Audit the datafusion-spark implementation of the $ARGUMENTS function for correctness against Apache Spark.

Baseline Version

The baseline is Spark 4.2.0. Every expected value written into a .slt file asserts Spark 4.2.0 behavior.

Spark 3.5.8, 4.0.4, and 4.1.3 are also read, but only to detect that behavior changed between versions. They never set an expectation. There is currently no mechanism for version-specific expectations in DataFusion's SLT files, which is tracked by https://github.com/apache/datafusion/issues/23887.

Older versions are read rather than ignored because datafusion-spark is meant to be reusable by engines that target more than one Spark release. Comet supports Spark 3.4 through 4.0. A function audited against 4.2.0 alone can be correct there and silently wrong for such a consumer, and nothing in the crate records which of the two it is. Finding the divergence is useful even though the test suite cannot yet assert it.

Overview

Study the Spark implementation across versions
Harvest Spark's own test coverage
Review the DataFusion implementation
Cross-check Comet and Sail
Review existing SLT coverage
Gap analysis
Establish ground truth with PySpark
Apply findings

Audit one function per invocation.

Paths in This Skill Are Hints

Every file path below was correct when this skill was written and several will rot. FunctionRegistry.scala, the golden-file directories, and datafusion/common/src/config.rs all move on someone else's schedule.

A path that does not resolve is a stop condition, not a step to skip. The failure mode this exists to prevent is a command that quietly matches nothing, an audit that reads the silence as "no ANSI branch" or "Spark does not test this", and a confident report built on an empty grep. A grep over a nonexistent directory and a grep that genuinely found nothing are the same output.

So when a path in this skill does not exist, or a command returns nothing where something was expected:

Say so out loud in the audit report. Name the path and the step.
Search for where the thing moved to, by content rather than by path. The grep patterns here are more durable than the directories they are pointed at.
If it cannot be found, mark every conclusion that depended on that step as unestablished. Do not let an unread source become an absence of findings.

The same rule covers a .slt file, an implementation file, or a Spark class that is not where this skill says it is.

Step 1: Study the Spark Implementation

Shallow-clone the four version tags. Skip any already present.

set -eu
for tag in v3.5.8 v4.0.4 v4.1.3 v4.2.0; do
  dir="/tmp/spark-${tag}"
  [ -d "$dir" ] || git clone --depth 1 --branch "$tag" \
    https://github.com/apache/spark.git "$dir"
done

Find the expression class in each version. Spark registers SQL function names in FunctionRegistry.scala, so start there to map $ARGUMENTS to its Scala class name, then locate the class.

for tag in v3.5.8 v4.0.4 v4.1.3 v4.2.0; do
  dir="/tmp/spark-${tag}"
  echo "=== $tag ==="
  grep -rn "\"$ARGUMENTS\"" \
    "$dir/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala"
done

With the class name in hand, read its source in each version:

for tag in v3.5.8 v4.0.4 v4.1.3 v4.2.0; do
  dir="/tmp/spark-${tag}"
  echo "=== $tag ==="
  find "$dir/sql/catalyst/src/main/scala" -name "*.scala" \
    | xargs grep -lE "case class <ClassName>[ (\[]" 2>/dev/null
done

A Scala case class name is always followed by a space, a (, or a [, so that bracket expression is the portable way to avoid matching a longer class name that starts with the same prefix. Do not reach for \b, which is a GNU and ugrep extension and is not available in POSIX or BSD grep.

If it is not in catalyst, widen the search to $dir/sql.

For each version, record:

eval / nullSafeEval / doGenCode logic
inputTypes and dataType
nullable, and how nulls propagate
ANSI branches (ansiEnabled, failOnError)
guards, require assertions, thrown exceptions
any SQLConf the expression reads
for string functions in 4.0+, whether it routes through CollationSupport.X.exec(..., collationId) or declares StringTypeWithCollation inputs

Produce a diff summary across 3.5.8 → 4.0.4 → 4.1.3 → 4.2.0. Note new or removed input types, changed edge-case behavior, new ANSI branches, new parameters, and collation changes.

Two changes affect nearly every function and are easy to miss:

ANSI mode became the default in Spark 4.0. A function whose behavior differs under ANSI has a different default behavior in 3.5 than in 4.x.
Collation support landed for many string functions in 4.0. Non-default collations are a divergence axis that does not exist in 3.5.

Step 2: Harvest Spark's Own Test Coverage

Spark's tests are a ready-made edge-case inventory. Two sources matter.

