blog: Apache DataFusion Comet 0.17.0 release post (#198)
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+---
+layout: post
+title: Apache DataFusion Comet 0.17.0 Release
+date: 2026-06-20
+author: pmc
+categories: [subprojects]
+---
+
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+
+[TOC]
+
+The Apache DataFusion PMC is pleased to announce version 0.17.0 of the [Comet](https://datafusion.apache.org/comet/) subproject.
+
+This release covers approximately five weeks of development work and is the result of merging 192 PRs from 19
+contributors. See the [change log] for more information.
+
+[change log]: https://github.com/apache/datafusion-comet/blob/main/dev/changelog/0.17.0.md
+
+## Fewer Fallbacks to Spark
+
+The headline feature of 0.17.0 is a new mechanism that keeps more of your query running inside Comet instead
+of falling back to Spark: the **JVM codegen dispatcher**.
+
+Comet has always fallen back to Spark row-based execution whenever an expression had no native Rust implementation, or where the
+Rust implementation could diverge from Spark on edge cases. A fallback is correct, but a columnar-to-row
+conversion is needed to feed the data into Spark's row-based operators, which adds overhead when processing billions of rows of data.
+
+The codegen dispatcher avoids the fallback to row-based processing by running Spark's own
+generated code (`doGenCode`) inside the Comet pipeline, operating directly on Arrow batches. The result is a
+JVM-implemented Arrow-native expression: the data stays in Arrow format, and because the expression is
+evaluated by Spark's own code, the result is guaranteed to match Spark exactly across every supported Spark
+version. When the dispatcher is disabled, Comet falls back as before.
+
+A dispatched expression is no faster than it would be in Spark, since it runs the same generated code. The
+benefit is that a single unsupported expression no longer forces an entire
+query stage back to row-based execution. The surrounding operators stay Arrow-native, and the
+stage avoids the columnar-to-row conversion and row-based Spark execution that a fallback would otherwise
+impose.
+
+This release puts the dispatcher to work across a wide surface:
+
+- **100% Spark-compatible regular expressions.** Regex expressions now dispatch to Spark's own
+ implementation, eliminating the long-standing compatibility gaps of a separate native regex engine.
+- **100% Spark-compatible JSON functions.** JSON expression handling follows the same approach, matching
+ Spark's behavior precisely.
+- **More scalar and structured-text functions**, including a batch of math and string functions, AES
+ encryption and decryption (`aes_encrypt` / `aes_decrypt` / `try_aes_decrypt`), `Upper` / `Lower` /
+ `InitCap`, `GetTimestamp`, `mask`, `try_to_number`, and an expanded set of date, time, and
+ timezone expressions.
+- **Collection and higher-order expressions**, including `array_intersect`, `array_except`, `array_join`,
+ `create_map`, and lambda-based higher-order functions such as `filter`.
+
+ 0.17.0 also changes how Comet treats expressions whose native Rust path is known to diverge
+ from Spark (marked `Incompatible`). Previously such an expression forced the entire projection back to Spark
+ unless the user opted into the divergent native behavior with the per-expression
+ `spark.comet.expression.<name>.allowIncompatible` flag. By default, that flag is unset, and the expression is
+ now routed through the codegen dispatcher and evaluated correctly inside Comet rather than triggering a
+ fallback. Setting it to `true` becomes a performance knob for users who accept the faster native path's
+ divergence. Expressions such as `from_unixtime`, and the `TimestampNTZ` branches of `hour`, `minute`, and
+ `second`, now stay in the pipeline by default.
+
+## User-Defined Functions in Java and Scala
+
+Building on the codegen dispatcher, 0.17.0 adds support for arbitrary user-defined functions written in Java
+and Scala, enabled by default. Eligible Spark `ScalaUDF` expressions are routed through the dispatcher and
+executed inside the Comet pipeline, so a project around a UDF no longer forces a fallback and a
+columnar-to-row conversion.
+
+The path has broad type coverage — scalars, arbitrarily nested complex types, and higher-order functions — and
+is backed by end-to-end, fuzz, and Iceberg test coverage. It can be disabled with
+`spark.comet.exec.scalaUDF.codegen.enabled=false`.
+
+## Arrow-Native, End to End
+
+The codegen dispatcher works because of a property that runs through all of Comet: queries stay **Arrow-native
+end to end**. Operators, expressions, shuffle, and broadcast all remain in Apache Arrow columnar format,
+avoiding the per-row overhead that Spark's row-based engine incurs from materializing and transitioning data
+one row at a time.
