EnableLocalDBFlatIndex setting (#2674)

17 files changed, 239 insertions, 128 deletions

diff --git a/ydb/core/protos/feature_flags.proto b/ydb/core/protos/feature_flags.proto
index bae93242a30..4e722d1acc8 100644
--- a/ydb/core/protos/feature_flags.proto
+++ b/ydb/core/protos/feature_flags.proto
@@ -131,4 +131,5 @@ message TFeatureFlags {
     optional bool EnableReplaceIfExistsForExternalEntities = 116 [ default = false];
     optional bool EnableCMSRequestPriorities = 117 [default = false];
     optional bool EnableKeyvalueLogBatching = 118 [default = false];
+    optional bool EnableLocalDBFlatIndex = 119 [default = true];
 }
diff --git a/ydb/core/tablet_flat/flat_executor.cpp b/ydb/core/tablet_flat/flat_executor.cpp
index 0a9e407ed71..e7d4d4e9e8c 100644
--- a/ydb/core/tablet_flat/flat_executor.cpp
+++ b/ydb/core/tablet_flat/flat_executor.cpp
@@ -4315,6 +4315,7 @@ ui64 TExecutor::BeginCompaction(THolder<NTable::TCompactionParams> params)
     comp->Epoch = snapshot->Subset->Epoch(); /* narrows requested to actual */
     comp->Layout.Final = comp->Params->IsFinal;
     comp->Layout.WriteBTreeIndex = AppData()->FeatureFlags.GetEnableLocalDBBtreeIndex();
+    comp->Layout.WriteFlatIndex = AppData()->FeatureFlags.GetEnableLocalDBFlatIndex();
     comp->Writer.StickyFlatIndex = !comp->Layout.WriteBTreeIndex;
     comp->Layout.MaxRows = snapshot->Subset->MaxRows();
     comp->Layout.ByKeyFilter = tableInfo->ByKeyFilter;
diff --git a/ydb/core/tablet_flat/flat_executor_ut.cpp b/ydb/core/tablet_flat/flat_executor_ut.cpp
index 18d8399728b..c9aba45a86a 100644
--- a/ydb/core/tablet_flat/flat_executor_ut.cpp
+++ b/ydb/core/tablet_flat/flat_executor_ut.cpp
@@ -5453,12 +5453,13 @@ Y_UNIT_TEST_SUITE(TFlatTableExecutorBTreeIndex) {
         }
     };
 
-    Y_UNIT_TEST(UseBtreeIndex_Default) {
+    Y_UNIT_TEST(EnableLocalDBBtreeIndex_Default) { // uses flat index
         TMyEnvBase env;
         TRowsModel rows;
 
         auto &appData = env->GetAppData();
         UNIT_ASSERT_VALUES_EQUAL(appData.FeatureFlags.HasEnableLocalDBBtreeIndex(), false);
+        UNIT_ASSERT_VALUES_EQUAL(appData.FeatureFlags.HasEnableLocalDBFlatIndex(), false);
         auto counters = MakeIntrusive<TSharedPageCacheCounters>(env->GetDynamicCounters());
         int readRows = 0, failedAttempts = 0;
 
@@ -5491,7 +5492,7 @@ Y_UNIT_TEST_SUITE(TFlatTableExecutorBTreeIndex) {
         UNIT_ASSERT_VALUES_EQUAL(failedAttempts, 288);
     }
 
-    Y_UNIT_TEST(UseBtreeIndex_True) {
+    Y_UNIT_TEST(EnableLocalDBBtreeIndex_True) { // uses b-tree index
         TMyEnvBase env;
         TRowsModel rows;
 
@@ -5529,7 +5530,85 @@ Y_UNIT_TEST_SUITE(TFlatTableExecutorBTreeIndex) {
         UNIT_ASSERT_VALUES_EQUAL(failedAttempts, 332);
     }
 
