Merge d414dea9b9 into 44b4bd38b9

src/Storages/MergeTree/MergeList.h

@@ -57,6 +57,7 @@ struct MergeInfo
 
 struct FutureMergedMutatedPart;
 using FutureMergedMutatedPartPtr = std::shared_ptr<FutureMergedMutatedPart>;
+using FutureParts = std::vector<FutureMergedMutatedPartPtr>;
 
 struct MergeListElement;
 using MergeListEntry = BackgroundProcessListEntry<MergeListElement, MergeInfo>;

src/Storages/MergeTree/MergeSelectors/AllMergeSelector.cpp

@@ -12,7 +12,7 @@ void registerAllMergeSelector(MergeSelectorFactory & factory)
     });
 }
 
-AllMergeSelector::PartsRange AllMergeSelector::select(
+AllMergeSelector::PartsRange AllMergeSelector::selectBest(
     const PartsRanges & parts_ranges,
     size_t /*max_total_size_to_merge*/)
 {

src/Storages/MergeTree/MergeSelectors/AllMergeSelector.h

@@ -11,7 +11,7 @@ class AllMergeSelector : public IMergeSelector
 {
 public:
     /// Parameter max_total_size_to_merge is ignored.
-    PartsRange select(
+    PartsRange selectBest(
         const PartsRanges & parts_ranges,
         size_t max_total_size_to_merge) override;
 };

src/Storages/MergeTree/MergeSelectors/MergeSelector.h

@@ -69,10 +69,17 @@ public:
     /** Function could be called at any frequency and it must decide, should you do any merge at all.
       * If better not to do any merge, it returns empty result.
       */
-    virtual PartsRange select(
+    virtual PartsRange selectBest(
         const PartsRanges & parts_ranges,
         size_t max_total_size_to_merge) = 0;
 
+    virtual PartsRanges selectUpToTopN(
+        const PartsRanges & parts_ranges,
+        size_t max_total_size_to_merge, size_t)
+    {
+        return {selectBest(parts_ranges, max_total_size_to_merge)};
+    }
+
     virtual ~IMergeSelector() = default;
 };
 

src/Storages/MergeTree/MergeSelectors/SimpleMergeSelector.cpp

@@ -7,7 +7,6 @@
 
 #include <cmath>
 #include <cassert>
-#include <iostream>
 #include <random>
 
 
@@ -39,6 +38,10 @@ struct Estimator
 {
     using Iterator = SimpleMergeSelector::PartsRange::const_iterator;
 
+    explicit Estimator(size_t top_n_ranges_)
+        : top_n_ranges(top_n_ranges_)
+    {}
+
     void consider(Iterator begin, Iterator end, size_t sum_size, size_t size_prev_at_left, const SimpleMergeSelector::Settings & settings)
     {
         double current_score = score(end - begin, sum_size, settings.size_fixed_cost_to_add);
@@ -56,17 +59,36 @@ struct Estimator
             while (end >= begin + 3 && (end - 1)->size < settings.heuristic_to_remove_small_parts_at_right_max_ratio * sum_size)
                 --end;
 
-        if (min_score == 0.0 || current_score < min_score)
-        {
-            min_score = current_score;
-            best_begin = begin;
-            best_end = end;
-        }
+        while (best_ranges.size() > top_n_ranges)
+            best_ranges.pop();
+
+        best_ranges.push(ScoredRange{current_score, begin, end});
     }
 
     SimpleMergeSelector::PartsRange getBest() const
     {
-        return SimpleMergeSelector::PartsRange(best_begin, best_end);
+        if (!best_ranges.empty())
+        {
+            const auto & top = best_ranges.top();
+            return SimpleMergeSelector::PartsRange(top.begin, top.end);
+        }
+        return SimpleMergeSelector::PartsRange{};
+    }
+
+    SimpleMergeSelector::PartsRanges getUpToTopN() const
+    {
+        SimpleMergeSelector::PartsRanges result;
+        result.reserve(best_ranges.size());
+        while (!best_ranges.empty())
+        {
+            const auto & top_range = best_ranges.top();
+            result.push_back(SimpleMergeSelector::PartsRange(top_range.begin, top_range.end));
+            best_ranges.pop();
+        }
+        if (result.empty())
+            result.push_back(SimpleMergeSelector::PartsRange{});
+
+        return result;
     }
 
     static double score(double count, double sum_size, double sum_size_fixed_cost)
@@ -89,9 +111,21 @@ struct Estimator
         return (sum_size + sum_size_fixed_cost * count) / (count - 1.9);
     }
 
