advitrocks9 commented on code in PR #50970:
URL: https://github.com/apache/arrow/pull/50970#discussion_r3849636367
##########
cpp/src/arrow/type.cc:
##########
@@ -1333,8 +1333,10 @@ namespace {
std::unordered_multimap<std::string_view, int> CreateNameToIndexMap(
const FieldVector& fields) {
std::unordered_multimap<std::string_view, int> name_to_index;
+ name_to_index.reserve(fields.size());
for (size_t i = 0; i < fields.size(); ++i) {
- name_to_index.emplace(fields[i]->name(), static_cast<int>(i));
+ const std::string_view name = fields[i]->name();
+ name_to_index.emplace_hint(name_to_index.find(name), name,
static_cast<int>(i));
Review Comment:
Done in 4cacc16.
##########
cpp/src/arrow/type.cc:
##########
@@ -2294,7 +2296,7 @@ Schema::Schema(FieldVector fields, std::shared_ptr<const
KeyValueMetadata> metad
impl_(new Impl(std::move(fields), Endianness::Native,
std::move(metadata))) {}
Schema::Schema(const Schema& schema)
- : detail::Fingerprintable(), impl_(new Impl(*schema.impl_)) {}
+ : Schema(schema.impl_->fields_, schema.impl_->endianness_,
schema.impl_->metadata_) {}
Review Comment:
Copying the Impl copies the multimap, and libc++'s multimap copy constructor
re-inserts each element without a hint, so it has the same quadratic problem.
See
https://github.com/llvm/llvm-project/blob/release/21.x/libcxx/include/unordered_map#L2319-L2324.
Rebuilding through CreateNameToIndexMap uses the hinted path.
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