Docs: Add PR review guide

.ai/skills/pr_review/SKILL.md

@@ -0,0 +1,35 @@
+---
+name: pr_review
+description: Review Apache DataFusion pull requests following the project's PR review guide. Use whenever asked to review a DataFusion PR or PR URL, and whenever creating a PR, to check the changes against the same criteria before submitting.
+---
+
+# DataFusion PR Review
+
+This skill describes the mechanics for doing PR reviews from the command line.
+
+When creating a PR, skip the "Collect PR context" step and instead check the
+changes against each area of the
+[PR review guide](../../../docs/source/contributor-guide/pr_review.md) before
+submitting.
+
+## Collect PR context
+
+- Check out the PR locally: `gh pr checkout <PR number>` (ask first if the
+  working tree has other work in progress).
+- Fetch the PR description, comments, and reviews:
+  `gh pr view <PR number> --json title,body,comments,reviews`
+- Fetch CI status: `gh pr checks <PR number>`.
+
+## Compute the diff
+
+```bash
+# find the remote that points at apache/datafusion (e.g. `apache`, `upstream`, or `origin`)
+UPSTREAM=$(git remote -v | grep -m1 'apache/datafusion' | cut -f1)
+git fetch $UPSTREAM main
+git diff $(git merge-base HEAD $UPSTREAM/main)
+```
+
+## Review checklist
+
+Work through each area from the
+[PR review guide](../../../docs/source/contributor-guide/pr_review.md).

AGENTS.md

@@ -5,6 +5,7 @@
 - [Quick Start Setup](docs/source/contributor-guide/development_environment.md#quick-start)
 - [Testing Quick Start](docs/source/contributor-guide/testing.md#testing-quick-start)
 - [Before Submitting a PR](docs/source/contributor-guide/index.md#before-submitting-a-pr)
+- [Reviewing Pull Requests](docs/source/contributor-guide/pr_review.md)
 - [Contributor Guide](docs/source/contributor-guide/index.md)
 - [Architecture Guide](docs/source/contributor-guide/architecture.md)
 

docs/source/contributor-guide/index.md

@@ -101,6 +101,11 @@ If you are concerned that a larger design will be lost in a string of small PRs,
 
 Note all commits in a PR are squashed when merged to the `main` branch so there is one commit per PR after merge.
 
+For larger PRs, it is often helpful to leave a review on your own PR with
+comments calling out important changes or specific important choices. These
+annotations can help reviewers quickly find areas they should focus on, thus
+speeding up review.
+
 ## Release Management and Backports
 
 Contributor-facing guidance for release branches, patch releases, and backports
@@ -135,22 +140,8 @@ do take priority over the conventional commit approach, allowing maintainers to
 
 ## Reviewing Pull Requests
 
-Some helpful links:
-
-- [PRs Waiting for Review] on GitHub
-- [Approved PRs Waiting for Merge] on GitHub
-
-[prs waiting for review]: https://github.com/apache/datafusion/pulls?q=is%3Apr+is%3Aopen+-review%3Aapproved+-is%3Adraft+
-[approved prs waiting for merge]: https://github.com/apache/datafusion/pulls?q=is%3Apr+is%3Aopen+review%3Aapproved+-is%3Adraft
-
-When reviewing PRs, our primary goal is to improve DataFusion and its community together. PR feedback should be constructive with the aim to help improve the code as well as the understanding of the contributor.
-
-Please ensure any issues you raise contains a rationale and suggested alternative -- it is frustrating to be told "don't do it this way" without any clear reason or alternate provided.
-
-Some things to specifically check:
-
-1. Is the feature or fix covered sufficiently with tests (see the [Testing](testing.md) section)?
-2. Is the code clear, and fits the style of the existing codebase?
+See the [Reviewing Pull Requests](pr_review.md) guide for what we look for
+when reviewing PRs and how to prepare your own for review.
 
