emkornfield commented on a change in pull request #11351:
URL: https://github.com/apache/arrow/pull/11351#discussion_r724492778
##########
File path: cpp/src/parquet/arrow/reader.cc
##########
@@ -842,36 +842,79 @@ Status GetReader(const SchemaField& field, const
std::shared_ptr<Field>& arrow_f
*out = nullptr;
return Status::OK();
}
- if (type_id == ::arrow::Type::LIST ||
- type_id == ::arrow::Type::MAP) { // Map can be reconstructed as list
of structs.
- if (type_id == ::arrow::Type::MAP &&
- child_reader->field()->type()->num_fields() != 2) {
- // This case applies if either key or value is filtered.
+
+ // These two types might not be equal if there column pruning occurred.
+ // further down the stack.
+ const std::shared_ptr<DataType> reader_child_type =
child_reader->field()->type();
+ const DataType& schema_child_type =
*(list_field->type()->field(0)->type());
+ if (type_id == ::arrow::Type::MAP) {
+ if (reader_child_type->num_fields() != 2 ||
+ !reader_child_type->field(0)->type()->Equals(
+ *schema_child_type.field(0)->type())) {
+ // This case applies if either key or value are completed filtered
+ // out so we can take the type as is or the key was partially
+ // so keeping it as a map no longer makes sence.
list_field =
list_field->WithType(::arrow::list(child_reader->field()));
+ } else if (!reader_child_type->field(1)->type()->Equals(
+ *schema_child_type.field(1)->type())) {
+ list_field = list_field->WithType(std::make_shared<::arrow::MapType>(
+ reader_child_type->field(
+ 0), // field 0 is unchanged baed on previous if statement
+ reader_child_type->field(1)));
+ }
+ // Map types are list<struct<key, value>> so use ListReader
+ // for reconstruction.
+ out->reset(new ListReader<int32_t>(ctx, list_field, field.level_info,
+ std::move(child_reader)));
+ } else if (type_id == ::arrow::Type::LIST) {
+ if (!reader_child_type->Equals(schema_child_type)) {
+ list_field = list_field->WithType(::arrow::list(reader_child_type));
}
+
out->reset(new ListReader<int32_t>(ctx, list_field, field.level_info,
std::move(child_reader)));
} else if (type_id == ::arrow::Type::LARGE_LIST) {
+ if (!reader_child_type->Equals(schema_child_type)) {
+ list_field =
list_field->WithType(::arrow::large_list(reader_child_type));
+ }
+
out->reset(new ListReader<int64_t>(ctx, list_field, field.level_info,
std::move(child_reader)));
-
} else if (type_id == ::arrow::Type::FIXED_SIZE_LIST) {
+ if (!reader_child_type->Equals(schema_child_type)) {
+ auto& fixed_list_type =
+ checked_cast<const
::arrow::FixedSizeListType&>(*list_field->type());
+ int32_t list_size = fixed_list_type.list_size();
+ list_field =
+ list_field->WithType(::arrow::fixed_size_list(reader_child_type,
list_size));
+ }
+
out->reset(new FixedSizeListReader(ctx, list_field, field.level_info,
std::move(child_reader)));
} else {
return Status::UnknownError("Unknown list type: ",
field.field->ToString());
}
} else if (type_id == ::arrow::Type::STRUCT) {
std::vector<std::shared_ptr<Field>> child_fields;
+ int arrow_field_idx = 0;
std::vector<std::unique_ptr<ColumnReaderImpl>> child_readers;
for (const auto& child : field.children) {
std::unique_ptr<ColumnReaderImpl> child_reader;
RETURN_NOT_OK(GetReader(child, ctx, &child_reader));
if (!child_reader) {
+ arrow_field_idx++;
// If all children were pruned, then we do not try to read this field
continue;
}
- child_fields.push_back(child.field);
+ std::shared_ptr<::arrow::Field> child_field = child.field;
+ const DataType& reader_child_type = *child_reader->field()->type();
+ const DataType& schema_child_type =
+ *arrow_field->type()->field(arrow_field_idx++)->type();
+ // These might not be equal if column pruning occurred.
+ if (!schema_child_type.Equals(reader_child_type)) {
+ child_field = child_field->WithType(child_reader->field()->type());
+ }
+ child_fields.push_back(child_field);
Review comment:
Good question. I don't think so. Ultimately indices is translated into
an [unordered
set](https://github.com/apache/arrow/blob/master/cpp/src/parquet/arrow/reader.cc#L211)
and only used for filtering columns. The only place iteration happens is
[here](https://github.com/apache/arrow/blob/master/cpp/src/parquet/arrow/reader.cc#L867)
which should have a deterministic order that is already aligned with the
schema. I think the only type of rearranging columns that is supported is at
the top level of selection (not nested types) which this code doesn't really
touch.
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