Unit tests:

dir=/tmp/spark-v4.2.0
find "$dir/sql" -name "*.scala" -path "*/test/*" \
  | xargs grep -ln "$ARGUMENTS" 2>/dev/null

Golden files, which carry Spark's own verified query results and are the highest-value source:

for tag in v3.5.8 v4.0.4 v4.1.3 v4.2.0; do
  dir="/tmp/spark-${tag}"
  echo "=== $tag ==="
  grep -rln "$ARGUMENTS" "$dir/sql/core/src/test/resources/sql-tests/results/" 2>/dev/null
done

The ANSI and non-ANSI golden directories swapped meaning in Spark 4.0. Read the version banner before trusting a path:

Version	ANSI-mode results	Non-ANSI results
3.5.8	`results/ansi/X.sql.out`	`results/X.sql.out`
4.x	`results/X.sql.out`	`results/nonansi/X.sql.out`

ANSI became the default in 4.0, so the unqualified path flipped from meaning non-ANSI to meaning ANSI. An auditor who learns the layout on 3.5.8 and then reads results/X.sql.out on 4.2.0 will record exactly inverted expectations, and they will look plausible. Confirm the mode by checking whether the file contains error output or NULL for a known-invalid input.

Read every match and extract:

input types covered
edge cases exercised: null, empty, overflow, negative, boundary, special characters
ANSI-mode cases
error cases and their exact messages

This list is the reference for the gap matrix in Step 6.

Step 3: Review the DataFusion Implementation

grep -rln "$ARGUMENTS" datafusion/spark/src/function/ --include="*.rs"

The implementation lives at datafusion/spark/src/function/<category>/<function>.rs. Read every other file that grep returns as well. In particular datafusion/spark/src/function/<category>/mod.rs carries the export_functions! registration and the user-facing doc string, and unimplemented-behavior TODO comments are written there too, not only in the function's own file. An audit that reads only <function>.rs will miss them and will leave a doc string that contradicts the code after a fix.

A .slt file under test_files/spark/ does not imply an implementation in datafusion/spark. SessionStateBuilderSpark::with_spark_features layers the Spark functions on top of with_default_features, so a name the spark crate does not define resolves to DataFusion's own function of that name. md5, coalesce, and reverse are served this way today, and date_format resolves through an alias on core's to_char. The query under audit still runs, and the code it runs is not in the directory the grep above searched. If that grep comes back empty, widen it before concluding the function is unimplemented:

grep -rln "\"$ARGUMENTS\"" \
  datafusion/functions/src datafusion/functions-nested/src \
  datafusion/functions-aggregate/src datafusion/functions-window/src \
  datafusion/functions-table/src --include="*.rs"

A hit may be in a file named after something else, because core registers aliases. date_format lives in to_char.rs. Read the aliases vector, not just the file name.

If that grep is also empty, the function is genuinely unimplemented and the .slt file is a porting stub with every query commented out. Check with grep -cE '^(query|statement)' <file>.slt, which returns zero for a stub. There is no implementation to audit in that case. Say so and stop, rather than reporting a clean audit of nothing.

Otherwise audit what the grep found against Spark with the rest of this step's checklist. The findings land differently. A core function has non-Spark users, so Spark's semantics cannot be imposed on it. Where the two agree, record the coverage and leave the core function alone. Where they diverge, the fix is a Spark-specific implementation under datafusion/spark/src/function/<category>/ that shadows the core one, never an edit to the core function. datafusion/spark/src/function/math/abs.rs shadowing datafusion/functions/src/math/abs.rs is the in-repo precedent. Say in the audit report which crate the audited code came from, because a reader who assumes datafusion/spark will look for a file that does not exist.

Check each of the following. The first two have no Comet analog and are the most common source of silent divergence.

Signature and coercion. Compare Signature (and coerce_types, if implemented) against Spark's inputTypes. Two failure directions, both real:

DataFusion rejects a type Spark accepts. The user gets a coercion error where Spark returns a value.
DataFusion accepts a type Spark rejects. The user gets a value where Spark raises an analysis error.

Note that Signature::exact is narrow. A Spark function accepting any string type needs all of Utf8, LargeUtf8, and Utf8View handled, and a function accepting any integral type needs the full width range.

Return nullability. Compare return_field_from_args (or return_type) against Spark's nullable. A function that reports non-nullable while Spark can return NULL produces wrong plans, not merely wrong values: downstream optimizer rules will fold away null checks that were load-bearing.