 ## Performance Improvements
 

docs/source/contributor-guide/pr_review.md

@@ -0,0 +1,234 @@
+<!---
+  Licensed to the Apache Software Foundation (ASF) under one
+  or more contributor license agreements.  See the NOTICE file
+  distributed with this work for additional information
+  regarding copyright ownership.  The ASF licenses this file
+  to you under the Apache License, Version 2.0 (the
+  "License"); you may not use this file except in compliance
+  with the License.  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing,
+  software distributed under the License is distributed on an
+  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+  KIND, either express or implied.  See the License for the
+  specific language governing permissions and limitations
+  under the License.
+-->
+
+# Reviewing Pull Requests
+
+When reviewing PRs, our primary goal is to improve DataFusion and its community
+together. PR feedback should be constructive and help improve the code as well
+as the understanding of the contributor.
+
+Review bandwidth is currently our most limited resource, and reviews from the
+broader community are both welcomed and encouraged. Reviewing PRs is a great way
+to learn the codebase, and you do not need to be a committer to leave valuable
+review feedback. In fact, one of the best ways to become a committer is to
+thoughtfully review other PRs.
+
+Please ensure any comments you leave contain a rationale and suggested
+alternative -- it is frustrating to be told "don't do it this way" without any
+clear reason or alternative provided.
+
+The criteria in this guide are also a useful checklist when preparing your own
+PR for review.
+
+## PR Review Mechanics
+
+Some helpful links:
+
+- [PRs Waiting for Review] on GitHub
+- [Approved PRs Waiting for Merge] on GitHub
+
+[prs waiting for review]: https://github.com/apache/datafusion/pulls?q=is%3Apr+is%3Aopen+-review%3Aapproved+-is%3Adraft+
+[approved prs waiting for merge]: https://github.com/apache/datafusion/pulls?q=is%3Apr+is%3Aopen+review%3Aapproved+-is%3Adraft
+
+The overall PR lifecycle (CI triggering, approval, the 24-hour rule for
+"major" PRs, and merging) is described in the
+[Pull Request Overview](index.md#pull-request-overview) section of the
+contributor guide.
+
+Practical tips:
+
+1. Check out the changes locally to explore them in your IDE or with an
+   agent, e.g. `gh pr checkout <PR number>` using the [GitHub CLI].
+2. There is normally no need to rerun tests locally that CI has already run.
+3. Leave comments on specific lines of the diff where possible, so the
+   discussion has context.
+4. If you review a PR but don't feel confident approving it, leaving comments
+   is still valuable: a partial review (e.g. "I reviewed the tests and they
+   look good") helps the next reviewer focus their time.
+5. Anything that does not need to block the current PR can be noted as a
+   potential follow-up (ideally by filing an issue), keeping the PR focused
+   and quick to merge.
+
+[github cli]: https://cli.github.com/
+
+## Review the PR Description
+
+The PR description is often what users and contributors will find when they have
+a question about the intention behind a change, or when the code itself is not
+clear. The PR description also becomes the extended commit message.
+
+Check that the description:
+
+1. Concisely describes the **problem being solved from the user's point of
+   view**.
+
+2. Follows the [PR template], and answers the template's questions.
+
+3. Accurately describes what the PR actually does.
+
+4. Explicitly calls out any user-facing or API changes (see
+   [Review the Code](#review-the-code) below).
+
+[pr template]: https://github.com/apache/datafusion/blob/main/.github/pull_request_template.md
+
+## Review the Code Comments
+
+The goal of code comments is to help future readers of the code understand what
+is not obvious from reading the code itself. Great comments make the code easier
+to reason about for readers with the expected background, and help future
+maintainers.
+
+Some practical guidelines for reviewing comments:
+
+1. The code has adequate comments, focused on the **rationale** for any
+   non-obvious change (the "why"), not a restatement of what the code does
+   (the "what"), which is typically clear from reading the code itself.
+2. Comments do not narrate irrelevant internal implementation details or the
+   history of how the change was developed (this is common in LLM-assisted
+   code, e.g. "// changed to use a HashMap" or "// this handles the case
+   mentioned above"). Such comments become irrelevant as soon as the PR merges.
+3. When comments refer to other structs, functions, or modules, they should use
+   [rustdoc intra-doc links] (e.g. `` [`SessionContext`] ``) rather than plain
+   text names, so that `cargo doc` link checking ensures the references stay
+   valid as the code evolves.
+4. New public APIs have doc comments, including examples where appropriate
+   (doc examples are also tested by CI, so they double as test coverage).
+5. When documenting modules, functions, or fields, start with simple examples
+   and intuitive explanations, and optionally add formal, math-like
+   definitions when necessary. This makes the implementation easier to reason
+   about.
+6. When something is confusing on first read, treat that as a good
+   opportunity to improve the comments.
+