+
+Within that Arrow-native pipeline, the work of an operator or expression runs in one of two ways:
+
+- **Rust-implemented**: native Rust code, executed through Apache DataFusion. This is what most people picture
+ when they think of Comet.
+- **JVM-implemented**: Scala or Java code that operates directly on Arrow batches, including the
+ codegen-dispatched expressions and UDFs described above.
+
+Because the data never leaves Arrow columnar format, there is no per-row materialization cost at the boundary
+between a Rust-implemented and a JVM-implemented step. A dispatched expression sits in the pipeline alongside
+Rust-implemented operators with no columnar-to-row transition between them. Expanding the JVM-implemented path
+in 0.17.0 lets Comet keep more work Arrow-native instead of handing it back to Spark.
+
+## Expanded Expression Coverage
+
+Partly through the codegen dispatcher and partly through new Rust implementations, Comet's expression coverage
+grew substantially in this release. More than 120 Spark expressions have gained support since 0.16.0, across
+most function families:
+
+- **Date and time** (~25): `convert_timezone`, `make_date`, `months_between`, `next_day`,
+ `from_utc_timestamp` / `to_utc_timestamp`, `date_from_unix_date`, the `timestamp_*` and `unix_*` second /
+ milli / micro conversions, and the current date/time/timezone functions.
+- **Math** (~24): `acosh`, `asinh`, `atanh`, `cbrt`, `csc`, `sec`, `hypot`, `log1p`, `bin`, `conv`,
+ `factorial`, `pmod`, `width_bucket`, `rint`, and more.
+- **String** (~16): `elt`, `find_in_set`, `format_number`, `format_string`, `levenshtein`, `locate`,
+ `overlay`, `soundex`, `split`, `substring_index`, `unbase64`, `to_char`, and `to_number`.
+- **XPath** (9): the full `xpath`, `xpath_boolean`, `xpath_double`, `xpath_int`, `xpath_long`, `xpath_string`
+ family.
+- **JSON, CSV, and XML** (9): `from_csv`, `to_csv`, `schema_of_csv`, `schema_of_json`, `json_object_keys`,
+ `json_array_length`, `from_xml`, `to_xml`, and `schema_of_xml`.
+- **Array and map** (11): `array_position`, `array_size`, `arrays_zip`, `slice`, `sort_array`, `sequence`,
+ `map_concat`, `map_contains_key`, and `map_from_entries`.
+- **Aggregate and window** (7): `any_value`, `count_if`, the `regr_*` regression aggregates, plus `lag` and
+ `lead`.
+- **Conditional and null handling** (7): `greatest`, `least`, `nullif`, `ifnull` / `nvl`, `nvl2`, and
+ `equal_null`.
+
+For the full list of supported expressions in this release, see the
+[Spark Expression Support](https://datafusion.apache.org/comet/user-guide/0.17/expressions.html) reference.
+
+Operator coverage grew too: 0.17.0 adds a native **broadcast nested loop join**, so queries with non-equi or
+cross join conditions can now stay in the Comet pipeline rather than falling back to Spark.
+
+## Performance
+
+### Removing an FFI Round Trip from Native Shuffle
+
+The most significant performance change in 0.17.0 targets the shuffle write path. When a native subtree feeds
+a Comet shuffle, Comet previously ran two separate native iterators per partition: one for the upstream
+subtree, and a second rooted at a synthetic `Scan("ShuffleWriterInput") -> ShuffleWriter` that consumed the
+batches back. The JVM never actually read this data, so the native-to-JVM-and-back hop was pure overhead.
+
+Although the Arrow C Data Interface is zero-copy, this particular round trip was not. The synthetic scan left
+its batches marked as not FFI-safe, so on import every batch was deep-copied into freshly allocated buffers.
+0.17.0 collapses the two iterators into a single native plan rooted at the shuffle writer, with the upstream
+subtree as a direct child. That removes the Arrow FFI export and import, the per-batch deep copy, and one
+`createPlan` / `releasePlan` pair per partition, while preserving all of the existing per-partition setup
+(broadcast alignment, subqueries, encryption) and input metrics reporting. The gain comes from removing the
+round trip, not from copying its data more efficiently.
+
+### A Single Arrow Stream on the Input Path
+
+0.17.0 also reworks the other side of the JVM/native boundary — the path that feeds JVM-sourced data _into_
+native execution. Previously each batch crossed via a bespoke `CometBatchIterator`, with a `hasNext` / `next`
+JNI pair per batch and every column imported through its own Arrow FFI array and schema. This release replaces
+that path with the **Arrow C Stream Interface**: the JVM exports each per-partition iterator once, and native
+imports the schema once and pulls each batch through a single C callback, taking ownership by reference count.