Null propagation. Does every input-null combination return what Spark returns? Check the order in which nullness and validity are tested. Spark expressions that are null intolerant (a NullIntolerant mixin in 3.5, a nullIntolerant override in 4.x) short circuit to NULL before validating the other arguments, so a NULL in one argument suppresses an error the other argument would otherwise raise, even under ANSI mode. An implementation that validates first and checks nullness second raises where Spark returns NULL.

Arrow kernel semantics are not Java semantics. A datafusion-spark function is usually a thin arrangement of Arrow kernels, and the kernels do not share Java's rules. Check each one the implementation leans on:

Comparison kernels order floats by total order, so -0.0 compares as less than 0.0 rather than equal to it. Any eq/lt used to detect a special value, such as a zero divisor, silently misses -0.0. Java compares them as equal. This is easy to miss because -0.0 almost never appears in hand-written test values, so write it in deliberately.
Arithmetic kernels are checked, not wrapping. Java wraps silently on overflow, so a checked Arrow kernel raises where Spark returns a wrapped value. Where Spark's own expression is written in Int arithmetic, a wrapping kernel is the faithful choice, not the reckless one.
Java promotes byte and short operands to int. A Spark expression whose Scala source reads (r + n) on Byte operands is evaluated at Int width and cannot overflow, while the same source on Int operands can. Reproducing it at Int8 width wraps where Spark does not.
Kernel coverage is type dependent. Arrow's rem raises on a zero divisor for integers and decimals but returns NaN for floats, so an ANSI check that relies on the kernel to raise silently does nothing on the float path.

Read the Scala source as arithmetic to be reproduced, not as pseudocode to be paraphrased. Then test each of these against a live Spark rather than reasoning about them.

ColumnarValue shape dispatch. A two-argument function has four shape combinations, and each needs its own branch: (Scalar, Scalar), (Array, Scalar), (Scalar, Array), (Array, Array). A missing arm falls through to a catch-all exec_err! and turns a working Spark query into a hard error. (Scalar, Array) is the one most often forgotten, because the natural test query puts the column first. Write a query for every combination and run it. Do not infer coverage from reading the match.

How to run an ad-hoc query. The only in-repo way to execute a datafusion-spark function is to add the query to datafusion/sqllogictest/test_files/spark/<category>/$ARGUMENTS.slt and run

cargo test --test sqllogictests -- spark/<category>/$ARGUMENTS

datafusion-cli does not register the datafusion-spark function library, so reaching for it fails in a way that looks like the function does not exist:

$ ./target/debug/datafusion-cli -c "SELECT next_day(arrow_cast(95026236,'Date32'),'Mon');"
Error during planning: Invalid function 'next_day'. Did you mean 'today'?

Giving the CLI an opt-in for the Spark library is tracked by https://github.com/apache/datafusion/issues/24146. Until that lands, treat the error above as a registration gap and not as evidence about the function.

For a throwaway probe that should not end up in the committed file, add a temporary .slt alongside it, run it by name, and delete it afterwards. To exercise a raw storage value that has no literal syntax, such as a Date32 past the last date DataFusion can render, wrap it in arrow_cast.

Cross the shapes with the NULL cases, and with ANSI mode. These are not three independent checklists to tick off separately, they are one product to enumerate. The bug this instruction exists to catch is a branch that gets NULL handling right in the row-wise path and wrong in the vectorized one, so the two shapes disagree with each other while each looks correct in isolation. For every shape, ask what happens when the other argument is NULL, and when it is NULL in only some rows. A shape that takes an array and a scalar has to make the scalar's validation depend on the array's null mask, because a per-row null intolerant function skips validation on the rows it short circuits. All-NULL, some-NULL, and no-NULL versions of the same array are three different tests.

Note that a (Array, Scalar) branch returning ColumnarValue::Scalar is not by itself a bug. In DataFusion a returned Scalar means "this value for every row" and the caller broadcasts it to the batch length. Before reporting one, run an N-row query and count the rows that come back. Report it only if the row count is actually wrong, or if the scalar's value depends on per-row input. Otherwise it is at most a readability cleanup, and reporting it as a correctness bug costs the audit credibility.

Overflow and underflow. Does it return Err, wrap, or panic? Spark's answer depends on ANSI mode. A panic is always a bug.

Type dispatch. Every type the signature admits must have a branch. An exec_err! fallthrough for a type the signature accepts is a bug.

