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PFOR splits a page into vectors, 1024 values by default. Each vector subtracts its minimum, bit-packs the offsets at the width a cost model picks, and stores the values that need more bits as exceptions with their positions. An offset array locates every vector, so a reader decodes a vector on first access and skipping does not unpack the ones in between. The reader checks each header field and exception position against the page before using it. Tests cover widths from 0 up to the full value width, partial vectors and malformed pages.
The writer properties gain a PFOR switch, global or per column, which the V2 values writer factory checks before BYTE_STREAM_SPLIT for INT32 and INT64 columns. It is off by default. The format has no PFOR value yet, so the metadata converter refuses to write one with a message that says so instead of failing in an enum lookup.
parquet.enable.pfor turns PFOR on for jobs configured through a Hadoop Configuration, exposed the same way as parquet.enable.bytestreamsplit. It defaults to off.
Encodes and decodes INT32 and INT64 columns drawn from constant, sequential, clustered, outlier-heavy, random and TPC-DS-like distributions, and prints each one's compression ratio. Surefire's benchmark exclusion keeps it out of normal test runs.
A sorted or slowly changing column packs much narrower as differences between successive values than as offsets from a frame. The writer now costs each vector both ways and keeps the cheaper, using a sampled estimate to skip the full delta pass where it cannot win. Bit 7 of the bit width byte, reserved until now, marks a delta vector, and such a vector stores its first value so it still decodes without the one before it. The reader patches exceptions before the prefix sum. The mode is on by default and can be turned off globally or per column.
parquet.enable.pfor.delta lets a job configured only through a Hadoop Configuration keep every PFOR vector on absolute values. It defaults to on and is consulted only where PFOR itself is enabled.
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FSST's proposal asks for encoding 10 and does not get it. ALP claimed 10 too, shipped here first, and has since been merged into parquet.thrift on parquet-format main -- so 10 is settled and not FSST's. FSST took 11 here, which is what the arrow-rs proof-of-concept predicted would happen once ALP landed. 11 is no longer free either. apache/parquet-format#617 proposes PFOR (Patched Frame of Reference) as encoding 11, and unlike the FSST proposal it arrives with two implementations behind it: apache/parquet-java#3775 and apache/arrow-rs#10977, both of which write 11. So FSST moves to 12 and 11 is reserved. The collision is not one a reader can detect and report: a decoder reads the encoding byte, believes it, and misreads the page body -- there is no magic or length that disagrees. That makes it worth vacating the slot now rather than after files exist. The number lives only on the enum member; every other site goes through Encoding.Fsst, so this is a one-line format change plus its documentation. Breaking for anyone who has persisted the numeric value, which the [Experimental] attribute on the member has always warned about. Adds EncodingWireNumberTests to pin all of the numbers, including that nothing answers to 11. Round-trip tests cannot see a renumber: this library would write and read its own files happily either way and only disagree with other implementations, which is exactly the failure mode that produced the move. Also corrects a stale README claim that FSST_16 is unimplemented and rejected; both symbol table widths have shipped since 2026-08-13. Claude-Session: https://claude.ai/code/session_01UX8ZZxcXf5q4EqqwhDntNA Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
With the vector minimum as the frame, one value far below a tight cluster forces a bit width wide enough to reach it, and no exception can help. The writer now searches 256 buckets of the vector's range for a frame that may sit above the minimum. A value below the frame wraps to a residual too wide for the packed width and becomes an ordinary exception. The minimum is costed first, so the chosen frame is never more expensive. The wire format and the reader are unchanged, and the delta mode searches the frame of its differences the same way.
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…239) * fix(parquet)!: move FSST off encoding 11, which PFOR now claims FSST's proposal asks for encoding 10 and does not get it. ALP claimed 10 too, shipped here first, and has since been merged into parquet.thrift on parquet-format main -- so 10 is settled and not FSST's. FSST took 11 here, which is what the arrow-rs proof-of-concept predicted would happen once ALP landed. 11 is no longer free either. apache/parquet-format#617 proposes PFOR (Patched Frame of Reference) as encoding 11, and unlike the FSST proposal it arrives with two implementations behind it: apache/parquet-java#3775 and apache/arrow-rs#10977, both of which write 11. So FSST moves to 12 and 11 is reserved. The collision is not one a reader can detect and report: a decoder reads the encoding byte, believes it, and misreads the page body -- there is no magic or length that disagrees. That makes it worth vacating the slot now rather than after files exist. The number lives only on the enum member; every other site goes through Encoding.Fsst, so this is a one-line format change plus its documentation. Breaking for anyone who has persisted the numeric value, which the [Experimental] attribute on the member has always warned about. Adds EncodingWireNumberTests to pin all of the numbers, including that nothing answers to 11. Round-trip tests cannot see a renumber: this library would write and read its own files happily either way and only disagree with other implementations, which is exactly the failure mode that produced the move. Also corrects a stale README claim that FSST_16 is unimplemented and rejected; both symbol table widths have shipped since 2026-08-13. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01UX8ZZxcXf5q4EqqwhDntNA * feat(parquet): PFOR encoding, plain and delta, behind EWPARQUET0005 Implements PFOR (Patched Frame of Reference) for INT32 and INT64 as proposed in apache/parquet-format#617, reader and writer, both modes. Frame of reference plus bit-packing, with the values that do not fit the chosen width stored separately as exceptions -- so one outlier stops widening the packing for everyone. Each 1024-value vector independently chooses whether to pack values or the differences between them, which is the difference from DELTA_BINARY_PACKED: a column sorted only in stretches gets the delta treatment on those stretches and frame-of-reference on the rest. The writer measures the result and falls back to PLAIN per page, so enabling the setting cannot make a file bigger. The page layout is close enough to ALP's that PforDecoder is shaped like AlpDecoder and the two read together: a 7-byte header, an offset array whose offsets are measured from its own start, then self-describing vectors. What PFOR adds is the delta flag in bit 7 of the width byte, a per-vector start value, and a prefix sum -- which must run AFTER the exceptions are patched, since an exception in a delta vector is a difference like any other. TWO PLACES WHERE THE SPEC CONTRADICTS ITSELF, both found by measuring. First, the frame. PforEncoding.md says it is the column's minimum. On the shape PFOR exists for -- a tight cluster with a low sentinel -- that is the wrong answer by a wide margin: the sentinel takes the frame, every ordinary value sits tens of thousands above it, and the width is set by the gap rather than the cluster. The spec's own Example 3 is that column and quotes a width only a frame ABOVE the minimum can produce. So the minimum is a candidate here, not the rule: the encoder buckets the range, slides a window over the bucket counts per candidate width, lowers the winner onto the smallest value it covers, and costs that frame exactly. The minimum is always among the candidates and the only one costed from a real histogram, so the search cannot do worse than the naive rule. arrow-rs does the same; parquet-java does not. MEASURED, on 200k INT64 date keys with a null sentinel: 0.65x against DELTA_BINARY_PACKED with the naive frame, 5.31x with the search. Second, width 0 in the delta mode. The spec says a reader may fill with the start value and "get the same answer for any frame". It does not: fill and the general path agree only when the frame is 0. No conforming writer can emit anything else, so it is unreachable from real data -- but arrow-rs takes the fill path and its own test pins an answer the general path disagrees with. This decodes by the general path, which is what the numbered decode steps say, and pins that. Compression, 200k INT64 per shape, uncompressed, vs DELTA_BINARY_PACKED: date keys + sentinel 5.31x, sorted in stretches 3.11x, tight cluster 1.74x, sorted ids 1.20x, timestamps 1.20x, sequence with gaps 1.06x, random 1.03x. With zstd the picture reverses everywhere except the outlier shapes, because DBP's output on clean sequential data is far more compressible than PFOR's tightly packed output. doc/parquet-pfor.md carries both tables and says so plainly rather than quoting only the flattering half. Tests sweep EVERY bit width, 0 to 32 and 0 to 64, at a vector length that is a multiple of eight and one that is not, rather than sampling: this is the class of bug that shipped once already in the RLE decoder (#236), and it is invisible at every width but the broken one. Three byte-for-byte golden vectors from arrow-rs's tests are the only assertions that could catch us writing a self-consistent page nobody else can read. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01UX8ZZxcXf5q4EqqwhDntNA * fix(parquet): strip the UTF-8 BOMs the PFOR commit introduced Three files were rewritten through a utf-8-sig encoder, which prepends a BOM. CI's "Check for UTF-8 BOMs" step is exactly there to catch it. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01UX8ZZxcXf5q4EqqwhDntNA * fix(parquet): bound a PFOR vector by the next offset, and reuse encoder scratch Both from Copilot's review of #239. A vector was sliced from its own offset to the END OF THE PAGE rather than to the next vector's offset. Every per-vector truncation check then measured against the wrong extent: a vector shorter than its own header claims read on into the following vectors' bytes, satisfied every length check, and returned their contents as data. Non-monotonic offsets were not checked at all, so a decreasing offset silently decoded overlapping vectors. Neither is a memory-safety problem -- the reads stay inside the page -- but both are silent. The value count comes from the page header, so the output is the right shape and the wrong data, with nothing raised to say so. The two new tests fail on the old code with "No exception was thrown", which is the whole point. The first attempt at the truncation test did not discriminate: shifting the next offset backwards also breaks the FOLLOWING vector, which threw for an unrelated reason and made the test pass against the bug. It now widens the first vector's declared bit width instead, leaving every offset alone, and asserts the precondition that makes it a regression test -- short measured against the next offset, long enough measured against the end of the page. Also makes the encoder's per-page scratch thread-static. Encoding a page allocated differences, residuals, exception positions, a histogram and the frame-search buckets every time, about eleven kilobytes of garbage per page of every integer column. Thread-static rather than pooled because column chunks encode in parallel and each thread encodes one page at a time, which is why ColumnChunkWriter.t_valuesBuffer is thread-static too. Not taken: the reviewer also suggested writing vectors into one contiguous page buffer instead of a byte[] per vector. That is a real cost, but the byte[]-per-vector shape is AlpEncoder's as well, and changing it in one encoder leaves the two inconsistent. Worth doing across both with a benchmark behind it, not blind in this PR. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01UX8ZZxcXf5q4EqqwhDntNA * fix(parquet): drop the "11 is reserved" comment now that PFOR occupies 11 Merge fallout. #238 left a comment above the FSST member reserving 11 for PFOR; this branch replaced it with the member itself, and merging main back in reinstated the comment alongside the member it describes. "Reserved rather than reused" now sits directly above Pfor = 11. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01UX8ZZxcXf5q4EqqwhDntNA --------- Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com> Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com> Co-authored-by: CurtHagenlocher <904803+CurtHagenlocher@users.noreply.github.com>
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