fix(blob): support blob-only writes and reject nested blob fields

src/paimon/common/data/blob_utils.cpp

@@ -82,13 +82,12 @@ Result<BlobUtils::SeparatedStructArrays> BlobUtils::SeparateBlobArray(
         return Status::Invalid(
             "SeparateBlobArray expects at least one non-inline blob field, but got none.");
     }
-    if (main_fields.empty()) {
-        return Status::Invalid("SeparateBlobArray expects at least one main field, but got none.");
-    }
 
     SeparatedStructArrays result;
-    PAIMON_ASSIGN_OR_RAISE_FROM_ARROW(result.main_array,
-                                      arrow::StructArray::Make(main_arrays, main_fields));
+    if (!main_fields.empty()) {
+        PAIMON_ASSIGN_OR_RAISE_FROM_ARROW(result.main_array,
+                                          arrow::StructArray::Make(main_arrays, main_fields));
+    }
     PAIMON_ASSIGN_OR_RAISE_FROM_ARROW(result.blob_array,
                                       arrow::StructArray::Make(blob_arrays, blob_fields));
     return result;

src/paimon/common/data/blob_utils.h

@@ -51,7 +51,8 @@ class PAIMON_EXPORT BlobUtils {
     };
 
     struct SeparatedStructArrays {
-        /// Non-blob fields (includes inline blob fields when inline_fields is provided)
+        /// Non-blob fields (includes inline blob fields when inline_fields is provided).
+        /// nullptr when all fields are stored in blob files.
         std::shared_ptr<arrow::StructArray> main_array;
         /// Blob fields that go to separate .blob files
         std::shared_ptr<arrow::StructArray> blob_array;

src/paimon/common/data/blob_utils_test.cpp

@@ -182,11 +182,13 @@ TEST_F(BlobUtilsTest, SeparateBlobArray) {
         BlobUtils::SeparateBlobArray(struct_array, /*inline_fields=*/{"f2_blob"}),
         "SeparateBlobArray expects at least one non-inline blob field, but got none.");
 
-    // All fields are blob with no inline -> no main field -> should return error
+    // All fields are blob with no inline -> no main array is needed
     auto all_blob_struct = arrow::StructArray::Make({blob_array_data}, {blob_field}).ValueOrDie();
     auto all_blob_sa = std::dynamic_pointer_cast<arrow::StructArray>(all_blob_struct);
-    ASSERT_NOK_WITH_MSG(BlobUtils::SeparateBlobArray(all_blob_sa, /*inline_fields=*/{}),
-                        "SeparateBlobArray expects at least one main field, but got none.");
+    ASSERT_OK_AND_ASSIGN(auto all_blob_separated,
+                         BlobUtils::SeparateBlobArray(all_blob_sa, /*inline_fields=*/{}));
+    ASSERT_EQ(nullptr, all_blob_separated.main_array);
+    ASSERT_TRUE(all_blob_separated.blob_array->Equals(*all_blob_sa));
 }
 
 TEST_F(BlobUtilsTest, SeparateBlobArrayWithPartialInline) {

src/paimon/core/append/append_only_writer.cpp

@@ -238,10 +238,14 @@ AppendOnlyWriter::RollingFileWriterResult AppendOnlyWriter::CreateRollingBlobWri
             options_.GetBlobTargetFileSize(), single_blob_file_writer_factory);
     };
 
+    WriterFactory main_writer_factory;
+    if (schemas.main_schema->num_fields() > 0) {
+        main_writer_factory =
+            GetDataFileWriterFactory(schemas.main_schema, schemas.main_schema->field_names());
+    }
     return std::make_unique<RollingBlobFileWriter>(
-        options_.GetTargetFileSize(/*has_primary_key=*/false),
-        GetDataFileWriterFactory(schemas.main_schema, schemas.main_schema->field_names()),
-        blob_schema, blob_writer_creator, arrow::struct_(write_schema_->fields()), inline_fields);
+        options_.GetTargetFileSize(/*has_primary_key=*/false), main_writer_factory, blob_schema,
+        blob_writer_creator, arrow::struct_(write_schema_->fields()), inline_fields);
 }
 
 Status AppendOnlyWriter::Sync() {

src/paimon/core/append/append_only_writer_test.cpp

@@ -741,6 +741,53 @@ TEST_F(AppendOnlyWriterTest, TestWriteWithSingleBlobField) {
     ASSERT_OK(writer->Close());
 }
 
