REVIEW 3 major objections 4 minor 39 references
This paper shows that FSST's greedy encoding and symbol selection are suboptimal, and that replacing them with a dynamic-programming encoder plus improved table construction raises average compression factors by 7.3% over FSST and 17.0% ove
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
OptFSST lifts FSST's average compression factor by 7.3% and FSST12's by 17.0% across 92 string columns using DP encoding, triple counting, and pruning; it also proves the symbol-table selection problem is NP-hard when the alphabet is part of the input.
T0 review reviewed 2026-08-02 challenge →
load-bearing objection The core result — DP-based encoding plus count3 and pruning gives 7.3%/17% average compression-factor gains on FSST/FSST12 — is plausible and honestly evaluated; the main blemish is the under-specified benchmark-filtering rule, which needs pinning down before those averages are reproducible. the 3 major comments →
OptFSST: Optimized FSST String Compression
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
Core claim
For a fixed symbol table T, the minimum compressed size of a string s satisfies the recurrence dp[i] = min(2 + dp[i+1], 1 + min_{s[i:j] in T} dp[j]), and OptFSST computes it with a trie walk, storing dp_choice to emit the optimal segmentation. This replaces FSST's greedy longest-match encoder. For table construction, OptFSST adds count3 triple counting to expose longer recurring symbols earlier and prunes overlapping candidates' counts so redundant symbols do not waste limited table entries. The paper proves that STS_l, selecting a dictionary of length-at-most-l symbols minimizing total encoded cost, is NP-hard for every fixed l >= 2 when the alphabet is part of the input, motivating these h
What carries the argument
The central mechanism is the DP recurrence with a trie-backed lookup: from each input position the encoder either escapes one byte (cost 2) or emits a table symbol matching the prefix (cost 1), choosing the minimum suffix cost. Because candidate symbols are stored in a trie, the DP enumerates only actually matching symbols up to the maximum symbol length, and the dp_choice array turns the cost table into an encoding. Supporting mechanisms are the third frequency counter count3, which packs three 9-bit codes into a 32-bit hash key to discover triples of consecutive emitted symbols, and pruning, which subtracts a selected concatenated symbol's count from its component symbols and pairs before
Load-bearing premise
The headline averages depend on a filtering rule that removes columns where dictionary encoding beats FSST, and the exact rule is not disclosed; if that selection were different, the improvement numbers could change materially.
What would settle it
Run OptFSST and FSST on a large unfiltered set of string columns, including dictionary-friendly columns, with identical training settings for both, and compute the mean compression-factor improvement; if the mean falls below a few percent or many columns regress, the claimed 7.3%/17.0% averages are an artifact of the benchmark filter. A second check: hold the symbol table fixed and compare DP encoding to greedy longest-match encoding on every string; if DP never improves on greedy, the central recurrence is not carrying the gain.
If this is right
- Systems that already store FSST-compressed columns can adopt OptFSST without changing the decompression format or the random-access property, because the compressed representation is unchanged.
- Gains in compression factor translate directly into smaller memory footprints and lower memory-bandwidth pressure per scan, which matters when analytical workloads are cache- and bandwidth-bound.
- OptFSST12's 1.2x decompression-speed gain means queries reading compressed string columns can get faster, not just smaller.
- The NP-hardness result for STS_l implies no polynomial-time optimal table construction exists for variable alphabet size, so the heuristic components are necessary rather than incidental.
- The per-column variation (89% of columns improve for OptFSST, 95% for OptFSST12) indicates the method helps most columns while a minority stay near parity or regress, so users should expect workload-dependent gains.
Where Pith is reading between the lines
- Combining OptFSST with prefix-extraction layout schemes could compound the two sources of redundancy (cross-string prefixes and intra-string symbols); the paper notes the schemes are complementary but does not measure the combination.
- The DP encoder is branch-heavy and parallelizable per string; the paper's hint about DPX-style instructions suggests a GPU implementation could substantially cut the reported compression-time cost, but this is untested.
