/streaming/io.py`:
- Around line 759-765: Add a unit benchmark covering the ParquetScanTask
specialization represented by the scan eligibility checks in the relevant
streaming I/O code. Include representative specialized parquet scans and measure
their execution, while preserving existing coverage for row-group boundaries,
sliced scans, hybrid evaluation, and fallback evaluation.
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I have a couple PRs open that have minor conflicts, but I'm keen to get this PR in first. Once this is in, and the multi-file hybrid scan python binings (see #22795). Once that's in I'll add the dispatch to multi-file hybrid scan for |
| for scan in node.scans: | ||
| for path in scan.paths: | ||
| for task in node.tasks: | ||
| for path in task.paths: |
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lots of this diff is just renaming "scan(s)" to "task(s)" to clearly distinguish between the full-table Scan node and the individual Scan "tasks" used to generate each chunk.
| if not cached: | ||
| continue | ||
| Scan._validate_cached_parquet_info(cached_paths, cached) | ||
| base_scan.cached_parquet_info = cached |
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We keep the cache on the base_scan instead of creating a separate cache on each task. In the future, this may also allow us to avoid re-reading the same footer metadata on the same rank when it isn't already cached. However, I didn't add that optimization yet, because we would probably need some kind of locking mechanism on the cache (because we have concurrent producers on the rank).
rjzamora
changed the title
ParquetScanTask for all parquet scan tasksParquetScanTask for all parquet scan tasks
| with pytest.raises( | ||
| AssertionError, | ||
| match=(r"Paths do not match cached parquet info."), | ||
| ): |
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Is there a new test that checks we raise in this case? I think this testing that we fail early when metadata is not prefetched
| n_rows: int | ||
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| class ScanTask(IR): |
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Should this still accept parquet_options?
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I was able to remove it from ScanTask, but it's worth noting that the base Scan.do_evaluate still expects a parquet_options argument for now.
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| ): | ||
| cached_parquet_info = task._fetch_parquet_info() |
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It looks like we're fetching the parquet footers inside an IR.do_evaluate here. Is this a change from what happens on main? On of the design goals of prefetching was to not have any dynamic fetching of metadata in cudf-polars: everything was explicitly done ahead of time when prefetching was enabled, or implicitly done by libcudf when it was disabled.
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Yeah, this is a good detail to discuss. When we "pre-fetch" the metadata, it must be before do_evaluate is called.
However, once we are in do_evaluate, we must read the metadata to decide what bounds we are reading. On main, this is where we use plc.io.parquet_metadata.read_parquet_metadata to fetch this information. This PR is indeed changing the behavior so that we just fetch the metadata in a form where we can calculate the bounds and we can pass it into the libcudf read call. On main, we are essentially fetching the metadata twice whenever we don't have it prefetched (I think).
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Description
Refactors streaming scan tasks so parquet IO has one task shape.
SplitScan/FusedScantask classes with a genericScanTask.StreamingScan.scanstoStreamingScan.tasksto distinguish the scan operation from the per-chunk work items.ParquetScanTask(ScanTask)for all parquet-backed streaming scan tasks.ParquetScanTask, including split-file row bounds, row-group alignment, and hybrid-scan reads.This is intended as an infrastructure cleanup for parquet streaming IO. It should not change runtime behavior.
Notes
ScanTaskpath.split_index=0andtotal_splits=1.ParquetScanTaskfor parquet-footer ordering extraction and broader hybrid-scan support.