-    Y_UNIT_TEST(UseBtreeIndex_True_TurnOff) {
+    Y_UNIT_TEST(EnableLocalDBBtreeIndex_True_EnableLocalDBFlatIndex_False) { // uses b-tree index
+        TMyEnvBase env;
+        TRowsModel rows;
+
+        auto &appData = env->GetAppData();
+        appData.FeatureFlags.SetEnableLocalDBBtreeIndex(true);
+        appData.FeatureFlags.SetEnableLocalDBFlatIndex(false);
+        auto counters = MakeIntrusive<TSharedPageCacheCounters>(env->GetDynamicCounters());
+        int readRows = 0, failedAttempts = 0;
+
+        env.FireDummyTablet(ui32(NFake::TDummy::EFlg::Comp));
+
+        auto policy = MakeIntrusive<TCompactionPolicy>();
+        policy->MinBTreeIndexNodeSize = 128;
+        env.SendSync(rows.MakeScheme(std::move(policy)));
+
+        env.SendSync(rows.VersionTo(TRowVersion(1, 10)).RowTo(0).MakeRows(1000, 950));
+        env.SendSync(rows.VersionTo(TRowVersion(2, 20)).RowTo(0).MakeRows(1000, 950));
+        
+        env.SendSync(new NFake::TEvCompact(TRowsModel::TableId));
+        env.WaitFor<NFake::TEvCompacted>();
+
+        // all pages are always kept in shared cache
+        UNIT_ASSERT_VALUES_EQUAL(counters->ActivePages->Val(), 334);
+
+        env.SendSync(new NFake::TEvExecute{ new TTxFullScan(readRows, failedAttempts) });
+        UNIT_ASSERT_VALUES_EQUAL(readRows, 1000);
+        UNIT_ASSERT_VALUES_EQUAL(failedAttempts, 0);
+
+        // restart tablet
+        env.SendSync(new TEvents::TEvPoison, false, true);
+        env.FireDummyTablet(ui32(NFake::TDummy::EFlg::Comp));
+
+        // after restart we have no pages in private cache
+        env.SendSync(new NFake::TEvExecute{ new TTxFullScan(readRows, failedAttempts) }, true);
+        UNIT_ASSERT_VALUES_EQUAL(readRows, 1000);
+        UNIT_ASSERT_VALUES_EQUAL(failedAttempts, 332);
+    }
+
+    Y_UNIT_TEST(EnableLocalDBBtreeIndex_False_EnableLocalDBFlatIndex_False) { // uses flat index
+        TMyEnvBase env;
+        TRowsModel rows;
+
+        auto &appData = env->GetAppData();
+        appData.FeatureFlags.SetEnableLocalDBBtreeIndex(false);
+        appData.FeatureFlags.SetEnableLocalDBFlatIndex(false);
+        auto counters = MakeIntrusive<TSharedPageCacheCounters>(env->GetDynamicCounters());
+        int readRows = 0, failedAttempts = 0;
+
+        env.FireDummyTablet(ui32(NFake::TDummy::EFlg::Comp));
+
+        auto policy = MakeIntrusive<TCompactionPolicy>();
+        policy->MinBTreeIndexNodeSize = 128;
+        env.SendSync(rows.MakeScheme(std::move(policy)));
+
+        env.SendSync(rows.VersionTo(TRowVersion(1, 10)).RowTo(0).MakeRows(1000, 950));
+        env.SendSync(rows.VersionTo(TRowVersion(2, 20)).RowTo(0).MakeRows(1000, 950));
+        
+        env.SendSync(new NFake::TEvCompact(TRowsModel::TableId));
+        env.WaitFor<NFake::TEvCompacted>();
+
+        // all pages are always kept in shared cache
+        UNIT_ASSERT_VALUES_EQUAL(counters->ActivePages->Val(), 290);
+
+        env.SendSync(new NFake::TEvExecute{ new TTxFullScan(readRows, failedAttempts) });
+        UNIT_ASSERT_VALUES_EQUAL(readRows, 1000);
+        UNIT_ASSERT_VALUES_EQUAL(failedAttempts, 0);
+
+        // restart tablet
+        env.SendSync(new TEvents::TEvPoison, false, true);
+        env.FireDummyTablet(ui32(NFake::TDummy::EFlg::Comp));
+
+        // after restart we have no pages in private cache
+        env.SendSync(new NFake::TEvExecute{ new TTxFullScan(readRows, failedAttempts) }, true);
+        UNIT_ASSERT_VALUES_EQUAL(readRows, 1000);
+        UNIT_ASSERT_VALUES_EQUAL(failedAttempts, 288);
+    }
+
+    Y_UNIT_TEST(EnableLocalDBBtreeIndex_True_TurnOff) { // uses b-tree index at first
         TMyEnvBase env;
         TRowsModel rows;
 
@@ -5569,7 +5648,7 @@ Y_UNIT_TEST_SUITE(TFlatTableExecutorBTreeIndex) {
         UNIT_ASSERT_VALUES_EQUAL(failedAttempts, 288);
     }
 
-    Y_UNIT_TEST(UseBtreeIndex_True_Generations) {
+    Y_UNIT_TEST(EnableLocalDBBtreeIndex_True_Generations) { // uses b-tree index
         TMyEnvBase env;
         TRowsModel rows;
 
