This rabbit hole started with a simple observation: VertiPaq seemed to like parquet produced by delta-rs more than parquet produced by DuckDB — and that drove me nuts. delta-rs was at the time a niche library for nerds; Fabric didn’t even have a Python notebook. The mental model was simple: write with Spark, get V-Order, get the best possible layout for Power BI.
It is 2026; Fabric is more widespread, and there are simply more patterns and use cases:
- New Fabric workspaces default Spark to the
writeHeavyresource profile, which does not write V-Order. - Customers — especially on smaller SKUs — routinely write with delta-rs from Python notebooks.
- Reading tables written by Snowflake, Databricks and BigQuery through Direct Lake is a production pattern.
So “what parquet is friendly to Vertipaq” is now a legitimate data engineering question — I can’t tell you how happy I was when I read this tweet 🙂
The short version
- Row groups of a few million rows : 2–6M rows per group; never go above 16M, VertiPaq’s segment ceiling.
- Dictionary-encode every column — and make the file footer say so.
- One global
ORDER BY, lowest-cardinality columns first, date up front. No clustering, no Z-ordering, none of that. - Delta vs Iceberg does not matter. Only the parquet inside the table matters.
Method
I treated VertiPaq as a black box and did what experimental science does with a phenomenon it doesn’t understand: change one variable, measure, repeat. Nothing here is confidential or internal — every number was measured from the outside, on tables anyone can rebuild.
One more thing changed this year: AI became genuinely useful for this kind of work, because it never gets bored. Sweeping writers × row-group sizes × file sizes × sort orders across hundreds of runs is exactly the tedium it doesn’t feel.
Two experiments.
- First: figure out why VertiPaq preferred delta-rs output.
- Second: run multiple writers and vary row-group count, file size and ordering, measuring cold, warm and hot query cost.
Caveat. Hot behaviour is well documented — after all, it is the same in-memory format as import mode — so I am more interested in cold runs (first touch of a fresh model), even though most real-world traffic is hot: a live model transcodes once and then serves from RAM.
The datasets are rather smallish — the biggest table here is around 600M rows. As a data analyst I have always dealt with small data, so I am optimising for the workload I actually care about.
How Direct Lake reads your parquet
The mechanism that explains almost every finding. Transcoding is per column, on demand: the first DAX query to touch a column that is not yet in memory converts that column only into VertiPaq’s in-memory format. The column’s per-row-group parquet dictionaries are merged into one global VertiPaq dictionary, and each row group of the column is loaded as one resident column segment, remapping parquet data IDs onto VertiPaq IDs on the way in. Every query after that scans the segments the transcode produced. Query latency — and capacity consumption — is therefore a property of how the parquet was written.
Findings
Dictionary encoding is the big one
VertiPaq is itself a dictionary-based engine. When a chunk arrives dictionary-encoded, the transcode merges the parquet dictionary into the column’s global one and remaps the data IDs — it never decodes the values. Anything else has to be decoded and re-hashed, value by value, at load time. On a single 144M-row DECIMAL(18,4) column, PLAIN measured 618.6 MB against 423.1 MB dictionary-encoded — ~200 MB extra and a re-encode, on one column.
The surprise is that the encoding alone isn’t enough: the declaration is part of the encoding. The engine takes the cheap remap path only when the footer’s encoding_stats prove a chunk is entirely dictionary-encoded without decoding its pages. DuckDB’s writer emitted no encoding_stats at all until duckdb#24957 (merged 2026-08-24, currently in main only). That PR measures the cold first-touch of a 142M-row dictionary string column falling from 10,857.5 ms to 689.3 ms — about 15×, with identical pages. The attribution was verified the hard way: synthesising only that footer field into an otherwise unmodified file reproduces the speedup. A second PR, duckdb#24645 (merged 2026-08-10), adds a data_page_size_limit option — before it, DuckDB often wrote one huge data page per column chunk.
