pitrou commented on code in PR #49296:
URL: https://github.com/apache/arrow/pull/49296#discussion_r2845443349
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cpp/src/parquet/decoder.cc:
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@@ -1618,16 +1618,27 @@ class DeltaBitPackDecoder : public
TypedDecoderImpl<DType> {
int values_decode = std::min(values_remaining_current_mini_block_,
static_cast<uint32_t>(max_values - i));
- if (decoder_->GetBatch(delta_bit_width_, buffer + i, values_decode) !=
- values_decode) {
- ParquetException::EofException();
- }
- for (int j = 0; j < values_decode; ++j) {
- // Addition between min_delta, packed int and last_value should be
treated as
- // unsigned addition. Overflow is as expected.
- buffer[i + j] = static_cast<UT>(min_delta_) + static_cast<UT>(buffer[i
+ j]) +
- static_cast<UT>(last_value_);
- last_value_ = buffer[i + j];
+ if (delta_bit_width_ == 0) {
+ // Fast path that avoids a back-to-back dependency between two
consecutive
+ // computations: we know all deltas decode to zero. We actually don't
+ // even need to decode them.
Review Comment:
Since the bit width is zero, there are zero bytes to decode, so we don't
need to advance it.
(also this is the purpose of the additional test)
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