ANSI handling. Compare against datafusion.execution.enable_ansi_mode (defined in datafusion/common/src/config.rs). The crate has scattered TODO comments about unimplemented ANSI paths. Treat every one found in this function as a finding, not as a known-acceptable state. Grep for them across every file Step 3's grep returned, including mod.rs:

grep -rn "TODO" datafusion/spark/src/function/<category>/$ARGUMENTS.rs \
                datafusion/spark/src/function/<category>/mod.rs

Wiring ANSI up is usually mechanical rather than a semantics decision. The flag is already threaded through ScalarFunctionArgs, so the implementation reads it directly:

let ScalarFunctionArgs { args, config_options, .. } = args;
let ansi_mode = config_options.execution.enable_ansi_mode;

datafusion/spark/src/function/math/abs.rs and datafusion/spark/src/function/math/modulus.rs are the in-repo precedents for both the plumbing and the test layout. DataFusion does not model Spark's error classes or SQLSTATE values, so say that in the audit either way.

Whether to match Spark's message text depends on who constructs the error:

The function constructs it (an exec_err! you write). Match Spark's text exactly, including trailing punctuation, and assert it in the .slt.
An Arrow kernel constructs it (rem, add, a cast). You do not control the wording. Assert only that an error is raised, note the divergence in a comment, and file an issue for the message text rather than contorting the implementation to restate an Arrow error in Spark's words.

Do not read the abs.rs precedent as licence to invent DataFusion-flavoured text for an error you do construct. abs reports Int32 overflow on abs(-2147483648) where Spark 4.2.0 reports [ARITHMETIC_OVERFLOW] overflow., which is a known divergence tracked by https://github.com/apache/datafusion/issues/24145. Follow its ANSI plumbing and test layout, not its wording.

abs also shows why the "who constructs it" question is worth asking rather than assuming. Its scalar path builds the message in the spark crate, but its array path borrows make_try_abs_function! from datafusion/functions/src/math/abs.rs, so that half of the text is owned by core DataFusion and shared with core's own abs. Wherever a spark function reuses a core macro or kernel, Spark-specific error text is not available without un-sharing the code first. Check where the message is actually built before promising to match Spark's.

Rust unit tests. Read the #[test] blocks in the same file. Per the crate README, direct invocation through test_scalar_function!() bypasses input coercion, so passing unit tests never establish end-to-end correctness. Note which cases are covered by unit tests only.

Step 4: Cross-Check Comet and Sail

The datafusion-spark README recommends checking both projects for existing implementations. Diff against them to surface edge cases they handle that this crate misses.

Neither project needs to be checked out. Both are searched over HTTP, by content rather than by path, because both have moved their directory layouts more than once:

for repo in apache/datafusion-comet lakehq/sail; do
  echo "=== $repo ==="
  gh api -X GET search/code -f q="$ARGUMENTS repo:$repo" --jq '.items[].path'
done

# Then read a specific hit
gh api "repos/<repo>/contents/<path from above>" --jq '.content' | base64 -d

If a clone happens to be on disk, grep it instead. It is faster and it is not rate limited. Do not silence the failure with 2>/dev/null, though: a grep over a directory that is not there prints an error, and swallowing it turns "Comet not checked out" into "Comet has no implementation".

[ -d ../datafusion-comet ] \
  && grep -rln "$ARGUMENTS" ../datafusion-comet/native/spark-expr/src/ --include="*.rs" \
  || echo "no local Comet clone, use the API above"

gh api search/code requires authentication and is rate limited to a handful of requests per minute. If it fails, say so in the audit report and mark Step 4 as not performed rather than reporting no differences found.

As of this writing the implementations live under crates/sail-function/src/scalar/<category>/spark_<function>.rs, and Sail also keeps behavior fixtures at python/pysail/tests/spark/function/features/<function>.feature. Treat both paths as hints, not as addresses.

Sail matters more than a generic peer here: many datafusion-spark functions were ported from Sail, so Sail's current code is often the same code with subsequent fixes applied. A difference between the two is a strong lead.

Treat both as peer implementations, not as authority. Each has its own audit backlog, and a shared assumption between them is not evidence of correctness. Spark's source and a running Spark are the only authorities.

Step 5: Review Existing SLT Coverage

ls datafusion/sqllogictest/test_files/spark/*/$ARGUMENTS.slt

Read the file and list the queries, the types exercised, which cases are commented out, and whether the file has an audit header comment from a previous audit.

Also read datafusion/sqllogictest/test_files/spark/README.md. It is the binding local convention for this tree and it prescribes things this skill does not repeat: every function must be tested on both scalar and array inputs, every literal needs an explicit cast because DataFusion and Spark infer types differently, test cases contain SELECT statements only, and a function whose behavior differs under ANSI is wrapped in

statement ok
set datafusion.execution.enable_ansi_mode = true;

# queries

statement ok
set datafusion.execution.enable_ansi_mode = false;

Reset the flag to false at the end of the block. The setting leaks to whatever follows it in the file otherwise.