-    double min_score = 0.0;
-    Iterator best_begin {};
-    Iterator best_end {};
+    struct ScoredRange
+    {
+        double score = 0.0;
+        Iterator begin {};
+        Iterator end {};
+
+        bool operator>(const ScoredRange & other) const
+        {
+            return score > other.score;
+        }
+    };
+
+
+    mutable std::priority_queue<ScoredRange, std::vector<ScoredRange>, std::greater<>> best_ranges;
+    size_t top_n_ranges;
 };
 
 
@@ -246,11 +280,11 @@ void selectWithinPartition(
 }
 
 
-SimpleMergeSelector::PartsRange SimpleMergeSelector::select(
+SimpleMergeSelector::PartsRange SimpleMergeSelector::selectBest(
     const PartsRanges & parts_ranges,
     size_t max_total_size_to_merge)
 {
-    Estimator estimator;
+    Estimator estimator(1);
 
     /// Precompute logarithm of settings boundaries, because log function is quite expensive in terms of performance
     const double min_size_to_lower_base_log = log(1 + settings.min_size_to_lower_base);
@@ -262,4 +296,21 @@ SimpleMergeSelector::PartsRange SimpleMergeSelector::select(
     return estimator.getBest();
 }
 
+SimpleMergeSelector::PartsRanges SimpleMergeSelector::selectUpToTopN(
+    const PartsRanges & parts_ranges,
+    size_t max_total_size_to_merge, size_t top_n_ranges)
+{
+    Estimator estimator(top_n_ranges);
+    /// Precompute logarithm of settings boundaries, because log function is quite expensive in terms of performance
+    const double min_size_to_lower_base_log = log(1 + settings.min_size_to_lower_base);
+    const double max_size_to_lower_base_log = log(1 + settings.max_size_to_lower_base);
+
+    for (const auto & part_range : parts_ranges)
+        selectWithinPartition(part_range, max_total_size_to_merge, estimator, settings, min_size_to_lower_base_log, max_size_to_lower_base_log);
+
+    return estimator.getUpToTopN();
+
+}
+
+
 }

src/Storages/MergeTree/MergeSelectors/SimpleMergeSelector.h

@@ -171,10 +171,15 @@ public:
 
     explicit SimpleMergeSelector(const Settings & settings_) : settings(settings_) {}
 
-    PartsRange select(
+    PartsRange selectBest(
         const PartsRanges & parts_ranges,
         size_t max_total_size_to_merge) override;
 
+    PartsRanges selectUpToTopN(
+        const PartsRanges & parts_ranges,
+        size_t max_total_size_to_merge, size_t top_n_ranges) override;
+
+
 private:
     const Settings settings;
 };

src/Storages/MergeTree/MergeSelectors/TTLMergeSelector.cpp

@@ -34,7 +34,7 @@ const String & getPartitionIdForPart(const ITTLMergeSelector::Part & part_info)
 }
 
 
-IMergeSelector::PartsRange ITTLMergeSelector::select(
+IMergeSelector::PartsRange ITTLMergeSelector::selectBest(
     const PartsRanges & parts_ranges,
     size_t max_total_size_to_merge)
 {

src/Storages/MergeTree/MergeSelectors/TTLMergeSelector.h

@@ -29,7 +29,7 @@ public:
     {
     }
 
-    PartsRange select(
+    PartsRange selectBest(
         const PartsRanges & parts_ranges,
         size_t max_total_size_to_merge) override;
 

src/Storages/MergeTree/MergeSelectors/TrivialMergeSelector.cpp

@@ -18,9 +18,9 @@ void registerTrivialMergeSelector(MergeSelectorFactory & factory)
     });
 }
 