- The fixed-alphabet polynomial result in Appendix B points to a pragmatic test: on low-cardinality alphabets, a near-optimal table search may be feasible and could outperform the heuristics on those columns.
- The benchmark filters out dictionary-encodable columns, so the 7.3%/17.0% averages describe the gain on columns where FSST is the right tool; on an unfiltered fleet of string columns the headline averages would likely be lower.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper presents OptFSST and OptFSST12, modifications of the FSST and FSST12 string compressors. OptFSST replaces FSST's greedy longest-match encoder with a dynamic program that finds the minimum-cost segmentation of each string for a fixed symbol table, using cost 1 for a symbol and cost 2 for an escaped byte (Section 3.1). It also adds a third frequency counter for triples of emitted symbols during table construction and a pruning rule that corrects stale gains of overlapping candidates (Sections 3.2 and 3.3). The paper proves that the underlying symbol-table selection problem STS_l is NP-hard for every fixed l>=2 when the alphabet is part of the input (Section 4, Appendix A). The evaluation on 92 real-world string columns reports average compression-factor improvements of 7.3% over FSST and 17.0% over FSST12, with a 3-4x compression-time slowdown, preserved decompression speed for OptFSST, and 1.2x better decompression speed for OptFSST12.
Significance. The DP-based encoding is a principled and clean improvement over greedy encoding for a fixed symbol table, and the NP-hardness proof in Appendix A is a solid justification for the heuristic table construction; I verified the reduction's main accounting and found no flaw. The evaluation is unusually honest for this area: it ablates each component separately, discloses tested-and-abandoned variants, and explains the OptFSST12 0.39x outlier. If the empirical claims survive a well-specified benchmark population, this is a useful incremental contribution for systems that use FSST-style random-access string compression. The main risk is not algorithmic but methodological: the headline averages depend on a benchmark-selection rule that is not operationalized in the manuscript.
major comments (3)
- [Section 5.1] The headline averages (7.3% and 17.0% in Section 5.2) are computed over a set of 92 columns whose selection is not reproducible. Section 5.1 states only that columns 'compressed better using dictionary encoding are filtered out' and that columns with fewer than 1000 strings are dropped. This leaves the central exclusion rule undefined: which dictionary encoder, compared by compression factor or size, with what threshold, and applied to what units? The per-dataset counts (23+22+31+13+3=92) exactly sum to the final total, making it unclear whether any columns were excluded and from which corpora. I request the exact selection rule, the number of excluded columns per source, the full column list in the artifact, and a sensitivity analysis around the dictionary-encoding filter. Without this, the average improvements and the 82/92 and 87/92 column counts are not independently verifiable.
- [Sections 3.1 and 5.2] The compression-factor metric is not defined, and the role of the symbol-table size is ambiguous. The DP recurrence in Section 3.1 minimizes only the encoded payload (1 byte per symbol, 2 per escaped byte). However, the Section 5.4 discussion of the OptFSST12 min 0.39x result says the 4096-entry symbol table's 'serialization alone exceeds twice the input size and dominates the reported compression factor', implying the reported compression factor includes the serialized symbol table. If so, the DP objective is not the reported objective, and a heuristic table construction can outweigh the DP gain on small columns. Please state the exact formula for compression factor, specify whether symbol-table bytes are included for all codecs, and report table sizes for baseline and OptFSST variants. This is needed to interpret the headline improvements and the outlier.
- [Section 5.2] The pruning component is retained even though its aggregate effect is reported as neutral ('no noticeable effect' on the mean) and the ablation shows it can hurt in a minority of columns. This is a defensible design choice given the majority-of-columns benefit, but the paper should quantify the distribution rather than only report the mean and best cases: e.g., how often does pruning degrade the compression factor and by how much? Since pruning is one of the three named contributions, a fuller characterization would strengthen the claim that it is a useful correction rather than a heuristic kept on the basis of the majority count.
minor comments (4)
- [Section 2] The citation 'GPU-aware FSST adaptation [5?]' contains an unresolved question mark and should be corrected to a proper reference.