Row-group size: a tension between cold and hot
There is no single best size, but both ends fail measurably. Every row group is one more dictionary merge and one more segment to set up per column, which is why tiny groups murder the cold tier: the same DuckDB in the same notebook was 3.5× slower cold (96,503 ms vs 27,785 ms) when a library default sliced a 144M-row table into 1,172 groups of ~123k rows. At the other end, 16M rows — VertiPaq’s segment ceiling — was the worst sorted geometry measured: nine segments starve the scan pool. Cold prefers slightly bigger groups than hot, but very big groups are bad for both.
Power BI doesn’t disclose how many cores it uses, so the practical rule is: enough row groups to keep the cores busy. On the 144M-row table, warm query time stepped down between 19 and 24 groups (≈5,700 ms → 3,221 ms), and 72 groups bought nothing over 24. Hence the plateau: 2–6M rows per group.
A global sort keeps paying after the data is in memory
Transcoding does not change row order — it is essentially a working data copy into memory. So a sort applied at write time survives into the resident segments, which is why ordering matters for hot runs too, not just for compression.
A single global ORDER BY with low-cardinality columns first (and a preference for date) produces long RLE runs. It is not V-Order — but in some cases it is good enough. When V-Order does engage, what it’s worth depends on the surface — column count × categorical skew — not row count: on a skewed 17-column taxi table it collapsed the most repetitive column to 3,371× fewer runs; on a near-unique 5-column table it left row order untouched and still shrank files 16%. One caution from the sweep: an alternative sort key cut file size a further 30% and bought statistically zero query time — sort for the columns your queries filter on, not for size on disk.
This is also the cleanest way to see what V-Order’s reorder actually is. A hand-written ORDER BY collapses the column you name and leaves the others fragmented; V-Order sorts by several columns at once, most repetitive first — the taxi measurement above is its signature, runs falling off exactly as an encoding-driven sort predicts.
VertiPaq doesn’t like ragged row groups
delta-rs closes a file the moment the size cap is hit, truncating the in-progress row group — measured writing groups at 0.43× their declared rows. A truncated group isn’t just small; it makes segment sizes uneven, exactly the non-uniform scan load you were sizing row groups to avoid. I’ve proposed a fix in delta-rs#4677 — still open, and opt-in — which rolls files only on row-group boundaries.
Dynamic row group size at write time are very hard.
I built a personal package, duckrun, using delta-rs and DuckDB and tried a clever optimisation. The plan: before writing a query’s result, estimate its row count and compute the perfect row-group size on a 1M–16M scale. It failed completely, because query planners are bad at estimating output size — DuckDB estimated ~14.9M rows for a table that actually held 143,980,961, 9.7× too low — so the “optimised” geometry was off by an order of magnitude. Derive geometry from an exact count (the table you’re rewriting, the Delta log) — never from an estimate. Not only that: in an initial version the cost of the estimation was nearly the same as writing the table 🙂
Anyway, I endup writing 6M as the default row group everywhere, i feel it is a good enough compromise
Takeaway
V-Order is usually understood as row reordering — and the reordering is real; the hard part is not the reordering itself but doing it fast (that’s the secret sauce basically), to be super clear, V-order is a local sort, not global
But it is not only that: a V-Order write also sets the row-group geometry and runs the encoding pass, keeping a declared dictionary on every column.
Two consequences follow:
If you write with a Fabric engine, turning V-Order on is a no-brainer, and honestly, it should be the default in my personal opinion.
measured here at ~8% of build compute for up to 2.8× less query capacity (1,332 vs 3,769 CU on identical data) — the write premium is paid once, while queries pay every day. I do wish it were simpler to turn on: changing the Spark write profile is not obvious, and a lot of users don’t even know it is there. As someone who used VertiPaq for more than a decade with zero knowledge of columnar data structures, I suspect there must be a better way.
If you write with anything else — Snowflake, Databricks, delta-rs, DuckDB — my hope is that there will be more public specifications on how to optimize parquet layout for VertiPaq.
Thanks to Krystian Sakowski for answering my silly questions 🙂
Small Data And self service 