be/src/olap/parallel_scanner_builder.cpp

// Licensed to the Apache Software Foundation (ASF) under one
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// distributed with this work for additional information
// regarding copyright ownership.  The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License.  You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied.  See the License for the
// specific language governing permissions and limitations
// under the License.

#include "parallel_scanner_builder.h"

#include <cstddef>

#include "cloud/cloud_storage_engine.h"
#include "cloud/cloud_tablet_hotspot.h"
#include "cloud/config.h"
#include "common/status.h"
#include "olap/base_tablet.h"
#include "olap/rowset/beta_rowset.h"
#include "olap/segment_loader.h"
#include "pipeline/exec/olap_scan_operator.h"
#include "vec/exec/scan/olap_scanner.h"

namespace doris {

using namespace vectorized;

Status ParallelScannerBuilder::build_scanners(std::list<ScannerSPtr>& scanners) {
    RETURN_IF_ERROR(_load());
    if (_scan_parallelism_by_segment) {
        return _build_scanners_by_segment(scanners);
    } else if (_is_dup_mow_key) {
        // Default strategy for DUP/MOW tables: split by rowids within segments
        return _build_scanners_by_rowid(scanners);
    } else {
        // TODO: support to split by key range
        return Status::NotSupported("split by key range not supported yet.");
    }
}

Status ParallelScannerBuilder::_build_scanners_by_rowid(std::list<ScannerSPtr>& scanners) {
    DCHECK_GE(_rows_per_scanner, _min_rows_per_scanner);

    for (auto&& [tablet, version] : _tablets) {
        DCHECK(_all_read_sources.contains(tablet->tablet_id()));
        auto& entire_read_source = _all_read_sources[tablet->tablet_id()];

        if (config::is_cloud_mode()) {
            // FIXME(plat1ko): Avoid pointer cast
            ExecEnv::GetInstance()->storage_engine().to_cloud().tablet_hotspot().count(*tablet);
        }

        // `rs_splits` in `entire read source` will be devided into several partitial read sources
        // to build several parallel scanners, based on segment rows number. All the partitial read sources
        // share the same delete predicates from their corresponding entire read source.
        TabletReadSource partitial_read_source;
        int64_t rows_collected = 0;
        for (auto& rs_split : entire_read_source.rs_splits) {
            auto reader = rs_split.rs_reader;
            auto rowset = reader->rowset();
            const auto rowset_id = rowset->rowset_id();

            const auto& segments_rows = _all_segments_rows[rowset_id];

            if (rowset->num_rows() == 0) {
                continue;
            }

            int64_t segment_start = 0;
            auto split = RowSetSplits(reader->clone());

            for (size_t i = 0; i != segments_rows.size(); ++i) {
                const size_t rows_of_segment = segments_rows[i];
                RowRanges row_ranges;
                int64_t offset_in_segment = 0;

                // try to split large segments into RowRanges
                while (offset_in_segment < rows_of_segment) {
                    const int64_t remaining_rows = rows_of_segment - offset_in_segment;
                    auto rows_need = _rows_per_scanner - rows_collected;

                    // 0.9: try to avoid splitting the segments into excessively small parts.
                    if (rows_need >= remaining_rows * 9 / 10) {
                        rows_need = remaining_rows;
                    }
                    DCHECK_LE(rows_need, remaining_rows);

                    // RowRange stands for range: [From, To), From is inclusive, To is exclusive.
                    row_ranges.add({offset_in_segment,
                                    offset_in_segment + static_cast<int64_t>(rows_need)});
                    rows_collected += rows_need;
                    offset_in_segment += rows_need;

                    // If collected enough rows, build a new scanner
                    if (rows_collected >= _rows_per_scanner) {
                        split.segment_offsets.first = segment_start,
                        split.segment_offsets.second = i + 1;
                        split.segment_row_ranges.emplace_back(std::move(row_ranges));

                        DCHECK_EQ(split.segment_offsets.second - split.segment_offsets.first,
                                  split.segment_row_ranges.size());

                        partitial_read_source.rs_splits.emplace_back(std::move(split));

