perf: combine_hashes for primitive multi-column rehash#22350
Closed
Dandandan wants to merge 1 commit into
Closed
Conversation
Aligns hash_array_primitive's rehash path with the existing pattern used by hash_array (generic variable-width) and hash_dictionary_array: hash the current value once with the query RandomState and combine it with the previous row hash via combine_hashes, instead of constructing a fresh foldhash::SeedableRandomState per row to fold the seed in. Measured on ClickBench q32 (`GROUP BY WatchID, ClientIP ORDER BY COUNT(*) DESC LIMIT 10`, partitioned dataset, dfbench, current main baseline 3988.27 ms over 3 iterations): a 10-iteration same-machine run with this change settled around 1.7 s with a warm tail of 1.4-1.5 s. Validation: - cargo test -p datafusion-common hash_utils - cargo clippy -p datafusion-common --all-targets --all-features -- -D warnings
Contributor
Author
|
run benchmarks |
|
🤖 Benchmark running (GKE) | trigger CPU Details (lscpu)Comparing perf/primitive-rehash-multi-column (4712451) to c8b784a (merge-base) diff using: clickbench_partitioned File an issue against this benchmark runner |
|
🤖 Benchmark running (GKE) | trigger CPU Details (lscpu)Comparing perf/primitive-rehash-multi-column (4712451) to c8b784a (merge-base) diff using: tpcds File an issue against this benchmark runner |
|
🤖 Benchmark running (GKE) | trigger CPU Details (lscpu)Comparing perf/primitive-rehash-multi-column (4712451) to c8b784a (merge-base) diff using: tpch File an issue against this benchmark runner |
|
🤖 Benchmark completed (GKE) | trigger Instance: CPU Details (lscpu)Details
Resource Usagetpch — base (merge-base)
tpch — branch
File an issue against this benchmark runner |
|
🤖 Benchmark completed (GKE) | trigger Instance: CPU Details (lscpu)Details
Resource Usagetpcds — base (merge-base)
tpcds — branch
File an issue against this benchmark runner |
|
🤖 Benchmark completed (GKE) | trigger Instance: CPU Details (lscpu)Details
Resource Usageclickbench_partitioned — base (merge-base)
clickbench_partitioned — branch
File an issue against this benchmark runner |
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
2 participants
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
Which issue does this PR close?
Rationale for this change
When multiple group keys participate in a grouped aggregate,
create_hasheswalks the columns left-to-right and rehashes each row by combining the current value's hash with the previously accumulated row hash. The generic variable-width path (hash_array) and the dictionary path (hash_dictionary_array) already do this withcombine_hashes(value.hash_one(random_state), *hash). The primitive path (hash_array_primitive) instead constructs a freshfoldhash::fast::SeedableRandomStateper row, seeded from the previous hash, and runs the value through that. That approach is heavier on the hot loop and isn't necessary for correctness — the same hash distribution is achievable withcombine_hashes.The motivating workload is ClickBench q32 on the partitioned dataset:
The two group keys are both primitive (
Int64/Int32), and the partial aggregate spends most of its time in group-id calculation. On a current-main baseline of 3988.27 ms (dfbench clickbench --query 32 --iterations 3), a 10-iteration same-machine run with this change settled around 1.7 s, with a warm tail of 1.4-1.5 s per iteration.Guardrail single-query runs of other primitive multi-key aggregates (q30, q31, q35) and the q18 path that benefits from primitive rehashing as a second column did not regress.
What changes are included in this PR?
Replaces the two
seeded_state(...).build_hasher() / value.hash_write(...) / hasher.finish()blocks inhash_array_primitive's rehash branch (no-null and nullable variants) with the samecombine_hashes(value.hash_one(random_state), *hash)pattern already used by the surrounding generic and dictionary paths.seeded_stateitself is unchanged and still used by the byte-view hash paths.Doc comment on
hash_array_primitiveupdated to match the new behavior.Are these changes tested?
Covered by existing tests in
datafusion-common::hash_utils, including the multi-column hash tests:cargo test -p datafusion-common hash_utilscargo clippy -p datafusion-common --all-targets --all-features -- -D warningsAre there any user-facing changes?
No. This is an internal performance change with no API or behavioral impact — hash distribution properties are unchanged because
combine_hashesis already the standard pattern in this file.