-TrivialMergeSelector::PartsRange TrivialMergeSelector::select(
+TrivialMergeSelector::PartsRanges TrivialMergeSelector::selectUpToTopN(
     const PartsRanges & parts_ranges,
-    size_t max_total_size_to_merge)
+    size_t max_total_size_to_merge, size_t top_n_ranges)
 {
     size_t num_partitions = parts_ranges.size();
     if (num_partitions == 0)
@@ -44,11 +44,11 @@ TrivialMergeSelector::PartsRange TrivialMergeSelector::select(
     size_t right = 0;
 
     std::vector<PartsRange> candidates;
-    while (candidates.size() < settings.num_ranges_to_choose)
+    while (candidates.size() < top_n_ranges)
     {
         const PartsRange & partition = parts_ranges[partition_idx];
 
-        if (1 + right - left == settings.num_parts_to_merge)
+        if (1 + right - left == top_n_ranges)
         {
             ++right;
 
@@ -59,7 +59,7 @@ TrivialMergeSelector::PartsRange TrivialMergeSelector::select(
             if (!max_total_size_to_merge || total_size <= max_total_size_to_merge)
             {
                 candidates.emplace_back(partition.data() + left, partition.data() + right);
-                if (candidates.size() == settings.num_ranges_to_choose)
+                if (candidates.size() == settings.num_ranges_to_consider)
                     break;
             }
 
@@ -82,6 +82,16 @@ TrivialMergeSelector::PartsRange TrivialMergeSelector::select(
             left = right;
     }
 
+    return candidates;
+}
+
+
+TrivialMergeSelector::PartsRange TrivialMergeSelector::selectBest(
+    const PartsRanges & parts_ranges,
+    size_t max_total_size_to_merge)
+{
+    auto candidates = selectUpToTopN(parts_ranges, max_total_size_to_merge, settings.num_ranges_to_consider);
+
     if (candidates.empty())
         return {};
 

src/Storages/MergeTree/MergeSelectors/TrivialMergeSelector.h

@@ -18,13 +18,17 @@ public:
     struct Settings
     {
         size_t num_parts_to_merge = 10;
-        size_t num_ranges_to_choose = 100;
+        size_t num_ranges_to_consider = 100;
     };
 
-    PartsRange select(
+    PartsRange selectBest(
         const PartsRanges & parts_ranges,
         size_t max_total_size_to_merge) override;
 
+    PartsRanges selectUpToTopN(
+        const PartsRanges & parts_ranges,
+        size_t max_total_size_to_merge, size_t top_n_ranges) override;
+
 private:
     const Settings settings;
 };

src/Storages/MergeTree/MergeTreeDataMergerMutator.cpp

@@ -82,6 +82,7 @@ namespace MergeTreeSetting
     extern const MergeTreeSettingsMergeSelectorAlgorithm merge_selector_algorithm;
     extern const MergeTreeSettingsBool merge_selector_enable_heuristic_to_remove_small_parts_at_right;
     extern const MergeTreeSettingsFloat merge_selector_base;
+    extern const MergeTreeSettingsUInt64 merge_selector_max_ranges_to_select_at_once;
     extern const MergeTreeSettingsUInt64 min_parts_to_merge_at_once;
 }
 
@@ -165,7 +166,7 @@ UInt64 MergeTreeDataMergerMutator::getMaxSourcePartSizeForMutation() const
 }
 
 SelectPartsDecision MergeTreeDataMergerMutator::selectPartsToMerge(
-    FutureMergedMutatedPartPtr future_part,
+    FutureParts & out_future_parts,
     bool aggressive,
     size_t max_total_size_to_merge,
     const AllowedMergingPredicate & can_merge_callback,
@@ -192,7 +193,7 @@ SelectPartsDecision MergeTreeDataMergerMutator::selectPartsToMerge(
         return SelectPartsDecision::CANNOT_SELECT;
     }
 