Many files in this tree were machine-generated from Sail's gold data. Their expected values were not necessarily verified against Spark, so an existing passing expectation is not evidence of correctness. Re-verify in Step 7.

Step 6: Gap Analysis

Build this matrix, filling each cell with yes, no, or n/a:

Dimension	Spark tests it	DataFusion SLT	Rust unit test	Gap?
All-scalar arguments
All-array arguments
Array first argument, scalar second
Scalar first argument, array second
NULL input, each argument independently
Empty string / array / map
Zero, negative zero, negatives
Overflow, underflow
Boundary values (INT_MIN, INT_MAX, i64 bounds)
NaN, Infinity, -Infinity, subnormal
Multibyte / special UTF-8
ANSI mode enabled
ANSI mode disabled
Every type in Spark's `inputTypes`
Types Spark rejects
Return nullability
Cross-version divergence

Rows marked n/a need a one-line justification, for example "no float inputs" against the NaN row.

Both ANSI rows matter independently. DataFusion defaults enable_ansi_mode to false while Spark 4.x defaults ANSI to true, so the mode users actually hit on Spark 4 is the one most likely to be untested here.

Step 7: Establish Ground Truth with PySpark

No expected value goes into a .slt file without being observed from a real Spark. Reading nullSafeEval and predicting the output is exactly how wrong golden values get written in the first place.

Set up the environment

Requires a JDK 17 or later on PATH. Use uv to create the venv rather than python3 -m venv: pyspark==4.2.0 requires Python 3.10 or later, and the system python3 on many machines is older. uv is already used by this repository and will fetch a suitable interpreter if none is installed.

Unset SPARK_HOME and PYSPARK_PYTHON before doing anything else. If either is set, a local Spark install on SPARK_HOME silently overrides the pip-installed pyspark JARs, and PYSPARK_PYTHON can point PySpark's workers at a different Python than the venv you just built. The SPARK_HOME case is not hypothetical: it reproduces reliably on a machine with any other Spark install on PATH, and it fails late and unhelpfully, inside getOrCreate(), as TypeError: 'JavaPackage' object is not callable. If you hit that exact error, this is the cause: check echo $SPARK_HOME and unset it.

unset does not persist between tool calls. Each shell invocation starts from the user's profile, which is where SPARK_HOME was exported in the first place. Unsetting it once in the setup block and then running the harness in a later call reintroduces the failure. Put unset SPARK_HOME PYSPARK_PYTHON in the same command as every JVM-starting invocation, not in a preceding one.

unset SPARK_HOME PYSPARK_PYTHON
uv venv --python 3.12 /tmp/pyspark-4.2.0-venv
uv pip install --python /tmp/pyspark-4.2.0-venv --quiet 'pyspark==4.2.0'
/tmp/pyspark-4.2.0-venv/bin/python -c "import pyspark; print(pyspark.__version__)"

The last command must print 4.2.0. If it prints anything else, stop and fix the environment: an audit against the wrong Spark version is worse than no audit, because it writes confident wrong expectations.

That check is necessary but not sufficient. It only reads the installed package's metadata: it never starts a JVM, so it passes cleanly even on a machine primed to hit the SPARK_HOME shadowing failure above. The check that actually proves which Spark you are talking to is the # Spark 4.2.0 tz=UTC banner the harness prints from a live SparkSession in the next section. Do not trust the environment until you have seen that banner.

The harness

Write this to the scratchpad as spark_ground_truth.py. It is deliberately not committed to the repo: pyspark is too heavy a dependency to add to the uv workspace in dev/, and datafusion/spark/scripts/ is claimed by other in-flight work.

Write it unconditionally, overwriting whatever is already there. A previous audit may have left an older copy in the same scratchpad, and silently reusing it means auditing with a harness whose fixes you do not have.

#!/usr/bin/env python3
"""Run SQL queries against a local PySpark and print SLT-ready results.

Usage: spark_ground_truth.py QUERIES.sql

Input is a file of SQL statements, each terminated by a semicolon at the end
of a line. Each query is run under both spark.sql.ansi.enabled=true and
=false, and the observed result is printed in a form that can be pasted into
a .slt file.

The session time zone is pinned to UTC so that results do not depend on the
auditing machine's local zone. DataFusion's sqllogictest runner leaves
execution.time_zone unset, which is effectively UTC, so anything time zone
sensitive would otherwise produce golden values that are wrong everywhere
except on the machine that generated them.