- [Section 3.1] The tie-breaking rule 'prefer longer symbols on equal DP cost' is presented as improving compression factors. Because all ties have the same output cost for the final encoding, the effect must come from the training loop's frequency counts; this should be stated explicitly so readers do not infer that the tie-break improves the DP optimum.
- [Section 5.1] The paper does not state which FSST/FSST12 version or parameter settings (sample size, number of generations, candidate budget) are used for the baseline. Please add the exact version and default parameters, or state that the released defaults are used.
- [Section 5.3] Table 1 reports end-to-end slowdowns, but the text in Section 5.3 gives only average compression speeds. It would be useful to report the median as well, since the violin plots show skewed distributions.
Circularity Check
No circularity found: DP optimality is a direct exact algorithm, NP-hardness proof is self-contained, and evaluation uses independent baselines.
full rationale
The paper's central claims are not circular. The DP recurrence in §3.1, dp[i] = min(2 + dp[i+1], 1 + min_{s[i:j] in T} dp[j]), is an exact optimal-substructure formulation under the FSST cost model (one byte per symbol, two bytes per escaped character); it is not a fitted parameter or a renamed prediction. The NP-hardness result in §4/Appendix A is proved by a self-contained reduction from Maximum Vertex Coverage on bipartite graphs, citing an external source [6], and the proof's forward and reverse directions are arithmetically checked. Symbol-table heuristics (third frequency counter, pruning) are presented as engineering improvements with ablations, not as predictions derived from the benchmark data. The evaluation compares against the independent cwida/fsst baseline and FSST12, and the ablation study isolates each contribution. No load-bearing self-citation exists: the present authors' prior work appears only as general background (e.g., [30]) and not as justification for core claims. The benchmark filtering rule in §5.1 ('Columns that are compressed better using dictionary encoding are filtered out') is under-specified and could affect reproducibility of the headline averages, but this is a sample-selection/measurement concern, not circularity in the sense of a result reducing to its inputs by construction. The disclosed outlier (OptFSST12 min 0.39, §5.4) is an honest limitation and does not indicate circularity.
Axiom & Free-Parameter Ledger
free parameters (3)
- DP tie-break: prefer longest symbol on equal cost =
'longest wins' via '<=' at Listing 1, line 29
- Candidate gain heuristic =
gain = length x observed frequency
- Training sample size / generation count =
not stated
axioms (3)
- standard math Maximum vertex coverage on bipartite graphs is NP-hard [6].
- domain assumption FSST format abstraction: a dictionary phrase costs 1 output byte, an escaped literal costs 2; symbols have length <= l (8 or 12); table size bounded by 255/4095.
- domain assumption Benchmark columns are representative of FSST's deployment population after exclusions.
Cite this review
Pith. "Pith review of OptFSST: Optimized FSST String Compression." pith.science (2026). https://pith.science/paper/Y3MHW7JQ
@misc{pith2026260711271,
author = {Pith},
title = {Pith review of: OptFSST: Optimized FSST String Compression},
year = {2026},
howpublished = {\url{https://pith.science/paper/Y3MHW7JQ}},
note = {Machine review of arXiv:2607.11271}
}
read the original abstract
Strings account for a substantial fraction of data in modern analytical systems, making lightweight compression with fast random access an important building block for efficient query processing. Fast Static Symbol Table (FSST) addresses this need by replacing frequent byte sequences with compact codes while preserving independent decompression of individual strings. However, FSST's compression effectiveness is limited by its greedy symbol selection and greedy encoding strategy, leaving encoding gains on the table. We present OptFSST, an optimized FSST variant that improves its compression factors while preserving its static-symbol-table design and random-access decompression. OptFSST optimally encodes the text using dynamic programming given a symbol table. Additionally, we show that a generalized version of the symbol-table selection problem is NP-hard when the alphabet is part of the input, motivating heuristic table construction for field-level compressors. Hence, we add in OptFSST (i) an additional frequency counter that accelerates the discovery of longer symbols and (ii) a pruning strategy that removes redundant and conflicting symbol candidates during table construction. We also extend the same techniques to FSST12, yielding OptFSST12. Our evaluation on 92 real-world string datasets shows that OptFSST improves the compression factors of FSST and FSST12 by up to 47.7% and 91.5%, with an average improvement of 7.3% and 17.0%, respectively, while retaining the fine-grained random-access properties. Notably, OptFSST12 improves FSST12's decompression speed by $1.2\times$ on average.