+TEST_F(AppendOnlyWriterTest, TestWriteWithOnlyBlobField) {
+    auto options =
+        CreateOptions({{Options::FILE_FORMAT, "orc"}, {Options::MANIFEST_FORMAT, "orc"}});
+    auto dir = UniqueTestDirectory::Create();
+    ASSERT_TRUE(dir);
+    auto path_factory = CreatePathFactory(dir->Str(), "orc", options);
+
+    auto blob_field = BlobUtils::ToArrowField("blob", false);
+    auto schema = arrow::schema({blob_field});
+    ASSERT_OK_AND_ASSIGN(auto writer,
+                         CreateAppendOnlyWriter(options, /*schema_id=*/0, schema,
+                                                /*write_cols=*/std::vector<std::string>{"blob"},
+                                                /*max_sequence_number=*/-1, path_factory,
+                                                compact_manager_, memory_pool_));
+
+    arrow::LargeBinaryBuilder blob_builder;
+    ASSERT_TRUE(blob_builder.Append("a", 1).ok());
+    ASSERT_TRUE(blob_builder.Append("bb", 2).ok());
+    auto blob_array = blob_builder.Finish().ValueOrDie();
+
+    ASSERT_OK(writer->Write(CreateStructBatch(schema, {blob_array})));
+    ASSERT_OK_AND_ASSIGN(CommitIncrement inc, writer->PrepareCommit(/*wait_compaction=*/true));
+    ASSERT_OK(writer->Close());
+
+    const auto& new_files = inc.GetNewFilesIncrement().NewFiles();
+    ASSERT_EQ(new_files.size(), 1);
+    ASSERT_TRUE(BlobUtils::IsBlobFile(new_files[0]->file_name));
+    ASSERT_EQ(new_files[0]->row_count, 2);
+    ASSERT_TRUE(new_files[0]->write_cols.has_value());
+    ASSERT_EQ(new_files[0]->write_cols.value(), std::vector<std::string>({"blob"}));
+    std::string blob_file_path = path_factory->ToPath(new_files[0]->file_name);
+    ASSERT_TRUE(options.GetFileSystem()->Exists(blob_file_path).value());
+
+    auto blob_reader = OpenFormatReader(blob_file_path, "blob");
+    ::ArrowSchema c_blob_schema;
+    ASSERT_TRUE(arrow::ExportSchema(*schema, &c_blob_schema).ok());
+    ASSERT_OK(blob_reader->SetReadSchema(&c_blob_schema, /*predicate=*/nullptr,
+                                         /*selection_bitmap=*/std::nullopt));
+    ASSERT_OK_AND_ASSIGN(auto actual_array, ReadResultCollector::CollectResult(blob_reader.get()));
+    auto expected_struct_array =
+        arrow::StructArray::Make({blob_array}, {blob_field->name()}).ValueOrDie();
+    auto expected_array = std::make_shared<arrow::ChunkedArray>(expected_struct_array);
+    ASSERT_TRUE(expected_array->Equals(actual_array)) << "Expected:\n"
+                                                      << expected_array->ToString() << "\nActual:\n"
+                                                      << actual_array->ToString();
+}
+
 TEST_F(AppendOnlyWriterTest, TestWriteWithMultipleBlobFields) {
     auto options =
         CreateOptions({{Options::FILE_FORMAT, "orc"}, {Options::MANIFEST_FORMAT, "orc"}});

src/paimon/core/io/rolling_blob_file_writer.cpp

@@ -16,6 +16,7 @@
 
 #include "paimon/core/io/rolling_blob_file_writer.h"
 
