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ARROW-9827: [C++][Dataset] Skip parsing RowGroup metadata statistics when there is no filter #8037

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ARROW-9827: [C++][Dataset] Skip parsing RowGroup metadata statistics when there is no filter #8037

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ARROW-9827: [C++][Dataset] Skip parsing RowGroup metadata statistics …
jorisvandenbossche Aug 24, 2020
105ddbe
still need to get vector of row groups if no filter is specified
jorisvandenbossche Aug 25, 2020
99d3aec
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jorisvandenbossche Sep 8, 2020
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25 changes: 19 additions & 6 deletions cpp/src/arrow/dataset/file_parquet.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -385,12 +385,25 @@ Result<ScanTaskIterator> ParquetFileFormat::ScanFile(std::shared_ptr<ScanOptions
GetReader(fragment->source(), options.get(), context.get()));

if (!parquet_fragment->HasCompleteMetadata()) {
// row groups were not already filtered; do this now
RETURN_NOT_OK(parquet_fragment->EnsureCompleteMetadata(reader.get()));
ARROW_ASSIGN_OR_RAISE(row_groups,
parquet_fragment->FilterRowGroups(*options->filter));
if (row_groups.empty()) {
return MakeEmptyIterator<std::shared_ptr<ScanTask>>();
// row groups were not already filtered; do this now (if there is a filter)
if (!options->filter->Equals(true)) {
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@jorisvandenbossche jorisvandenbossche Aug 24, 2020

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@bkietz ideally we would also skip this if the flilter only involves the partition_expression and not any actual columns of the file. What is the best way to check this? (simplify the filter first with partition_expression ?)

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@bkietz bkietz Aug 28, 2020

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Simplifying the filter by the partition expression would work, but it's not safe without loading the physical schema since we need to validate the filter against that. Otherwise a filter without implicit casts (for example) would result in an assertion failure. Let's leave that optimization for a follow up; the right way to do this is probably to rewrite Expression::Assume to return a Result (allowing it to fail non-catastrophically even without a schema against which to validate), but that's a much larger project

RETURN_NOT_OK(parquet_fragment->EnsureCompleteMetadata(reader.get()));
ARROW_ASSIGN_OR_RAISE(row_groups,
parquet_fragment->FilterRowGroups(*options->filter));
if (row_groups.empty()) {
return MakeEmptyIterator<std::shared_ptr<ScanTask>>();
}
} else {
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@bkietz bkietz Aug 28, 2020

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Suggested change
} else {
} else {
// since we are not scanning this fragment with a filter, don't bother loading statistics

// since we are not scanning this fragment with a filter, don't bother loading
// statistics
row_groups = parquet_fragment->row_groups();
if (row_groups.empty()) {
// empty vector represents all row groups
std::shared_ptr<parquet::FileMetaData> metadata =
reader->parquet_reader()->metadata();
int num_row_groups = metadata->num_row_groups();
row_groups = RowGroupInfo::FromCount(num_row_groups);
}
}
}

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