+[rustdoc intra-doc links]: https://doc.rust-lang.org/rustdoc/write-documentation/linking-to-items-by-name.html
+
+## Review the Test Coverage
+
+Check that the feature or fix is covered sufficiently with tests (see the
+[Testing](testing.md) guide for more details): the PR should include tests for
+any new functionality, and a bug fix should include a test that reproduces the
+reported problem.
+
+Guidelines for evaluating tests:
+
+1. Prefer `sqllogictest` (`.slt`) tests or DataFrame API tests where
+   possible, as they exercise **user visible behavior** and are less coupled
+   to internal implementation details than unit tests.
+2. Verify tests cover edge cases and common failure scenarios, not just the
+   common successful path. However, it is NOT necessary to test every possible
+   error path, especially if it is difficult to trigger or unlikely to occur in
+   practice.
+3. Verify test coverage of changed code using the `codecov` check on the PR,
+   or by running [`cargo llvm-cov`] locally for an HTML report. Use judgement
+   about any uncovered lines -- the goal is confidence in the change, not
+   slavishly hitting some coverage number.
+4. Avoid tests with lots of repeated boilerplate: when many tests share
+   near-identical setup, it is hard to understand what is different
+   (and thus what is actually being tested) between them. Make the _difference_
+   between cases obvious.
+5. Check that tests assert on specific expected values or plans (e.g. via
+   `insta` snapshots or `.slt` expected output) rather than merely checking
+   "no error occurred".
+
+[`cargo llvm-cov`]: https://github.com/taiki-e/cargo-llvm-cov
+
+## Review the Code
+
+Check that:
+
+1. The code is clear and fits the style of the existing codebase.
+2. New APIs are consistent with existing public APIs and patterns; where a
+   similar mechanism already exists, the PR should extend it rather than
+   introduce a parallel one.
+3. Any changes to the public API follow the [API health policy].
+4. The change is appropriately scoped: unrelated refactoring, formatting
+   churn, or drive-by changes make review longer and are better as separate
+   PRs.
+5. New errors are actionable, mention the offending item, and use
+   the right error variant (e.g. `plan_err!` for user-triggerable errors vs
+   `internal_err!` for invariant violations).
+
+[api health policy]: api-health.md
+
+## Review the Performance
+
+Performance is a key DataFusion feature. See [Performance Improvements](index.md#performance-improvements)
+for the project policy: an improvement should be "enough" to justify any
+added code complexity, and performance PRs should come with benchmark
+results.
+
+When reviewing:
+
+1. Find any relevant existing benchmarks and run them against `main`:
+   the [system level SQL benchmarks] are run with `bench.sh` (see the
+   [benchmarks README]), and microbenchmarks (e.g. in
+   `datafusion/functions/benches`) are run with `cargo bench`.
+2. Be aware that benchmarking on a machine where other
+   work is being done will make results hard to reproduce. Prefer a quiet,
+   dedicated machine and repeated runs.
+3. If the PR claims a performance improvement, check that the reported
+   results are reproducible and that the benchmark exercises the changed
+   code path.
+
+[system level sql benchmarks]: https://github.com/apache/datafusion/tree/main/benchmarks
+[benchmarks readme]: https://github.com/apache/datafusion/blob/main/benchmarks/README.md
+
+## Best Practices for Reviewers
+
+Here are some suggested best practices to follow when reviewing PRs.
+
+### Review Tone: Thank Contributors and Praise Good Work Specifically
+
+Open reviews by thanking the author by name, and when a PR is well done, say
+specifically what makes it good -- positive feedback encourages people to keep
+contributing and helps them understand what is valued in the project.
+
+### State Approval Conditions Explicitly
+
+If you are not ready to approve, list concretely what you would need to see
+before approving (e.g. "benchmark results and an upgrade guide entry") so
+the author has a clear path to merge.
+
+### Defer Non-Blocking Work to Follow-On Issues
+
+Explicitly defer non-critical suggestions to a follow-on PR and file
+(or ask the author to file) issues for them, so good PRs merge quickly
+without scope creep.
+
+Similarly, when a PR mixes refactoring with behavior changes or fixes a narrow
+problem with a broad mechanism, ask for it to be split or scoped down rather
+than reviewing it as-is.
+
+### Verify Tests Actually Pin the Bug ("Ablation Testing")
+
+For bug fixes, revert the fix locally and check that the new test fails
+without it -- several tests have been found during review that pass even
+with the fix reverted.
+
+### Narrate What You Verified When Approving
+
+Rather than a bare "LGTM", say what you actually checked ("traced the state
+transitions by hand", "confirmed the hasher change cannot affect ordering")
+so it is clear what was verified and what was not.
+
+### Invite Additional Committers on Core Changes
+
+For changes to core, widely shared code, leave the PR open for other
+committers to look at and cc those who know the area, even after you have
+approved.

docs/source/index.rst

@@ -158,6 +158,7 @@ To get started, see
    :caption: Contributor Guide
 
    contributor-guide/index
+   contributor-guide/pr_review
    contributor-guide/communication
    contributor-guide/development_environment
    contributor-guide/architecture