Method initStreamIfNeeded

src/Storages/MergeTree/MergeTreeIndexReader.cpp:85–122

MergeTreeIndexReader::~MergeTreeIndexReader() = default;

void MergeTreeIndexReader::initStreamIfNeeded()
{
    if (!streams.empty())
        return;

    auto index_format = index->getDeserializedFormat(part->checksums, index->getFileName(), &part->getDataPartStorage());
    auto index_name = index->getFileName();
    auto last_mark = getLastMark(all_mark_ranges);

    for (const auto & substream : index_format.substreams)
    {
        auto full_stream_name = index_name + substream.suffix;
        auto stream_name_opt = DB::IMergeTreeDataPart::getStreamNameOrHash(full_stream_name, substream.extension, part->checksums);

        /// If the stream doesn't exist (neither original nor hashed name), use the full name
        /// and let it fail later when trying to open the file. This preserves the original error
        /// behavior and compatibility - the error message will indicate the missing file path.
        auto stream_name = stream_name_opt.value_or(full_stream_name);

        auto stream = makeIndexReaderStream(
            stream_name,
            substream.extension,
            part,
            marks_count,
            all_mark_ranges,
            mark_cache,
            uncompressed_cache,
            patchSettings(settings, substream.type));

        stream->adjustRightMark(last_mark);
        stream->seekToStart();

        streams[substream.type] = stream.get();
        stream_holders.emplace_back(std::move(stream));
    }

    version = index_format.version;
}

void MergeTreeIndexReader::read(size_t mark, const IMergeTreeIndexCondition * condition, MergeTreeIndexGranulePtr & granule)
{