-    auto res = selectPartsToMergeFromRanges(future_part, aggressive, max_total_size_to_merge, merge_with_ttl_allowed,
+    auto res = selectPartsToMergeFromRanges(out_future_parts, aggressive, max_total_size_to_merge, merge_with_ttl_allowed,
                                             metadata_snapshot, info.parts_ranges, info.current_time, out_disable_reason);
 
     if (res == SelectPartsDecision::SELECTED)
@@ -201,7 +202,8 @@ SelectPartsDecision MergeTreeDataMergerMutator::selectPartsToMerge(
     String best_partition_id_to_optimize = getBestPartitionToOptimizeEntire(info.partitions_info);
     if (!best_partition_id_to_optimize.empty())
     {
-        return selectAllPartsToMergeWithinPartition(
+        auto future_part = std::make_shared<FutureMergedMutatedPart>();
+        res = selectAllPartsToMergeWithinPartition(
             future_part,
             can_merge_callback,
             best_partition_id_to_optimize,
@@ -210,6 +212,10 @@ SelectPartsDecision MergeTreeDataMergerMutator::selectPartsToMerge(
             txn,
             out_disable_reason,
             /*optimize_skip_merged_partitions=*/true);
+
+        out_future_parts.emplace_back(future_part);
+
+        return res;
     }
 
     if (!out_disable_reason.text.empty())
@@ -258,11 +264,11 @@ MergeTreeDataMergerMutator::PartitionIdsHint MergeTreeDataMergerMutator::getPart
 
     for (size_t i = 0; i < all_partition_ids.size(); ++i)
     {
-        auto future_part = std::make_shared<FutureMergedMutatedPart>();
+        FutureParts out_future_parts;
         PreformattedMessage out_disable_reason;
         /// This method should have been const, but something went wrong... it's const with dry_run = true
         auto status = const_cast<MergeTreeDataMergerMutator *>(this)->selectPartsToMergeFromRanges(
-                future_part, /*aggressive*/ false, max_total_size_to_merge, merge_with_ttl_allowed,
+                out_future_parts, /*aggressive*/ false, max_total_size_to_merge, merge_with_ttl_allowed,
                 metadata_snapshot, ranges_per_partition[i], info.current_time, out_disable_reason,
                 /* dry_run */ true);
         if (status == SelectPartsDecision::SELECTED)
@@ -483,7 +489,7 @@ MergeTreeDataMergerMutator::MergeSelectingInfo MergeTreeDataMergerMutator::getPo
 }
 
 SelectPartsDecision MergeTreeDataMergerMutator::selectPartsToMergeFromRanges(
-    FutureMergedMutatedPartPtr future_part,
+    FutureParts & out_future_parts,
     bool aggressive,
     size_t max_total_size_to_merge,
     bool merge_with_ttl_allowed,
@@ -495,7 +501,18 @@ SelectPartsDecision MergeTreeDataMergerMutator::selectPartsToMergeFromRanges(
 {
     Stopwatch select_parts_from_ranges_timer;
     const auto data_settings = data.getSettings();
-    IMergeSelector::PartsRange parts_to_merge;
+    auto get_parts_from_range = [] (const IMergeSelector::PartsRange & parts_to_merge)
+    {
+        MergeTreeData::DataPartsVector parts;
+        parts.reserve(parts_to_merge.size());
+        for (const IMergeSelector::Part & part_info : parts_to_merge)
+        {
+            const MergeTreeData::DataPartPtr & part = part_info.getDataPartPtr();
+            parts.push_back(part);
+        }
+        return parts;
+    };
+
 
     if (metadata_snapshot->hasAnyTTL() && merge_with_ttl_allowed && !ttl_merges_blocker.isCancelled())
     {
@@ -512,10 +529,13 @@ SelectPartsDecision MergeTreeDataMergerMutator::selectPartsToMergeFromRanges(
         TTLDeleteMergeSelector drop_ttl_selector(params_drop);
 
         /// The size of the completely expired part of TTL drop is not affected by the merge pressure and the size of the storage space
-        parts_to_merge = drop_ttl_selector.select(parts_ranges, (*data_settings)[MergeTreeSetting::max_bytes_to_merge_at_max_space_in_pool]);
+        auto parts_to_merge = drop_ttl_selector.selectBest(parts_ranges, (*data_settings)[MergeTreeSetting::max_bytes_to_merge_at_max_space_in_pool]);
         if (!parts_to_merge.empty())
         {
+            auto future_part = std::make_shared<FutureMergedMutatedPart>();
             future_part->merge_type = MergeType::TTLDelete;
+            future_part->assign(get_parts_from_range(parts_to_merge));
+            out_future_parts.emplace_back(future_part);
         }
         else if (!(*data_settings)[MergeTreeSetting::ttl_only_drop_parts])
         {
@@ -529,9 +549,14 @@ SelectPartsDecision MergeTreeDataMergerMutator::selectPartsToMergeFromRanges(
             };
             TTLDeleteMergeSelector delete_ttl_selector(params_delete);
 
