fatemehp commented on code in PR #14142:
URL: https://github.com/apache/arrow/pull/14142#discussion_r989295024
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cpp/src/parquet/column_reader.cc:
##########
@@ -1329,6 +1332,185 @@ class TypedRecordReader : public
ColumnReaderImplBase<DType>,
return records_read;
}
+ // Throw away levels from start_levels_position to levels_position_.
+ // Will update levels_position_ and levels_written_ accordingly and move
+ // the levels to left to fill in the gap. It will not shrink the size
+ // of the buffer or overwrite the positions after levels_written_.
+ // This is inefficient, though necessary to consume levels that we have
+ // already read into the buffer and we want to Skip.
+ void ThrowAwayLevels(int64_t start_levels_position) {
+ ARROW_DCHECK_LE(levels_position_, levels_written_);
+ ARROW_DCHECK_LE(start_levels_position, levels_position_);
+ int64_t gap = levels_position_ - start_levels_position;
+
+ for (int64_t i = levels_position_; i < levels_written_; ++i) {
+ *(def_levels() + i - gap) = *(def_levels() + i);
+ *(rep_levels() + i - gap) = *(rep_levels() + i);
+ }
+
+ levels_written_ -= gap;
+ levels_position_ -= gap;
+ }
+
+ // Skip records that we have in our buffer. This function is only for
+ // non-repeated fields.
+ int64_t SkipRecordsInBufferNonRepeated(int64_t num_records) {
+ ARROW_DCHECK_EQ(this->max_rep_level_, 0);
+ if (!this->has_values_to_process()) return 0;
+
+ int64_t remaining_records = levels_written_ - levels_position_;
+ int64_t skipped_records = std::min(num_records, remaining_records);
+ int64_t start_levels_position = levels_position_;
+ // Since there is no repetition, number of levels equals number of records.
+ levels_position_ += skipped_records;
+
+ // We skipped the levels by incrementing 'levels_position_'. For values
+ // we do not have a buffer, so we need to read them and throw them away.
+ // First we need to figure out how many present/not-null values there are.
+ std::shared_ptr<::arrow::ResizableBuffer> valid_bits;
+ valid_bits = AllocateBuffer(this->pool_);
+ PARQUET_THROW_NOT_OK(
+ valid_bits->Resize(bit_util::BytesForBits(skipped_records), true));
+ ValidityBitmapInputOutput validity_io;
+ validity_io.values_read_upper_bound = skipped_records;
+ validity_io.valid_bits = valid_bits->mutable_data();
+ validity_io.valid_bits_offset = 0;
+ DefLevelsToBitmap(def_levels() + start_levels_position,
+ skipped_records,
+ this->leaf_info_, &validity_io);
+ int64_t values_to_read = validity_io.values_read - validity_io.null_count;
+
+ // Now that we have figured out number of values to read, we do not need
+ // these levels anymore. Updates levels_position_ and levels_written.
+ ThrowAwayLevels(start_levels_position);
+ ReadAndThrowAway(values_to_read);
+
+ // Mark the levels as read in the underlying column reader.
+ this->ConsumeBufferedValues(skipped_records);
+
+ return skipped_records;
+ }
+
+ // Skip records for repeated fields. Returns number of skipped records.
+ int64_t SkipRecordsRepeated(int64_t num_records) {
+ ARROW_DCHECK_GT(this->max_rep_level_, 0);
+
+ // For repeated fields, we are technically reading and throwing away the
+ // levels and values since we do not know the record boundaries in advance.
+ // Keep filling the buffer and skipping until we reach the desired number
+ // of records or we run out of values in the column chunk.
+ int64_t skipped_records = 0;
+ int64_t level_batch_size = std::max<int64_t>(kMinLevelBatchSize,
num_records);
+ // If 'at_record_start_' is false, but (skip_records == num_records), it
+ // means that for the last record that was counted, we have not seen all
+ // of it's values yet.
+ while (!at_record_start_ || skipped_records < num_records) {
+ // Is there more data to read in this row group?
+ // HasNextInternal() will advance to the next page if necessary.
+ if (!this->HasNextInternal()) {
+ if (!at_record_start_) {
+ // We ended the row group while inside a record that we haven't seen
+ // the end of yet. So increment the record count for the last record
+ // in the row group
+ ++skipped_records;
+ at_record_start_ = true;
+ }
+ break;
+ }
+
+ // Read some more levels.
+ int64_t batch_size = std::min(level_batch_size,
available_values_current_page());
+ // No more data in column. This must be an empty page.
+ // If we had exhausted the last page, HasNextInternal() must have
advanced
+ // to the next page. So there must be available values to process.
+ if (batch_size == 0) {
+ break;
+ }
+
+ // For skip we will read the levels and append them to the end
+ // of the def_levels and rep_levels just like for read.
+ ReserveLevels(batch_size);
+
+ int16_t* def_levels = this->def_levels() + levels_written_;
+ int16_t* rep_levels = this->rep_levels() + levels_written_;
+
+ int64_t levels_read = 0;
+ levels_read = this->ReadDefinitionLevels(batch_size, def_levels);
+ if (this->ReadRepetitionLevels(batch_size, rep_levels) !=
levels_read) {
+ throw ParquetException("Number of decoded rep / def levels did not
match");
+ }
+
+ levels_written_ += levels_read;
+
+ // Look at the buffered levels, delimit them based on
+ // (rep_level == 0), report back how many records are in there, and
+ // fill in how many not-null values (def_level == max_def_level_).
+ // DelimitRecords updates levels_position_.
+ int64_t start_levels_position = levels_position_;
+ int64_t remaining_records = num_records - skipped_records;
+ int64_t values_seen = 0;
+ skipped_records += DelimitRecords(remaining_records, &values_seen);
+ if (ReadAndThrowAway(values_seen) != values_seen) {
+ throw ParquetException("Could not read and throw away requested
values");
+ }
+ // Mark those levels and values as consumed in the the underlying
page.
+ // This must be done before we throw away levels since it updates
+ // levels_position_ and levels_written_.
+ this->ConsumeBufferedValues(levels_position_ - start_levels_position);
+ // Updated levels_position_ and levels_written_.
+ ThrowAwayLevels(start_levels_position);
+ }
+
+ return skipped_records;
+ }
+
+ // Read 'num_values' values and throw them away.
+ int64_t ReadAndThrowAway(int64_t num_values) {
+ int64_t values_left = num_values;
+ int64_t batch_size = kMinLevelBatchSize; // ReadBatch with a smaller
memory footprint
+ int64_t values_read = 0;
+
+ // This will be enough scratch space to accommodate 16-bit levels or any
+ // value type
+ int value_size = type_traits<DType::type_num>::value_byte_size;
+ std::shared_ptr<ResizableBuffer> scratch = AllocateBuffer(
+ this->pool_, batch_size * std::max<int>(sizeof(int16_t), value_size));
+ do {
+ batch_size = std::min<int>(batch_size, values_left);
+ values_read = this->ReadValues(
+ batch_size, reinterpret_cast<T*>(scratch->mutable_data()));
+ values_left -= values_read;
+ } while (values_read > 0 && values_left > 0);
+ return num_values - values_left;
+ }
+
+ int64_t SkipRecords(int64_t num_records) override {
Review Comment:
No, here is what we have now: Consider a non-repeated field, and that there
are 10 pages with 100 values each. SkipRecords(900) will skip "decoding" the
first 9 pages. It will still look at the page headers to find out how many
values there are per page.
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