Figures
Reference graph
Works this paper leans on
-
[1]
Abadi, Samuel Madden, and Nabil Hachem
Daniel J. Abadi, Samuel Madden, and Nabil Hachem. 2008. Column-stores vs. row-stores: how different are they really?. InProceedings of the ACM SIGMOD International Conference on Management of Data, SIGMOD 2008, Vancouver, BC, Canada, June 10-12, 2008, Jason Tsong-Li Wang (Ed.). ACM, 967–980. https: //doi.org/10.1145/1376616.1376712
arXiv 2008
-
[2]
Azim Afroozeh and Peter Boncz. 2025. The FastLanes File Format.Proc. VLDB Endow.18, 11 (2025), 4629–4643. https://doi.org/10.14778/3749646.3749718
arXiv 2025
-
[3]
Azim Afroozeh, Lotte Felius, and Peter Boncz. 2024. Accelerating GPU Data Processing using FastLanes Compression. In20th International Workshop on Data Management on New Hardware (DaMoN). ACM. https://doi.org/10.1145/3662010. 3663450
-
[4]
2025.FSST+: Enhancing String Compression Through Common Prefix Extraction
Yan Lanna Alexandre. 2025.FSST+: Enhancing String Compression Through Common Prefix Extraction. Master’s thesis. Vrije Universiteit Amsterdam and Universiteit van Amsterdam, Amsterdam, The Netherlands
2025
-
[5]
Peter Hofstee
Tim Anema, Joost Hoozemans, Zaid Al-Ars, and H. Peter Hofstee. 2025. High Throughput GPU-Accelerated FSST String Compression. In16th International Workshop on Accelerating Analytics and Data Management Systems Using Modern Processor and Storage Architectures (ADMS)
2025
-
[6]
Nicola Apollonio and Bruno Simeone. 2014. The maximum vertex coverage problem on bipartite graphs.Discret. Appl. Math.165 (2014), 37–48. https: //doi.org/10.1016/J.DAM.2013.05.015
-
[7]
Peter Boncz. 2020. cwida/fsst. https://github.com/cwida/fsst. Original release: 2020-03-03. Accessed: 2026-01-10
2020
-
[8]
Peter Boncz, Thomas Neumann, and Viktor Leis. 2020. FSST: Fast Random Access String Compression.Proc. VLDB Endow.13, 11 (2020), 2649–2661. http: //www.vldb.org/pvldb/vol13/p2649-boncz.pdf
2020
-
[9]
Boncz and Martin L
Peter A. Boncz and Martin L. Kersten. 1995. Monet: An Impressionist Sketch of an Advanced Database System. InProceedings of the IEEE BIWIT Workshop. San Sebastian, Spain, 240–251
1995
-
[10]
Tijmen Bruineman. 2025. [Compression] Introduce DICT_FSST Compression Method. https://github.com/duckdb/duckdb/pull/15637. Accessed: 2026-01-23
2025
-
[11]
ClickHouse. 2022. ClickBench. https://github.com/ClickHouse/ClickBench. Original release: 2022-07-11. Accessed: 2026-01-23
2022
-
[12]
Yann Collet. 2011. LZ4. https://github.com/lz4/lz4. Original release: 2011-03-25. Accessed: 2026-01-10
2011
-
[13]
