westonpace commented on a change in pull request #11991:
URL: https://github.com/apache/arrow/pull/11991#discussion_r780473574
##########
File path: cpp/src/arrow/dataset/scanner.cc
##########
@@ -823,10 +584,77 @@ Result<int64_t> AsyncScanner::CountRows() {
return total.load();
}
+Result<std::shared_ptr<RecordBatchReader>> AsyncScanner::ToRecordBatchReader()
{
+ ARROW_ASSIGN_OR_RAISE(auto it, ScanBatches());
+ return
std::make_shared<ScannerRecordBatchReader>(options()->projected_schema,
+ std::move(it));
+}
+
const std::shared_ptr<Dataset>& AsyncScanner::dataset() const { return
dataset_; }
+Status NestedFieldRefsNotImplemented() {
+ // TODO(ARROW-11259) Several functions (for example, IpcScanTask::Make)
assume that
+ // only top level fields will be materialized.
+ return Status::NotImplemented("Nested field references in scans.");
+}
+
} // namespace
+Result<ProjectionDescr> ProjectionDescr::FromStructExpression(
+ const compute::Expression& projection, const Schema& dataset_schema) {
+ ARROW_ASSIGN_OR_RAISE(compute::Expression bound_expression,
+ projection.Bind(dataset_schema));
+
+ if (bound_expression.type()->id() != Type::STRUCT) {
+ return Status::Invalid("Projection ", projection.ToString(),
+ " cannot yield record batches");
+ }
+ std::shared_ptr<Schema> projection_schema =
+ ::arrow::schema(checked_cast<const
StructType&>(*bound_expression.type()).fields(),
+ dataset_schema.metadata());
+
+ return ProjectionDescr{std::move(bound_expression),
std::move(projection_schema)};
+}
+
+Result<ProjectionDescr> ProjectionDescr::FromExpressions(
+ const std::vector<compute::Expression>& exprs, std::vector<std::string>
names,
+ const Schema& dataset_schema) {
+ compute::MakeStructOptions project_options{std::move(names)};
+
+ for (size_t i = 0; i < exprs.size(); ++i) {
+ if (auto ref = exprs[i].field_ref()) {
+ if (!ref->name()) return NestedFieldRefsNotImplemented();
+
+ // set metadata and nullability for plain field references
+ ARROW_ASSIGN_OR_RAISE(auto field, ref->GetOne(dataset_schema));
+ project_options.field_nullability[i] = field->nullable();
+ project_options.field_metadata[i] = field->metadata();
+ }
+ }
+
+ return ProjectionDescr::FromStructExpression(
+ call("make_struct", std::move(exprs), std::move(project_options)),
dataset_schema);
Review comment:
Good catch. Fixed. All the callers were moving into it anyways.
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