Method getColumnSample

src/Storages/MergeTree/IMergeTreeDataPart.cpp:3077–3122

}

ColumnPtr IMergeTreeDataPart::getColumnSample(const NameAndTypePair & column) const
{
    const size_t total_mark = getMarksCount();
    /// If column doesn't have dynamic subcolumns or part has no data, just create column using it's type.
    if (!column.type->hasDynamicSubcolumns() || !total_mark)
        return column.type->createColumn();

    /// Otherwise, read sample column with 0 rows from the part, so it will load dynamic structure.
    NamesAndTypesList cols;
    cols.emplace_back(column);

    const auto metadata_ptr = storage.getInMemoryMetadataPtr(storage.getContext(), false);
    StorageSnapshotPtr storage_snapshot_ptr = std::make_shared<StorageSnapshot>(storage, metadata_ptr);

    /// We need to read only prefixes, so no data will be read.
    /// Use read settings without prefetches/filesystem cache/etc.
    MergeTreeReaderSettings settings = MergeTreeReaderSettings::createFromSettings(getReadSettingsForMetadata());
    settings.can_read_part_without_marks = true;
    /// Use prefixes deserialization thread pool to read prefixes faster.
    /// In JSON type there might be hundreds of small files that needs to be read.
    /// Set this to value from storage settings rather than true by default as this could cause starvation on huge JSON.
    settings.use_prefixes_deserialization_thread_pool = storage.getContext()->getSettingsRef()[Setting::merge_tree_use_prefixes_deserialization_thread_pool];
    settings.read_only_column_sample = true;

    auto alter_conversions = std::make_shared<AlterConversions>();
    auto part_info = std::make_shared<LoadedMergeTreeDataPartInfoForReader>(shared_from_this(), alter_conversions);

    MergeTreeReaderPtr reader = createMergeTreeReader(
        part_info,
        cols,
        storage_snapshot_ptr,
        storage.getSettings(),
        MarkRanges{MarkRange(0, total_mark)},
        /*virtual_fields=*/ {},
        /*uncompressed_cache=*/{},
        storage.getContext()->getMarkCache().get(),
        nullptr,
        settings,
        ValueSizeMap{},
        ReadBufferFromFileBase::ProfileCallback{});

    Columns result;
    result.resize(1);
    reader->readRows(0, total_mark, false, 0, 0, result);
    return result[0];
}

bool isCompactPart(const MergeTreeDataPartPtr & data_part)
{