Statement splitting is naive: it splits on a semicolon followed by a newline.
That keeps a semicolon inside a string literal intact, as in
SELECT ascii(';'), as long as the literal is not the last thing on the line.
A query whose string literal ends a line with a semicolon still splits
wrongly. Put such a query on a single line with trailing text after the
literal, or run it on its own.
"""

import sys

from pyspark.sql import SparkSession


def format_value(value):
    """Render one cell the way sqllogictest expects it."""
    if value is None:
        return "NULL"
    if isinstance(value, bool):
        return "true" if value else "false"
    return str(value)


def run_query(spark, sql):
    try:
        rows = spark.sql(sql).collect()
    except Exception as exc:  # noqa: BLE001 - any failure is a result here
        first_line = str(exc).strip().split("\n")[0]
        return f"ERROR: {type(exc).__name__}: {first_line}"
    if not rows:
        return "(no rows)"
    return "\n".join("\t".join(format_value(v) for v in row) for row in rows)


def main(path):
    spark = (
        SparkSession.builder.appName("slt-ground-truth")
        .master("local[1]")
        .getOrCreate()
    )
    spark.conf.set("spark.sql.session.timeZone", "UTC")
    spark.sparkContext.setLogLevel("ERROR")
    print(f"# Spark {spark.version} tz={spark.conf.get('spark.sql.session.timeZone')}")

    raw = open(path).read()
    if not raw.endswith("\n"):
        raw += "\n"
    queries = [q.strip() for q in raw.split(";\n") if q.strip()]

    for sql in queries:
        for ansi in (True, False):
            spark.conf.set("spark.sql.ansi.enabled", str(ansi).lower())
            print(f"### ansi={str(ansi).lower()}")
            print(f"# {sql}")
            print(run_query(spark, sql))
            print()

    spark.stop()


if __name__ == "__main__":
    if len(sys.argv) != 2:
        sys.exit("usage: spark_ground_truth.py QUERIES.sql")
    main(sys.argv[1])

Using it

Write every candidate query, including the ones already in the .slt file, to a scratch .sql file and run:

cd <scratchpad>
unset SPARK_HOME PYSPARK_PYTHON
/tmp/pyspark-4.2.0-venv/bin/python spark_ground_truth.py queries.sql \
  > ground_truth.txt 2> spark_logs.txt

Keep the streams separate. Spark writes log4j output, including structured JSON error records, on a stream that will interleave with the results if you merge them with 2>&1. The merged form is what an agent reaches for by reflex and it makes the output unreadable exactly on the error cases that matter most.

Verify the banner line reads # Spark 4.2.0 tz=UTC before trusting any output. Both halves matter. The version half proves you are not talking to a Spark install that shadowed the pip-installed one, and the time zone half proves the session is not silently inheriting the auditing machine's local zone. DataFusion's sqllogictest runner leaves execution.time_zone unset, which behaves as UTC, so a session on any other zone generates golden values that only reproduce on the machine that generated them. This is invisible for functions that take a DATE and never touch a time zone, and it is a silent corruption for to_utc_timestamp, date_format, from_unixtime, and every other timestamp function. If the banner shows anything but tz=UTC, stop and fix the harness before recording a single expected value.

Wrap date, timestamp, and interval results in CAST(... AS STRING). The harness calls collect(), which converts every cell to a Python object, and Python's datetime.date cannot represent everything Spark's DateType can. A value outside Python's range surfaces as a line like

ERROR: ValueError: year 10000 is out of range

which reads exactly like a Spark error and is not one. Spark returned a value fine. Any ValueError or OverflowError in the output is a harness artifact, never a Spark result. Re-run the query wrapped in a cast to see what Spark actually produced:

SELECT CAST(next_day(DATE '9999-12-31', 'Mon') AS STRING);
-- +10000-01-03

Re-verify expectations that already exist in the .slt file. Many files in this tree were machine-generated and their values were never checked against Spark. An existing expectation is a claim, not evidence.

Write the queries in Spark SQL. The .slt file needs DataFusion syntax, which differs: DataFusion uses '2015-01-14'::DATE where Spark SQL uses DATE '2015-01-14'. Translate when transcribing, and keep the semantics identical. If a query cannot be expressed in both dialects, note it as a finding rather than silently testing something different.

Never use `--complete`

The sqllogictest runner supports --complete, which fills in expected results with whatever DataFusion currently returns.

Never run it on files under test_files/spark/. On an audit it would silently cement the bug being audited as the golden answer, which inverts the purpose of the whole exercise. Expected values come from PySpark and are written by hand.