-            parts_to_merge = delete_ttl_selector.select(parts_ranges, max_total_size_to_merge);
+            parts_to_merge = delete_ttl_selector.selectBest(parts_ranges, max_total_size_to_merge);
             if (!parts_to_merge.empty())
+            {
+                auto future_part = std::make_shared<FutureMergedMutatedPart>();
                 future_part->merge_type = MergeType::TTLDelete;
+                future_part->assign(get_parts_from_range(parts_to_merge));
+                out_future_parts.emplace_back(future_part);
+            }
         }
 
         if (parts_to_merge.empty() && metadata_snapshot->hasAnyRecompressionTTL())
@@ -547,13 +572,19 @@ SelectPartsDecision MergeTreeDataMergerMutator::selectPartsToMergeFromRanges(
 
             TTLRecompressMergeSelector recompress_ttl_selector(params);
 
-            parts_to_merge = recompress_ttl_selector.select(parts_ranges, max_total_size_to_merge);
+            parts_to_merge = recompress_ttl_selector.selectBest(parts_ranges, max_total_size_to_merge);
             if (!parts_to_merge.empty())
+            {
+                auto future_part = std::make_shared<FutureMergedMutatedPart>();
                 future_part->merge_type = MergeType::TTLRecompress;
+                future_part->assign(get_parts_from_range(parts_to_merge));
+                out_future_parts.emplace_back(future_part);
+            }
         }
     }
 
-    if (parts_to_merge.empty())
+    /// Nothing is selected with specialized selectors
+    if (out_future_parts.empty())
     {
         auto merge_selector_algorithm = (*data_settings)[MergeTreeSetting::merge_selector_algorithm];
 
@@ -586,32 +617,34 @@ SelectPartsDecision MergeTreeDataMergerMutator::selectPartsToMergeFromRanges(
             merge_settings = simple_merge_settings;
         }
 
-        parts_to_merge = MergeSelectorFactory::instance().get(merge_selector_algorithm, merge_settings)->select(parts_ranges, max_total_size_to_merge);
+        auto parts_to_merge_ranges = MergeSelectorFactory::instance().get(merge_selector_algorithm, merge_settings)->selectUpToTopN(parts_ranges, max_total_size_to_merge, (*data_settings)[MergeTreeSetting::merge_selector_max_ranges_to_select_at_once]);
 
-        /// Do not allow to "merge" part with itself for regular merges, unless it is a TTL-merge where it is ok to remove some values with expired ttl
-        if (parts_to_merge.size() == 1)
-            throw Exception(ErrorCodes::LOGICAL_ERROR, "Merge selector returned only one part to merge");
-
-        if (parts_to_merge.empty())
+        for (const auto & parts_to_merge : parts_to_merge_ranges)
         {
-            ProfileEvents::increment(ProfileEvents::MergerMutatorSelectPartsForMergeElapsedMicroseconds, select_parts_from_ranges_timer.elapsedMicroseconds());
-            out_disable_reason = PreformattedMessage::create("Did not find any parts to merge (with usual merge selectors) in {}ms", select_parts_from_ranges_timer.elapsedMicroseconds() / 1000);
-            return SelectPartsDecision::CANNOT_SELECT;
+            /// Do not allow to "merge" part with itself for regular merges, unless it is a TTL-merge where it is ok to remove some values with expired ttl
+            if (parts_to_merge.size() == 1)
+                throw Exception(ErrorCodes::LOGICAL_ERROR, "Merge selector returned only one part to merge");
+
+            if (parts_to_merge.empty())
+            {
+                chassert(parts_to_merge_ranges.size() == 1);
+                ProfileEvents::increment(ProfileEvents::MergerMutatorSelectPartsForMergeElapsedMicroseconds, select_parts_from_ranges_timer.elapsedMicroseconds());
+                out_disable_reason = PreformattedMessage::create("Did not find any parts to merge (with usual merge selectors) in {}ms", select_parts_from_ranges_timer.elapsedMicroseconds() / 1000);
+                return SelectPartsDecision::CANNOT_SELECT;
+            }
+            else
+            {
+                auto future_part = std::make_shared<FutureMergedMutatedPart>();
+                future_part->merge_type = MergeType::Regular;
+                auto parts = get_parts_from_range(parts_to_merge);
+                ProfileEvents::increment(ProfileEvents::MergerMutatorSelectRangePartsCount, parts.size());
+                LOG_DEBUG(log, "Selected {} parts from {} to {}", parts.size(), parts.front()->name, parts.back()->name);
+                future_part->assign(std::move(parts));
+                out_future_parts.emplace_back(future_part);
+            }
         }
     }
-
-    MergeTreeData::DataPartsVector parts;
-    parts.reserve(parts_to_merge.size());
-    for (IMergeSelector::Part & part_info : parts_to_merge)
-    {
-        const MergeTreeData::DataPartPtr & part = part_info.getDataPartPtr();
-        parts.push_back(part);
-    }
-
-    LOG_DEBUG(log, "Selected {} parts from {} to {} in {}ms", parts.size(), parts.front()->name, parts.back()->name, select_parts_from_ranges_timer.elapsedMicroseconds() / 1000);
-    ProfileEvents::increment(ProfileEvents::MergerMutatorSelectRangePartsCount, parts.size());
-
-    future_part->assign(std::move(parts));
+    LOG_DEBUG(log, "Selected {} parts ranges in {}ms", out_future_parts.size(), select_parts_from_ranges_timer.elapsedMicroseconds() / 1000);
     ProfileEvents::increment(ProfileEvents::MergerMutatorSelectPartsForMergeElapsedMicroseconds, select_parts_from_ranges_timer.elapsedMicroseconds());
     return SelectPartsDecision::SELECTED;
 }