                        scanners.emplace_back(_build_scanner(
                                tablet, version, _key_ranges,
                                {.rs_splits = std::move(partitial_read_source.rs_splits),
                                 .delete_predicates = entire_read_source.delete_predicates,
                                 .delete_bitmap = entire_read_source.delete_bitmap}));

                        partitial_read_source = {};
                        split = RowSetSplits(reader->clone());
                        row_ranges = RowRanges();

                        segment_start = offset_in_segment < rows_of_segment ? i : i + 1;
                        rows_collected = 0;
                    }
                }

                // The non-empty `row_ranges` means there are some rows left in this segment not added into `split`.
                if (!row_ranges.is_empty()) {
                    DCHECK_GT(rows_collected, 0);
                    DCHECK_EQ(row_ranges.to(), rows_of_segment);
                    split.segment_row_ranges.emplace_back(std::move(row_ranges));
                }
            }

            DCHECK_LE(rows_collected, _rows_per_scanner);
            if (rows_collected > 0) {
                split.segment_offsets.first = segment_start;
                split.segment_offsets.second = segments_rows.size();
                DCHECK_GT(split.segment_offsets.second, split.segment_offsets.first);
                DCHECK_EQ(split.segment_row_ranges.size(),
                          split.segment_offsets.second - split.segment_offsets.first);
                partitial_read_source.rs_splits.emplace_back(std::move(split));
            }
        } // end `for (auto& rowset : rowsets)`

        DCHECK_LE(rows_collected, _rows_per_scanner);
        if (rows_collected > 0) {
            DCHECK_GT(partitial_read_source.rs_splits.size(), 0);
#ifndef NDEBUG
            for (auto& split : partitial_read_source.rs_splits) {
                DCHECK(split.rs_reader != nullptr);
                DCHECK_LT(split.segment_offsets.first, split.segment_offsets.second);
                DCHECK_EQ(split.segment_row_ranges.size(),
                          split.segment_offsets.second - split.segment_offsets.first);
            }
#endif
            scanners.emplace_back(
                    _build_scanner(tablet, version, _key_ranges,
                                   {.rs_splits = std::move(partitial_read_source.rs_splits),
                                    .delete_predicates = entire_read_source.delete_predicates,
                                    .delete_bitmap = entire_read_source.delete_bitmap}));
        }
    }

    return Status::OK();
}

// Build scanners so that each segment is exclusively scanned by a single scanner.
// This guarantees the number of scanners equals the number of segments across all rowsets
// for the involved tablets. It preserves delete predicates and key ranges, and clones
// RowsetReader per scanner to avoid sharing between scanners.
Status ParallelScannerBuilder::_build_scanners_by_segment(std::list<ScannerSPtr>& scanners) {
    DCHECK_GE(_rows_per_scanner, _min_rows_per_scanner);

    for (auto&& [tablet, version] : _tablets) {
        DCHECK(_all_read_sources.contains(tablet->tablet_id()));
        auto& entire_read_source = _all_read_sources[tablet->tablet_id()];

        if (config::is_cloud_mode()) {
            // FIXME(plat1ko): Avoid pointer cast
            ExecEnv::GetInstance()->storage_engine().to_cloud().tablet_hotspot().count(*tablet);
        }

        // Collect segments into scanners based on rows count instead of one scanner per segment
        TabletReadSource partitial_read_source;
        int64_t rows_collected = 0;

        for (auto& rs_split : entire_read_source.rs_splits) {
            auto reader = rs_split.rs_reader;
            auto rowset = reader->rowset();
            const auto rowset_id = rowset->rowset_id();

            const auto& segments_rows = _all_segments_rows[rowset_id];
            if (segments_rows.empty() || rowset->num_rows() == 0) {
                continue;
            }

            int64_t segment_start = 0;
            auto split = RowSetSplits(reader->clone());

            for (size_t i = 0; i < segments_rows.size(); ++i) {
                const size_t rows_of_segment = segments_rows[i];

