Copilot commented on code in PR #50422:
URL: https://github.com/apache/arrow/pull/50422#discussion_r3553413993
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cpp/src/parquet/column_reader.cc:
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@@ -1137,40 +1231,40 @@ int64_t TypedColumnReaderImpl<DType>::ReadBatch(int64_t
batch_size, int16_t* def
return total_values;
}
-template <typename DType>
-void TypedColumnReaderImpl<DType>::InitScratchForSkip() {
- if (this->scratch_for_skip_ == nullptr) {
- int value_size = type_traits<DType::type_num>::value_byte_size;
- this->scratch_for_skip_ = AllocateBuffer(
- this->pool_, kSkipScratchBatchSize * std::max<int>(sizeof(int16_t),
value_size));
- }
-}
-
template <typename DType>
int64_t TypedColumnReaderImpl<DType>::Skip(int64_t num_values_to_skip) {
int64_t values_to_skip = num_values_to_skip;
// Optimization: Do not call HasNext() when values_to_skip == 0.
while (values_to_skip > 0 && HasNext()) {
// If the number of values to skip is more than the number of undecoded
values, skip
- // the Page.
+ // the whole Page without decoding levels or values.
const int64_t available_values = this->available_values_current_page();
if (values_to_skip >= available_values) {
values_to_skip -= available_values;
this->ConsumeBufferedValues(available_values);
} else {
- // We need to read this Page
- // Jump to the right offset in the Page
- int64_t values_read = 0;
- InitScratchForSkip();
- ARROW_DCHECK_NE(this->scratch_for_skip_, nullptr);
- do {
- int64_t batch_size = std::min(kSkipScratchBatchSize, values_to_skip);
- values_read = ReadBatch(static_cast<int>(batch_size),
- scratch_for_skip_->mutable_data_as<int16_t>(),
- scratch_for_skip_->mutable_data_as<int16_t>(),
- scratch_for_skip_->mutable_data_as<T>(),
&values_read);
- values_to_skip -= values_read;
- } while (values_read > 0 && values_to_skip > 0);
+ // Skip within the current Page. Since `values_to_skip <
available_values`, the
+ // whole batch fits inside this Page and no page boundary is crossed.
+ const int batch_size = static_cast<int>(values_to_skip);
+
+ // Advance the definition levels, counting how many correspond to present
+ // (non-null) values that must be skipped in the data decoder.
+ int64_t non_null_values_to_skip = batch_size;
+ if (this->max_def_level() > 0) {
+ const auto count =
+ this->definition_level_decoder_.CountUpTo(this->max_def_level(),
batch_size);
+ non_null_values_to_skip = count.matching_count;
+ ARROW_DCHECK_EQ(count.processed_count, batch_size);
+ }
+ // Advance the repetition levels; their values are not needed.
+ if (this->max_rep_level() > 0) {
+ this->repetition_level_decoder_.Skip(batch_size);
+ }
+ // Skip the corresponding data values.
+ this->current_decoder_.Skip(non_null_values_to_skip);
+
Review Comment:
`TypedColumnReaderImpl::Skip` ignores the return values of both
`repetition_level_decoder_.Skip(batch_size)` and
`current_decoder_.Skip(non_null_values_to_skip)`. If either skips fewer than
requested (e.g. truncated/corrupt page), the method still calls
`ConsumeBufferedValues(batch_size)` and decrements `values_to_skip` by
`batch_size`, which can silently desynchronize the reader. Please validate the
counts and throw on mismatch.
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