src/Storages/MergeTree/MergeTreeDataMergerMutator.h

@@ -96,7 +96,7 @@ public:
 
     /// The third step of selecting parts to merge: takes ranges that we can merge, and selects parts that we want to merge
     SelectPartsDecision selectPartsToMergeFromRanges(
-        FutureMergedMutatedPartPtr future_part,
+        FutureParts & out_future_parts,
         bool aggressive,
         size_t max_total_size_to_merge,
         bool merge_with_ttl_allowed,
@@ -125,7 +125,7 @@ public:
       *  - A part that already merges with something in one place, you can not start to merge into something else in another place.
       */
     SelectPartsDecision selectPartsToMerge(
-        FutureMergedMutatedPartPtr future_part,
+        FutureParts & out_future_parts,
         bool aggressive,
         size_t max_total_size_to_merge,
         const AllowedMergingPredicate & can_merge,

src/Storages/MergeTree/MergeTreeSettings.cpp

@@ -59,6 +59,7 @@ namespace ErrorCodes
     /** Merge selector settings. */ \
     DECLARE(UInt64, merge_selector_blurry_base_scale_factor, 0, "Controls when the logic kicks in relatively to the number of parts in partition. The bigger the factor the more belated reaction will be.", 0) \
     DECLARE(UInt64, merge_selector_window_size, 1000, "How many parts to look at once.", 0) \
+    DECLARE(UInt64, merge_selector_max_ranges_to_select_at_once, 10, "How many ranges merge selector will return per one invocation. Work only for SimpleMergeSelector and StochasticSimpleMergeSelector.", 0) \
     \
     /** Merge settings. */ \
     DECLARE(UInt64, merge_max_block_size, 8192, "How many rows in blocks should be formed for merge operations. By default has the same value as `index_granularity`.", 0) \

src/Storages/StorageMergeTree.cpp

@@ -994,10 +994,12 @@ MergeMutateSelectedEntryPtr StorageMergeTree::selectPartsToMerge(
 {
     auto data_settings = getSettings();
 
-    auto future_part = std::make_shared<FutureMergedMutatedPart>();
+    FutureParts future_parts;
+
+    UUID part_uuid = UUIDHelpers::Nil;
 
     if ((*storage_settings.get())[MergeTreeSetting::assign_part_uuids])
-        future_part->uuid = UUIDHelpers::generateV4();
+        part_uuid = UUIDHelpers::generateV4();
 