+#include <map>
 #include <memory>
 #include <string>
 #include <utility>
@@ -27,7 +28,6 @@
 #include "arrow/c/bridge.h"
 #include "arrow/c/helpers.h"
 #include "fmt/format.h"
-#include "fmt/ranges.h"
 #include "paimon/common/data/blob_utils.h"
 #include "paimon/common/utils/arrow/status_utils.h"
 #include "paimon/common/utils/scope_guard.h"
@@ -55,7 +55,7 @@ RollingBlobFileWriter::RollingBlobFileWriter(
 Status RollingBlobFileWriter::Write(::ArrowArray* record) {
     ScopeGuard guard([this]() -> void { this->Abort(); });
     // Open the current writer if write the first record or roll over happen before.
-    if (PAIMON_UNLIKELY(current_writer_ == nullptr)) {
+    if (writer_factory_ != nullptr && PAIMON_UNLIKELY(current_writer_ == nullptr)) {
         PAIMON_RETURN_NOT_OK(OpenCurrentWriter());
     }
     if (PAIMON_UNLIKELY(blob_writer_ == nullptr)) {
@@ -69,12 +69,14 @@ Status RollingBlobFileWriter::Write(::ArrowArray* record) {
     PAIMON_ASSIGN_OR_RAISE(BlobUtils::SeparatedStructArrays separated_arrays,
                            BlobUtils::SeparateBlobArray(struct_array, inline_fields_));
     // Write main (non-blob) data
-    ::ArrowArray c_main_array;
-    PAIMON_RETURN_NOT_OK_FROM_ARROW(
-        arrow::ExportArray(*separated_arrays.main_array, &c_main_array));
-    ScopeGuard array_lifecycle_guard(
-        [&c_main_array]() -> void { ArrowArrayRelease(&c_main_array); });
-    PAIMON_RETURN_NOT_OK(current_writer_->Write(&c_main_array));
+    if (current_writer_ != nullptr) {
+        ::ArrowArray c_main_array;
+        PAIMON_RETURN_NOT_OK_FROM_ARROW(
+            arrow::ExportArray(*separated_arrays.main_array, &c_main_array));
+        ScopeGuard array_lifecycle_guard(
+            [&c_main_array]() -> void { ArrowArrayRelease(&c_main_array); });
+        PAIMON_RETURN_NOT_OK(current_writer_->Write(&c_main_array));
+    }
 
     // Write blob data via MultipleBlobFileWriter (each blob field independently)
     ::ArrowArray c_blob_array;
@@ -84,28 +86,31 @@ Status RollingBlobFileWriter::Write(::ArrowArray* record) {
     PAIMON_RETURN_NOT_OK(blob_writer_->Write(&c_blob_array));
 
     record_count_ += record_count;
-    PAIMON_ASSIGN_OR_RAISE(bool need_rolling_file, NeedRollingFile());
-    if (need_rolling_file) {
-        PAIMON_RETURN_NOT_OK(CloseCurrentWriter());
+    if (current_writer_ != nullptr) {
+        PAIMON_ASSIGN_OR_RAISE(bool need_rolling_file, NeedRollingFile());
+        if (need_rolling_file) {
+            PAIMON_RETURN_NOT_OK(CloseCurrentWriter());
+        }
     }
     guard.Release();
     return Status::OK();
 }
 
 Status RollingBlobFileWriter::CloseCurrentWriter() {
-    if (current_writer_ == nullptr) {
-        return Status::OK();
-    }
     if (blob_writer_ == nullptr) {
         return Status::OK();
     }
-    PAIMON_ASSIGN_OR_RAISE(std::shared_ptr<DataFileMeta> main_data_file_meta, CloseMainWriter());
+    std::shared_ptr<DataFileMeta> main_data_file_meta;
+    if (current_writer_ != nullptr) {
+        PAIMON_ASSIGN_OR_RAISE(main_data_file_meta, CloseMainWriter());
+    }
     PAIMON_ASSIGN_OR_RAISE(std::vector<std::shared_ptr<DataFileMeta>> blob_metas,
                            CloseBlobWriter());
-    PAIMON_RETURN_NOT_OK(
-        ValidateFileConsistency(main_data_file_meta, blob_metas, blob_schema_->num_fields()));
 
-    results_.push_back(main_data_file_meta);
+    if (main_data_file_meta != nullptr) {
+        PAIMON_RETURN_NOT_OK(ValidateFileConsistency(main_data_file_meta, blob_metas));
+        results_.push_back(main_data_file_meta);
+    }
     results_.insert(results_.end(), blob_metas.begin(), blob_metas.end());
 
     current_writer_.reset();
@@ -137,29 +142,25 @@ Result<std::vector<std::shared_ptr<DataFileMeta>>> RollingBlobFileWriter::CloseB
 
