GH-48977: [C++] Fix quadratic field name index construction on libc++ - #50970
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Pull request overview
Optimizes schema field-name index construction on libc++ to avoid quadratic behavior with duplicate names.
Changes:
- Uses reserved, hinted multimap insertion.
- Optimizes schema copying and
SchemaBuilder. - Adds distinct- and duplicate-name benchmarks.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.
| File | Description |
|---|---|
cpp/src/arrow/type.cc |
Optimizes name-index construction and schema copying. |
cpp/src/arrow/type_benchmark.cc |
Adds schema construction benchmarks. |
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pitrou
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Aug 24, 2026
pitrou
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Aug 25, 2026
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@pitrou good to merge? I believe the 2 failures are unrelated known issues |
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You're right, those are unrelated. Thank you @advitrocks9 ! |
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Rationale for this change
Building a schema with 500,000 identically named fields takes about 300 seconds.
CreateNameToIndexMapfills anunordered_multimapwith one bareemplaceper field. libc++ keeps equal keys contiguous, so each unhinted insert walks to the end of the equal range before splicing, making the loop quadratic. libstdc++ splices next to the first match and is already linear.What changes are included in this PR?
emplace_hint(find(name), ...), the shape arvidjonasson worked out on the issue from the LLVM bug kou linked.reserve(fields.size())goes in alongside to pay for the extrafind.SchemaBuilder::Impl::AppendFieldhad the same pattern, andSchema's copy constructor now rebuilds fromfields_instead of copying the multimap node by node.Are these changes tested?
arrow-type-testpasses 157/157. Nothing observable changes, so no new unit test.type_benchmark.ccgains a schema construction case:duplicate_names/10000goes from 117 ms to 0.41 ms,distinct_namesis level to 7% faster.Are there any user-facing changes?
Fields sharing a name come back from
GetAllFieldsByNamein a different order. That order isn't a contract:GetAllFieldIndicessorts, both pyarrow callers reject more than one field, and the two standard libraries already disagree.arrow::schemaconstruction performance degrades to O(n^2) on libc++ for duplicate/unnamed fields #48977