dtim-upc. 2021. NextiaJD. https://github.com/dtim-upc/NextiaJD. Original release: 2021-05-17. Accessed: 2026-01-23
2021
-
[14]
Facebook. 2015. Zstandard. https://github.com/facebook/zstd. Original release: 2015-01-24. Accessed: 2026-01-10
2015
-
[15]
Francesco Gargiulo and Rossano Venturini. 2025. OnPair: Short Strings Com- pression for Fast Random Access.CoRRabs/2508.02280 (2025). https://doi.org/ 10.48550/ARXIV.2508.02280 arXiv:2508.02280
work page internal anchor Pith review Pith/arXiv arXiv doi:10.48550/arxiv.2508.02280 2025
-
[16]
Bogdan Ghita, Peter Boncz, and Duarte Tomé. 2019. Public BI Benchmark – Part
2019
- [17]
-
[18]
Google. 2011. snappy. https://github.com/google/snappy. Original release: 2011-03-07. Accessed: 2026-05-13
2011
-
[19]
Paul Groß, Daniel ten Wolde, and Peter Boncz. 2019. Adaptive Factorization Using Linear-Chained Hash Tables. (2019)
2019
-
[20]
Dong He, Supun C. Nakandala, Dalitso Banda, Rathijit Sen, Karla Saur, Kwanghyun Park, Carlo Curino, Jesús Camacho-Rodríguez, Konstantinos Karana- sos, and Matteo Interlandi. 2022. Query Processing on Tensor Computa- tion Runtimes.Proceedings of the VLDB Endowment15, 11 (2022), 2811–2825. https://doi.org/10.14778/3551793.3551833
arXiv 2022
-
[21]
Sven Hepkema, Azim Afroozeh, Charlotte Felius, Peter Boncz, and Stefan Mane- gold. 2025. G-ALP: Rethinking Light-weight Encodings for GPUs. In21st In- ternational Workshop on Data Management on New Hardware (DaMoN). ACM. https://doi.org/10.1145/3736227.3736242
arXiv 2025
-
[22]
Holloway, Vijayshankar Raman, Garret Swart, and David J
Allison L. Holloway, Vijayshankar Raman, Garret Swart, and David J. DeWitt
-
[23]
Violeta Kastreva, Philip Whittington, Dennis Komm, and Tiago Pimentel. 2026. Tokenisation over Bounded Alphabets is Hard. InThe Fourteenth International Conference on Learning Representations. https://openreview.net/forum?id= Xhf9YqwlM4
2026
-
[24]
Matteo Interlandi, Nicolas Bruno, Brandon Haynes, Carlo Curino, Rathijit Sen, Yinan Li, et al. 2026. CoddSpeed: Hardware Accelerated Query Processing in Microsoft Fabric. InCompanion of the International Conference on Management of Data. ACM, 359–372. https://doi.org/10.1145/3788853.3803077
arXiv 2026
-
[25]
Harald Lang, Tobias Mühlbauer, Florian Funke, Peter Boncz, Thomas Neumann, and Alfons Kemper. 2016. Data Blocks: Hybrid OLTP and OLAP on Compressed 10 Storage using both Vectorization and Compilation. InProceedings of the 2016 International Conference on Management of Data, SIGMOD Conference 2016, San Francisco, CA, USA, June 26 - July 01, 2016, Fatma Özc...