 Status RollingBlobFileWriter::ValidateFileConsistency(
     const std::shared_ptr<DataFileMeta>& main_data_file_meta,
-    const std::vector<std::shared_ptr<DataFileMeta>>& blob_tagged_metas, int32_t blob_field_count) {
-    if (blob_tagged_metas.empty()) {
-        return Status::OK();
-    }
-    // With multiple blob fields, each blob field produces its own set of files.
-    // total_blob_row_count should be exactly main_row_count * blob_field_count.
-    int64_t main_row_count = main_data_file_meta->row_count;
-    int64_t expected_blob_row_count = main_row_count * blob_field_count;
-    int64_t total_blob_row_count = 0;
+    const std::vector<std::shared_ptr<DataFileMeta>>& blob_tagged_metas) {
+    std::map<std::string, int64_t> blob_field_row_counts;
     for (const auto& blob_tagged_meta : blob_tagged_metas) {
-        total_blob_row_count += blob_tagged_meta->row_count;
+        if (!blob_tagged_meta->write_cols || blob_tagged_meta->write_cols->empty()) {
+            return Status::Invalid(
+                fmt::format("This is a bug: Blob file {} must contain a write column.",
+                            blob_tagged_meta->file_name));
+        }
+        blob_field_row_counts[blob_tagged_meta->write_cols->at(0)] += blob_tagged_meta->row_count;
     }
-    if (total_blob_row_count != expected_blob_row_count) {
-        std::vector<std::string> blob_file_names;
-        for (const auto& blob_tagged_meta : blob_tagged_metas) {
-            blob_file_names.push_back(blob_tagged_meta->file_name);
+
+    int64_t main_row_count = main_data_file_meta->row_count;
+    for (const auto& [field_name, row_count] : blob_field_row_counts) {
+        if (row_count != main_row_count) {
+            return Status::Invalid(fmt::format(
+                "This is a bug: The row count of main file and blob file does not match. Main "
+                "file: {} (row count: {}), blob field name: {} (row count: {})",
+                main_data_file_meta->file_name, main_row_count, field_name, row_count));
         }
-        return Status::Invalid(fmt::format(
-            "This is a bug: The row count of main file and blob files does not match. "
-            "Main file: {} (row count: {}), blob field count: {}, "
-            "expected blob row count: {}, blob files: {} (actual total row count: {})",
-            main_data_file_meta->file_name, main_row_count, blob_field_count,
-            expected_blob_row_count, fmt::join(blob_file_names, ", "), total_blob_row_count));
     }
     return Status::OK();
 }

src/paimon/core/io/rolling_blob_file_writer.h

@@ -38,7 +38,8 @@ namespace paimon {
 /// between them.
 ///
 /// Multiple blob fields are supported. Each blob field is written to its own set of blob files
-/// independently via MultipleBlobFileWriter.
+/// independently via MultipleBlobFileWriter. For blob-only writes, the main writer factory may be
+/// nullptr and only blob files are produced.
 ///
 /// <pre>
 /// For example,
@@ -76,8 +77,7 @@ class RollingBlobFileWriter
  private:
     static Status ValidateFileConsistency(
         const std::shared_ptr<DataFileMeta>& main_data_file_meta,
-        const std::vector<std::shared_ptr<DataFileMeta>>& blob_tagged_metas,
-        int32_t blob_field_count);
+        const std::vector<std::shared_ptr<DataFileMeta>>& blob_tagged_metas);
 
     Status CloseCurrentWriter();
 

src/paimon/core/io/rolling_blob_file_writer_test.cpp

@@ -82,11 +82,15 @@ TEST_F(RollingBlobFileWriterTest, ValidateFileConsistency) {
         /*delete_row_count=*/0, /*embedded_index=*/nullptr, FileSource::Append(),
         /*value_stats_cols=*/std::nullopt, /*external_path=*/std::nullopt, /*first_row_id=*/3,
         /*write_cols=*/std::vector<std::string>({"blob"}));
-    ASSERT_OK(RollingBlobFileWriter::ValidateFileConsistency(file_meta1, {file_meta2, file_meta3},
-                                                             /*blob_field_count=*/1));
-    ASSERT_NOK_WITH_MSG(RollingBlobFileWriter::ValidateFileConsistency(file_meta1, {file_meta2},
-                                                                       /*blob_field_count=*/2),
-                        "This is a bug: The row count of main file and blob files does not match.");
+    ASSERT_OK(RollingBlobFileWriter::ValidateFileConsistency(file_meta1, {file_meta2, file_meta3}));
+    ASSERT_NOK_WITH_MSG(RollingBlobFileWriter::ValidateFileConsistency(file_meta1, {file_meta2}),
+                        "This is a bug: The row count of main file and blob file does not match.");
+
+    file_meta2->write_cols = std::vector<std::string>({"blob1"});
+    file_meta3->write_cols = std::vector<std::string>({"blob2"});
+    ASSERT_NOK_WITH_MSG(
+        RollingBlobFileWriter::ValidateFileConsistency(file_meta1, {file_meta2, file_meta3}),
+        "This is a bug: The row count of main file and blob file does not match.");
 }
 
 }  // namespace paimon::test