+This removes the per-batch, per-column FFI export and JNI round trips for all JVM-sourced inputs, lets the
+old `CometBatchIterator` and a now-unnecessary deep copy be deleted, and is the input-side counterpart to the
+shuffle-write change above. TPC-DS at 1TB is about 9% faster in 0.17.0 than in 0.16.0, and these two FFI
+changes are the largest contributor to that gain.
+
+### Lower Per-Batch Overhead in Arrow Vectors
+
+Two changes reduce repeated work when reading Arrow columns across the JNI boundary. Comet now caches the
+validity buffer address on `CometDecodedVector` and the offset buffer address for variable-width vectors on
+`CometPlainVector`, so these addresses are resolved once per vector rather than on every access.
+
+### Faster Statistical Aggregates
+
+The variance, standard deviation, covariance, and correlation aggregates now use DataFusion's
+`GroupsAccumulator` interface, which is substantially more efficient for grouped aggregation than the
+row-accumulator path they used previously.
+
+Additional smaller improvements include bulk-NULL handling in `split` and `substring` (skipping a per-row
+allocation), and a new `interleave_time` shuffle metric with tuned output buffer sizing to make shuffle cost
+easier to attribute.
+
+## Preparing for the 1.0.0 Release
+
+Much of the correctness work in this release is part of an ongoing push toward a **1.0.0 release**.
+Planning for 1.0.0 is being tracked in [issue #4082], where the community is discussing what the milestone
+should mean. The criteria under consideration go well beyond correctness and include:
+
+- Demonstrated cost savings for TPC-H and TPC-DS at SF1000 (1TB)
+- Thorough documentation of compatibility status
+- A review of all configuration options, renaming some for consistency
+- Consistent logging
+- A documented policy for how long each Spark version will be supported
+- A documented process for preventing major performance regressions
+- A documented semantic-versioning policy and what it means for Comet going forward
+
+[issue #4082]: https://github.com/apache/datafusion-comet/issues/4082
+
+Correctness is central to that list: reaching 1.0.0 means being able to state precisely which Spark
+expressions Comet accelerates, and how faithfully it matches Spark on each one. Two efforts in 0.17.0 move
+toward that goal.
+
+First, the codegen dispatcher guarantees an exact match for every dispatched expression, since Spark's own
+generated code does the evaluation. That raises our confidence that Comet matches Spark across the supported
+versions, and it makes the Spark Expression Support reference something the project can stand behind as it
+approaches 1.0.0.
+
+Second, this release included a systematic audit of Comet's existing expression implementations against Spark,
+detailed below.
+
+### Expression Audit
+
+The audit compared Comet's expression implementations against Apache Spark 3.4.3, 3.5.8, 4.0.1, and 4.1.1,
+covering the hash, JSON, collection, map, predicate, bitwise, conditional, array, struct, math, and cast
+expression families, along with cast behavior. Each audit compared Comet's behavior to Spark across all four
+versions and expanded test coverage where gaps were found.
+
+This edge-case-by-edge-case comparison surfaced most of the behavior differences fixed in this
+release. By working through each expression's corner cases and writing targeted Comet SQL tests for them, the
+audit caught divergences that broader testing does not reliably exercise. Comet also runs the full Apache
+Spark SQL test suite against each supported Spark version as part of CI, which continues to catch some
+cross-version differences, but the audit's focused approach found considerably more.
+
+## Compatibility
+
+Supported platforms include:
+
+- **Spark 3.4.3** with Java 11/17 and Scala 2.12/2.13
+- **Spark 3.5.8** with Java 11/17 and Scala 2.12/2.13
+- **Spark 4.0.2** with Java 17 and Scala 2.13
+- **Spark 4.1.2** with Java 17 and Scala 2.13
+
+See the [Spark Version Compatibility] page for known limitations specific to each version.
+
+[Spark Version Compatibility]: https://datafusion.apache.org/comet/user-guide/latest/compatibility/spark-versions.html
+
+This release builds on **DataFusion 53.1** and **Arrow 58.3**.
+
+## Get Started with Comet 0.17.0
+
+Ready to try it out? Follow the [Comet 0.17.0 Installation Guide](https://datafusion.apache.org/comet/user-guide/0.17/installation.html)
+to get up and running, then point Comet at your existing Spark workloads, including Scala and Java UDFs and
+Spark 4 with ANSI mode enabled, and see the speedup for yourself.