     /// You must call destructor with unlocked `currently_processing_in_background_mutex`.
     CurrentlyMergingPartsTaggerPtr merging_tagger;
@@ -1087,7 +1089,7 @@ MergeMutateSelectedEntryPtr StorageMergeTree::selectPartsToMerge(
             if (max_source_parts_size > 0)
             {
                 select_decision = merger_mutator.selectPartsToMerge(
-                    future_part,
+                    future_parts,
                     aggressive,
                     max_source_parts_size,
                     can_merge,
@@ -1124,9 +1126,12 @@ MergeMutateSelectedEntryPtr StorageMergeTree::selectPartsToMerge(
                     break;
             }
 
+            auto future_part = std::make_shared<FutureMergedMutatedPart>();
             select_decision = merger_mutator.selectAllPartsToMergeWithinPartition(
                 future_part, can_merge, partition_id, final, metadata_snapshot, txn, out_disable_reason, optimize_skip_merged_partitions);
 
+            future_parts.push_back(future_part);
+
             /// If final - we will wait for currently processing merges to finish and continue.
             if (final
                 && select_decision != SelectPartsDecision::SELECTED
@@ -1142,7 +1147,9 @@ MergeMutateSelectedEntryPtr StorageMergeTree::selectPartsToMerge(
                 }
             }
             else
+            {
                 break;
+            }
         }
     }
 
@@ -1160,12 +1167,14 @@ MergeMutateSelectedEntryPtr StorageMergeTree::selectPartsToMerge(
         return {};
     }
 
+    auto best_future_part = future_parts.front();
+    best_future_part->uuid = part_uuid;
     /// Account TTL merge here to avoid exceeding the max_number_of_merges_with_ttl_in_pool limit
-    if (isTTLMergeType(future_part->merge_type))
+    if (isTTLMergeType(best_future_part->merge_type))
         getContext()->getMergeList().bookMergeWithTTL();
 
-    merging_tagger = std::make_unique<CurrentlyMergingPartsTagger>(future_part, MergeTreeDataMergerMutator::estimateNeededDiskSpace(future_part->parts, true), *this, metadata_snapshot, false);
-    return std::make_shared<MergeMutateSelectedEntry>(future_part, std::move(merging_tagger), std::make_shared<MutationCommands>());
+    merging_tagger = std::make_unique<CurrentlyMergingPartsTagger>(best_future_part, MergeTreeDataMergerMutator::estimateNeededDiskSpace(best_future_part->parts, true), *this, metadata_snapshot, false);
+    return std::make_shared<MergeMutateSelectedEntry>(best_future_part, std::move(merging_tagger), std::make_shared<MutationCommands>());
 }
 
 bool StorageMergeTree::merge(

src/Storages/StorageReplicatedMergeTree.cpp

@@ -3921,9 +3921,6 @@ void StorageReplicatedMergeTree::mergeSelectingTask()
         bool merge_with_ttl_allowed = merges_and_mutations_queued.merges_with_ttl < (*storage_settings_ptr)[MergeTreeSetting::max_replicated_merges_with_ttl_in_queue] &&
             getTotalMergesWithTTLInMergeList() < (*storage_settings_ptr)[MergeTreeSetting::max_number_of_merges_with_ttl_in_pool];
 
-        auto future_merged_part = std::make_shared<FutureMergedMutatedPart>();
-        if ((*storage_settings.get())[MergeTreeSetting::assign_part_uuids])
-            future_merged_part->uuid = UUIDHelpers::generateV4();
 
         bool can_assign_merge = max_source_parts_size_for_merge > 0;
         PartitionIdsHint partitions_to_merge_in;
@@ -3938,11 +3935,20 @@ void StorageReplicatedMergeTree::mergeSelectingTask()
                 merge_pred.emplace(queue.getMergePredicate(zookeeper, partitions_to_merge_in));
         }
 
