docs: add scalar expression optimization guide, skill, and performance audits - #4933
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andygrove
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July 15, 2026 13:51
…e audits Add a contributor guide page documenting how to optimize native scalar expressions in datafusion-comet-spark-expr: benchmark-first workflow, criterion benchmark shapes, a catalog of proven techniques, correctness rules, and the no-regression gate. Add a matching optimize-comet-expression skill that points at the guide as the shared source of truth. Extend the per-expression audit pages to record performance audits alongside correctness audits, and backfill entries for the recently tuned cast, to_json, parse_url, size, and unhex expressions.
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…d noise-vs-regression lessons
comphead
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Thanks @andygrove lets give it a try
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Which issue does this PR close?
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Rationale for this change
Comet has accumulated a campaign of scalar expression performance optimizations (for example
spark_cast_int_to_int,to_json,parse_url,spark_size,spark_unhex, and many more in flight). The recurring methodology and pitfalls have lived only in individual PR descriptions. This PR captures that knowledge in one place so both human contributors and automated agents follow the same approach, and so we can track which expressions have already been tuned.What changes are included in this PR?
Optimizing Scalar Expressions(added to the Debugging and Performance section) covering the benchmark-first workflow, criterion benchmark shapes that expose regressions, a catalog of proven techniques mapped to the PRs that landed them, correctness rules (bit-identical output, null-slot error handling, eval-mode semantics), and the no-regression gate.optimize-comet-expressionskill that points at the guide as the shared source of truth and adds the agent execution loop and submission gate.Performance (tuned ...)line naming the technique, speedup, PR, and benchmark file.cast(int-to-int),to_json,parse_url,size, andunhexoptimizations.How are these changes tested?
Documentation and skill content only; no code changes. Verified the new page is wired into the contributor guide toctree and that the audit entries land under the correct expression headings.