                // Check if adding this segment would exceed rows_per_scanner
                // 0.9: try to avoid splitting the segments into excessively small parts.
                if (rows_collected > 0 && (rows_collected + rows_of_segment > _rows_per_scanner &&
                                           rows_collected < _rows_per_scanner * 9 / 10)) {
                    // Create a new scanner with collected segments
                    split.segment_offsets.first = segment_start;
                    split.segment_offsets.second =
                            i; // Range is [segment_start, i), including all segments from segment_start to i-1

                    DCHECK_GT(split.segment_offsets.second, split.segment_offsets.first);

                    partitial_read_source.rs_splits.emplace_back(std::move(split));

                    scanners.emplace_back(_build_scanner(
                            tablet, version, _key_ranges,
                            {.rs_splits = std::move(partitial_read_source.rs_splits),
                             .delete_predicates = entire_read_source.delete_predicates,
                             .delete_bitmap = entire_read_source.delete_bitmap}));

                    // Reset for next scanner
                    partitial_read_source = {};
                    split = RowSetSplits(reader->clone());
                    segment_start = i;
                    rows_collected = 0;
                }

                // Add current segment to the current scanner
                rows_collected += rows_of_segment;
            }

            // Add remaining segments in this rowset to a scanner
            if (rows_collected > 0) {
                split.segment_offsets.first = segment_start;
                split.segment_offsets.second = segments_rows.size();
                DCHECK_GT(split.segment_offsets.second, split.segment_offsets.first);
                partitial_read_source.rs_splits.emplace_back(std::move(split));
            }
        }

        // Add remaining segments across all rowsets to a scanner
        if (rows_collected > 0) {
            DCHECK_GT(partitial_read_source.rs_splits.size(), 0);
#ifndef NDEBUG
            for (auto& split : partitial_read_source.rs_splits) {
                DCHECK(split.rs_reader != nullptr);
                DCHECK_LT(split.segment_offsets.first, split.segment_offsets.second);
            }
#endif
            scanners.emplace_back(
                    _build_scanner(tablet, version, _key_ranges,
                                   {.rs_splits = std::move(partitial_read_source.rs_splits),
                                    .delete_predicates = entire_read_source.delete_predicates,
                                    .delete_bitmap = entire_read_source.delete_bitmap}));
        }
    }

    return Status::OK();
}

/**
 * Load rowsets of each tablet with specified version, segments of each rowset.
 */
Status ParallelScannerBuilder::_load() {
    _total_rows = 0;
    size_t idx = 0;
    for (auto&& [tablet, version] : _tablets) {
        const auto tablet_id = tablet->tablet_id();
        _all_read_sources[tablet_id] = _read_sources[idx];
        const auto& read_source = _all_read_sources[tablet_id];
        for (auto& rs_split : read_source.rs_splits) {
            auto rowset = rs_split.rs_reader->rowset();
            RETURN_IF_ERROR(rowset->load());
            const auto rowset_id = rowset->rowset_id();

            auto beta_rowset = std::dynamic_pointer_cast<BetaRowset>(rowset);
            std::vector<uint32_t> segment_rows;
            RETURN_IF_ERROR(beta_rowset->get_segment_num_rows(&segment_rows, &_builder_stats));
            auto segment_count = rowset->num_segments();
            for (int64_t i = 0; i != segment_count; i++) {
                _all_segments_rows[rowset_id].emplace_back(segment_rows[i]);
            }
            _total_rows += rowset->num_rows();
        }
        idx++;
    }

    _rows_per_scanner = _total_rows / _max_scanners_count;
    _rows_per_scanner = std::max<size_t>(_rows_per_scanner, _min_rows_per_scanner);

    return Status::OK();
}

std::shared_ptr<OlapScanner> ParallelScannerBuilder::_build_scanner(
        BaseTabletSPtr tablet, int64_t version, const std::vector<OlapScanRange*>& key_ranges,
        TabletReadSource&& read_source) {
    OlapScanner::Params params {_state,  _scanner_profile.get(), key_ranges, std::move(tablet),
                                version, std::move(read_source), _limit,     _is_preaggregation};
    return OlapScanner::create_shared(_parent, std::move(params));
}

} // namespace doris