+        UUID part_uuid = UUIDHelpers::Nil;
+
+        if ((*storage_settings.get())[MergeTreeSetting::assign_part_uuids])
+            part_uuid = UUIDHelpers::generateV4();
+
+        FutureParts future_parts;
         PreformattedMessage out_reason;
         if (can_assign_merge &&
-            merger_mutator.selectPartsToMerge(future_merged_part, false, max_source_parts_size_for_merge, *merge_pred,
+            merger_mutator.selectPartsToMerge(future_parts, false, max_source_parts_size_for_merge, *merge_pred,
                 merge_with_ttl_allowed, NO_TRANSACTION_PTR, out_reason, &partitions_to_merge_in) == SelectPartsDecision::SELECTED)
         {
+            auto future_merged_part = future_parts.front();
+            future_merged_part->uuid = part_uuid;
+
             create_result = createLogEntryToMergeParts(
                 zookeeper,
                 future_merged_part->parts,
@@ -3986,7 +3992,7 @@ void StorageReplicatedMergeTree::mergeSelectingTask()
 
                 create_result = createLogEntryToMutatePart(
                     *part,
-                    future_merged_part->uuid,
+                    part_uuid,
                     desired_mutation_version->first,
                     desired_mutation_version->second,
                     merge_pred->getVersion());
@@ -5943,22 +5949,24 @@ bool StorageReplicatedMergeTree::optimize(
             }
             ReplicatedMergeTreeMergePredicate can_merge = queue.getMergePredicate(zookeeper, std::move(partition_ids_hint));
 
-            auto future_merged_part = std::make_shared<FutureMergedMutatedPart>();
+            auto part_uuid = UUIDHelpers::Nil;
             if ((*storage_settings.get())[MergeTreeSetting::assign_part_uuids])
-                future_merged_part->uuid = UUIDHelpers::generateV4();
+                part_uuid = UUIDHelpers::generateV4();
 
             constexpr const char * unknown_disable_reason = "unknown reason";
             PreformattedMessage disable_reason = PreformattedMessage::create(unknown_disable_reason);
             SelectPartsDecision select_decision = SelectPartsDecision::CANNOT_SELECT;
 
+            FutureParts future_parts;
             if (partition_id.empty())
             {
                 select_decision = merger_mutator.selectPartsToMerge(
-                    future_merged_part, /* aggressive */ true, (*storage_settings_ptr)[MergeTreeSetting::max_bytes_to_merge_at_max_space_in_pool],
+                    future_parts, /* aggressive */ true, (*storage_settings_ptr)[MergeTreeSetting::max_bytes_to_merge_at_max_space_in_pool],
                     can_merge, /* merge_with_ttl_allowed */ false, NO_TRANSACTION_PTR, disable_reason);
             }
             else
             {
+                auto future_merged_part = std::make_shared<FutureMergedMutatedPart>();
                 select_decision = merger_mutator.selectAllPartsToMergeWithinPartition(
                     future_merged_part,
                     can_merge,
@@ -5968,6 +5976,9 @@ bool StorageReplicatedMergeTree::optimize(
                     NO_TRANSACTION_PTR,
                     disable_reason,
                     query_context->getSettingsRef()[Setting::optimize_skip_merged_partitions]);
+
+                if (select_decision == SelectPartsDecision::SELECTED)
+                    future_parts.push_back(future_merged_part);
             }
 
             /// If there is nothing to merge then we treat this merge as successful (needed for optimize final optimization)
@@ -5994,6 +6005,8 @@ bool StorageReplicatedMergeTree::optimize(
                 return handle_noop(message_fmt, disable_reason.text);
             }
 
+            auto future_merged_part = future_parts.front();
+            future_merged_part->uuid = part_uuid;
             ReplicatedMergeTreeLogEntryData merge_entry;
             CreateMergeEntryResult create_result = createLogEntryToMergeParts(
                 zookeeper, future_merged_part->parts,

src/Storages/examples/merge_selector.cpp

@@ -51,7 +51,7 @@ int main(int, char **)
 
     while (parts.size() > 1)
     {
-        IMergeSelector::PartsRange selected_parts = selector.select(partitions, 0);
+        IMergeSelector::PartsRange selected_parts = selector.selectBest(partitions, 0);
 
         if (selected_parts.empty())
         {

src/Storages/examples/merge_selector2.cpp

@@ -48,7 +48,7 @@ int main(int, char **)
 
     while (parts.size() > 1)
     {
-        IMergeSelector::PartsRange selected_parts = selector.select(partitions, 100ULL * 1024 * 1024 * 1024);
+        IMergeSelector::PartsRange selected_parts = selector.selectBest(partitions, 100ULL * 1024 * 1024 * 1024);
 
         if (selected_parts.empty())
         {