arXiv 2016
-
[26]
Maximilian Kuschewski, David Sauerwein, Adnan Alhomssi, and Viktor Leis
-
[27]
2024.NVIDIA Blackwell Architecture Technical Brief
NVIDIA Corporation. 2024.NVIDIA Blackwell Architecture Technical Brief. https: //resources.nvidia.com/l/en-us-blackwell-architecture
2024
-
[28]
Wesley Pace, Chen She, Lei Xu, et al. 2025. Lance: Efficient Random Access in Columnar Storage through Adaptive Structural Encodings.CoRRabs/2504.15247 (2025). https://doi.org/10.48550/arXiv.2504.15247 arXiv:2504.15247
-
[29]
Jigao Luo, Qi Chen, and Carsten Binnig. 2026. Do GPUs Really Need New Tabular File Formats?. In22nd International Workshop on Data Management on New Hardware (DaMoN). ACM. https://doi.org/10.1145/3789237.3809125
arXiv 2026
-
[30]
Alexander van Renen, Dominik Horn, Pascal Pfeil, Kapil Vaidya, Wenjian Dong, Murali Narayanaswamy, Zhengchun Liu, Gaurav Saxena, Andreas Kipf, and Tim Kraska. 2024. Why TPC Is Not Enough: An Analysis of the Amazon Redshift Fleet.Proc. VLDB Endow.17, 11 (2024), 3694–3706. https://doi.org/10.14778/ 3681954.3682031
arXiv 2024
-
[31]
Vortex Team. 2026. vortex-data/vortex: An Extensible, State-of-the-Art Columnar File Format. https://github.com/vortex-data/vortex. Accessed: 2026-02-12
2026
-
[32]
Vijayshankar Raman, Gopi K. Attaluri, Ronald Barber, Naresh Chainani, David Kalmuk, Vincent KulandaiSamy, Jens Leenstra, Sam Lightstone, Shaorong Liu, Guy M. Lohman, Tim Malkemus, René Müller, Ippokratis Pandis, Berni Schiefer, David Sharpe, Richard Sidle, Adam J. Storm, and Liping Zhang. 2013. DB2 with BLU Acceleration: So Much More than Just a Column St...
arXiv 2013
-
[33]
Bowen Wu, Wei Cui, Carlo Curino, Matteo Interlandi, and Rathijit Sen. 2025. Terabyte-Scale Analytics in the Blink of an Eye.Proc. VLDB Endow.19, 2 (2025), 141–155. https://www.vldb.org/pvldb/vol19/p141-sen.pdf
2025
-
[34]
Patel, Andrew Pavlo, and Huanchen Zhang
Xinyu Zeng, Ruijun Meng, Martin Prammer, Wes McKinney, Jignesh M. Patel, Andrew Pavlo, and Huanchen Zhang. 2025. F3: The Open-Source Data File Format for the Future.Proc. ACM Manag. Data3, 4 (2025), 245:1–245:27. https: //doi.org/10.1145/3749163
-
[35]
Philip Whittington, Gregor Bachmann, and Tiago Pimentel. 2026. Tokenisation is NP-Complete. InTokenization Workshop. https://openreview.net/forum?id= zGMXftuVZz
2026
-
[36]
Marcin Zukowski, Sándor Héman, Niels Nes, and Peter Boncz. 2006. Super- Scalar RAM-CPU Cache Compression. InProceedings of the 22nd International Conference on Data Engineering, ICDE 2006, 3-8 April 2006, Atlanta, GA, USA, Ling Liu, Andreas Reuter, Kyu-Young Whang, and Jianjun Zhang (Eds.). IEEE Computer Society, 59. https://doi.org/10.1109/ICDE.2006.150 ...
-
[38]
Jacob Ziv and Abraham Lempel. 1977. A universal algorithm for sequential data compression.IEEE Trans. Inf. Theory23, 3 (1977), 337–343. https://doi.org/10. 1109/TIT.1977.1055714
arXiv 1977
-
[2007]
How to barter bits for chronons: compression and bandwidth trade offs for database scans. InProceedings of the ACM SIGMOD International Conference on Management of Data, Beijing, China, June 12-14, 2007, Chee Yong Chan, Beng Chin Ooi, and Aoying Zhou (Eds.). ACM, 389–400. https://doi.org/10.1145/1247480. 1247525
doi:10.1145/1247480 2007
-
[2023]
BtrBlocks: Efficient Columnar Compression for Data Lakes.Proc. ACM Manag. Data1, 2 (2023), 118:1–118:26. https://doi.org/10.1145/3589263
doi:10.1145/3589263 2023
This paper was first reviewed by deepseek-v4-flash on August 2, 2026.
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.