If PySpark cannot be installed

Continue the audit using values derived from the Spark source and the golden files from Step 2, and mark every such expectation as unverified in both the audit report and a comment in the .slt file:

# UNVERIFIED: derived from Spark source, not confirmed against a running Spark

Never present a derived value as a verified one. An audit that overstates its confidence is worse than one that admits a gap.

Step 8: Apply Findings

Findings do not survive as prose. A recommendation in a PR description is buried the moment the PR merges. Every finding becomes code, a test, or a filed issue.

Fix what can be fixed

Divergences from Spark 4.2.0 are fixed in the audit PR. Attempt the fix regardless of size. Fall back to filing an issue only when the fix needs a semantics decision a human should make, for example choosing which of two defensible behaviors DataFusion should adopt when Spark's own behavior is inconsistent across versions.

Every fix is locked in by an SLT test that fails before the fix and passes after. A test that passes before the fix was testing something else.

Verify both directions before committing. Stash only the implementation change, so the new .slt query runs against the unfixed code:

# Stash the implementation only, leaving the new .slt query in place
git stash push -- datafusion/spark/src/function/<category>/<function>.rs
cargo test --test sqllogictests -- spark/<category>/$ARGUMENTS   # expect FAIL
git stash pop
cargo test --test sqllogictests -- spark/<category>/$ARGUMENTS   # expect PASS

This verifies SLT tests only. Rust unit tests live inside the implementation file, so stashing that file stashes the tests with it and the run proves nothing about them. For a unit test, get the fail-before evidence by reverting the implementation body by hand, or by writing the test first and watching it fail before the fix exists.

Read the failure list from the first run and confirm it names the queries you added. Do not judge the run by an exit code you captured through a pipe: cargo test ... | head reports the exit status of head, not of cargo, so a piped command that "succeeded" tells you nothing. Either run it unpiped or inspect the External error: N errors in file ... block directly.

If the first run passes, the test is not exercising the fix. Either the query was stashed along with the implementation, or the assertion does not actually cover the divergent case. Investigate before continuing: a test that passes against the unfixed code locks in nothing.

If the fix also changed a mod.rs doc string or a helper in another file, that file is not covered by the stash above. Add it to the git stash push argument list, or the "expect FAIL" run may fail for the wrong reason.

Run this before committing the fix. If git stash push reports No local changes to save, the change is already committed and the check is invalid, because the "expect FAIL" run just tested the fixed code. Redo it against the pre-fix commit by checking that one file out of history and back, which touches only the working tree and never moves HEAD or a branch:

# <pre-fix-sha> is the commit before the one that introduced the fix
git checkout <pre-fix-sha> -- datafusion/spark/src/function/<category>/<function>.rs
cargo test --test sqllogictests -- spark/<category>/$ARGUMENTS   # expect FAIL
git checkout HEAD -- datafusion/spark/src/function/<category>/<function>.rs
cargo test --test sqllogictests -- spark/<category>/$ARGUMENTS   # expect PASS

Find <pre-fix-sha> with git log --oneline -- <the implementation file> and take the commit before the fix. If the fix also touched other files, list every one of them in both git checkout commands.

Do not reach for git reset, git revert, or git rebase here. None of them are needed and all of them can leave the branch in a state the audit then has to repair.

Capture what cannot be fixed

A deferred divergence becomes a commented-out SLT query carrying the correct Spark 4.2.0 result and the issue link:

# Spark 4.2.0 raises ILLEGAL_DAY_OF_WEEK under ANSI mode; DataFusion returns NULL
# https://github.com/apache/datafusion/issues/23889   <- the real number, not a placeholder
# query error
# SELECT next_day('2015-01-14'::DATE, 'NOT_A_DAY'::string);

Uncommenting the block is the contract for verifying the eventual fix. Note that DataFusion .slt files use # for comments, not --.

File the issue first, then write the block. The block cannot be written until its issue number exists, so do the filing described below before typing the comment. Never commit a placeholder in place of the number. A string like <ISSUE-URL-PENDING>, NNNNN, or TBD inside a URL ships a dead link, and because the surrounding block is commented out no test ever catches it.

If the issue cannot be filed at all, for example because gh is not authenticated or the user asked that nothing be filed yet, omit the link line entirely. A block with no link is honest and self-contained. Then say so explicitly in the audit report: name the divergence, state that no issue was filed and why, and ask the user to file it. Do not leave the reader to discover the gap from a broken URL.

When you do assert an error, remember that the runner normalizes whitespace in the query error regex, so literal leading, trailing, and repeated spaces in the expected message cannot be written out. An input like ' MO ' produces Illegal input for day of week: MO . with two spaces, and a regex containing those two spaces is collapsed to one before matching. Write \s+ instead, which carries no literal whitespace and survives normalization:

query error Illegal input for day of week:\s+MO \.
SELECT next_day('2015-01-14'::DATE, ' MO '::string);

That asserts the padding is present without pinning its exact width. Anchor the right-hand end of the match on a non-whitespace character from Spark's message, such as the trailing period in Spark 4.x error templates, so the assertion still fails if the padding runs into the rest of the message. If even that cannot be expressed, fall back to the stable prefix and say in a comment why the assertion stops where it does.

Match Spark's message text exactly, including trailing punctuation. Spark 4.x error templates in error-conditions.json generally end in a period, and the pre-4.0 exception strings they replaced generally did not. Copying the older string leaves the implementation asserting a version the audit does not claim, and a query error prefix match will not catch it.

Before filing, search for an existing issue:

gh issue list --repo apache/datafusion --search "$ARGUMENTS <symptom> in:title,body" --state all --limit 5

If a candidate comes back, open it with gh issue view <N> --repo apache/datafusion and confirm it actually describes this divergence and is still open. A closed issue cited as a known divergence is worse than no citation, because the reader follows the link and finds a fix that already shipped.

If no issue exists, file one with the bug label plus spark. Title format: [Bug] <function> <one-line symptom>. The body includes the Spark version range affected, a minimal repro in both Spark SQL and DataFusion SQL, the divergent result, and the relevant file and line.

Capture cross-version divergences

A behavior difference between Spark versions cannot be tested today. Capture it in two places.

A comment at the query site naming the versions:

# Spark 3.5.8 returns NULL here; Spark 4.0.4 and later throw under ANSI mode.
# Expectation below asserts 4.2.0. Version-specific expectations are tracked by
# https://github.com/apache/datafusion/issues/23887

And its own filed issue, so the divergence is scheduled once the version mechanism lands.

Record the audit

Add a header comment block to the .slt file, directly below the license header and the existing provenance comment:

# Audited against Spark 4.2.0 on YYYY-MM-DD.
# Expected values verified with pyspark==4.2.0.
# Known divergences are captured as commented-out queries below.

Use today's real date. If PySpark was unavailable, say so on the second line instead of claiming verification.

Attribution

Every issue and PR the skill files ends with:

Surfaced by the audit-datafusion-spark-expression skill.

Do not mention Claude, AI, or automated generation anywhere in an issue or PR.

Before committing

Per the repository CLAUDE.md:

cargo fmt --all
cargo clippy --all-targets --all-features -- -D warnings
cargo test -p datafusion-spark --lib $ARGUMENTS
cargo test --test sqllogictests -- spark/<category>/$ARGUMENTS

The --lib run is not optional. cargo test --test sqllogictests does not compile or run the #[test] blocks inside datafusion/spark/src/, so a refactor that renames or changes the signature of a helper those tests call passes the SLT gate and fails CI.

Do not use cargo build -p datafusion-spark as a quick compile check. The crate does not currently build in isolation and the resulting error is unrelated to your change. Use cargo test -p datafusion-spark --lib or the sqllogictest target instead.

If the audit touched documentation:

./ci/scripts/doc_prettier_check.sh --write --allow-dirty

Follow .github/pull_request_template.md for the PR.

Output Format

Present the audit as:

Function summary. What $ARGUMENTS does, its input and output types, and its null behavior.
Spark version differences. Across 3.5.8, 4.0.4, 4.1.3, and 4.2.0. Say explicitly if there are none.
DataFusion implementation notes. Including signature, coercion, and nullability findings.
Gap matrix. From Step 6, plus implementation gaps.
Findings. Split into fixed in this PR, deferred with a filed issue, and low-risk coverage gaps.
Offer to add the low-risk coverage tests.

Low-risk coverage gaps are the only category that pauses for user input. Correctness findings are always either fixed or filed.

The line between the two is the code you touched. Coverage for a path this PR changed goes into the .slt file without asking: it is part of the fix. The offer is for cases on paths the PR did not touch and that already look correct, where adding tests is optional scope the user may not want. Phrase the offer as:

Spark exercises the following cases that have no DataFusion test, on code paths that appear correct. Would you like me to add them?

[list each case]

Tone and Style

Write in clear, concise prose.
Use backticks around function names, file paths, type names, and config keys.
Acknowledge what is already well covered before raising gaps.
Avoid em dashes and semicolons. Use separate sentences.
Do not overstate confidence. An unverified expectation is labeled as one.