diff --git a/ydb/core/tablet_flat/flat_page_conf.h b/ydb/core/tablet_flat/flat_page_conf.h
index eb03ae558f7..3b37d2ab8ff 100644
--- a/ydb/core/tablet_flat/flat_page_conf.h
+++ b/ydb/core/tablet_flat/flat_page_conf.h
@@ -84,6 +84,7 @@ namespace NPage {
         bool Final = true;
         bool CutIndexKeys = true;
         bool WriteBTreeIndex = false;
+        bool WriteFlatIndex = true;
         ui32 MaxLargeBlob = 8 * 1024 * 1024 - 8; /* Maximum large blob size */
         ui32 LargeEdge = Max<ui32>();   /* External blob edge size      */
         ui32 SmallEdge = Max<ui32>();   /* Outer blobs edge bytes limit */
diff --git a/ydb/core/tablet_flat/flat_part_btree_index_iter.h b/ydb/core/tablet_flat/flat_part_btree_index_iter.h
index 656c06d2960..40bb92aa206 100644
--- a/ydb/core/tablet_flat/flat_part_btree_index_iter.h
+++ b/ydb/core/tablet_flat/flat_part_btree_index_iter.h
@@ -113,8 +113,8 @@ public:
         : Part(part)
         , Env(env)
         , GroupId(groupId)
-        , GroupInfo(part->Scheme->GetLayout(groupId))
-        , Meta(groupId.IsHistoric() ? part->IndexPages.BTreeHistoric[groupId.Index] : part->IndexPages.BTreeGroups[groupId.Index])
+        , GroupInfo(Part->Scheme->GetLayout(GroupId))
+        , Meta(Part->IndexPages.GetBTree(GroupId))
         , State(Reserve(Meta.LevelCount + 1))
     {
         const static TCellsIterable EmptyKey(static_cast<const char*>(nullptr), TColumns());
diff --git a/ydb/core/tablet_flat/flat_part_charge_btree_index.h b/ydb/core/tablet_flat/flat_part_charge_btree_index.h
index 4207f85f69f..fb9356c53ff 100644
--- a/ydb/core/tablet_flat/flat_part_charge_btree_index.h
+++ b/ydb/core/tablet_flat/flat_part_charge_btree_index.h
@@ -71,7 +71,7 @@ public:
 
         bool ready = true, overshot = true, hasValidRowsRange = Groups || IncludeHistory;
         const TRowId sliceBeginRowId = beginRowId, sliceEndRowId = endRowId;
-        const auto& meta = Part->IndexPages.BTreeGroups[0];
+        const auto& meta = Part->IndexPages.GetBTree({});
         Y_ABORT_UNLESS(beginRowId < endRowId);
         Y_ABORT_UNLESS(endRowId <= meta.RowCount);
 
@@ -221,7 +221,7 @@ public:
         
         bool ready = true, overshot = true, hasValidRowsRange = Groups || IncludeHistory;
         const TRowId sliceBeginRowId = beginRowId, sliceEndRowId = endRowId;
-        const auto& meta = Part->IndexPages.BTreeGroups[0];
+        const auto& meta = Part->IndexPages.GetBTree({});
         Y_ABORT_UNLESS(beginRowId < endRowId);
         Y_ABORT_UNLESS(endRowId <= meta.RowCount);
 
@@ -444,7 +444,7 @@ private:
 private:
     bool DoGroup(TGroupId groupId, TRowId beginRowId, TRowId endRowId, TRowId firstChildBeginRowId, ui64 bytesLimit) const noexcept {
         bool ready = true;
-        const auto& meta = groupId.IsHistoric() ? Part->IndexPages.BTreeHistoric[groupId.Index] : Part->IndexPages.BTreeGroups[groupId.Index];
+        const auto& meta = Part->IndexPages.GetBTree(groupId);
 
         TVector<TNodeState> level, nextLevel(::Reserve(3));
         ui64 firstChildPrevBytes = bytesLimit ? GetPrevBytes(meta, firstChildBeginRowId) : 0;
@@ -500,7 +500,7 @@ private:
 
     bool DoGroupReverse(TGroupId groupId, TRowId beginRowId, TRowId endRowId, TRowId lastChildEndRowId, ui64 bytesLimit) const noexcept {
         bool ready = true;
-        const auto& meta = groupId.IsHistoric() ? Part->IndexPages.BTreeHistoric[groupId.Index] : Part->IndexPages.BTreeGroups[groupId.Index];
+        const auto& meta = Part->IndexPages.GetBTree(groupId);
 
         // level's nodes is in reverse order
         TVector<TNodeState> level, nextLevel(::Reserve(3));
@@ -584,7 +584,7 @@ private:
         const TKeyCellDefaults* keyDefaults = Part->Scheme->HistoryKeys.Get();
 
         const TGroupId groupId(0, true);
-        const auto& meta = Part->IndexPages.BTreeHistoric[0];
+        const auto& meta = Part->IndexPages.GetBTree(TGroupId{0, true});
         TRowId beginRowId = 0, endRowId = meta.RowCount;
         
         TVector<TNodeState> level, nextLevel(::Reserve(3));
diff --git a/ydb/core/tablet_flat/flat_part_charge_create.cpp b/ydb/core/tablet_flat/flat_part_charge_create.cpp
index 30b9a525475..f10010deb62 100644
--- a/ydb/core/tablet_flat/flat_part_charge_create.cpp
+++ b/ydb/core/tablet_flat/flat_part_charge_create.cpp
@@ -5,7 +5,7 @@
 namespace NKikimr::NTable {
 
 THolder<ICharge> CreateCharge(IPages *env, const TPart &part, TTagsRef tags, bool includeHistory) {
-    if (part.IndexPages.BTreeGroups) {
+    if (part.IndexPages.HasBTree()) {
         return MakeHolder<TChargeBTreeIndex>(env, part, tags, includeHistory);
     } else {
         return MakeHolder<TCharge>(env, part, tags, includeHistory);
diff --git a/ydb/core/tablet_flat/flat_part_dump.cpp b/ydb/core/tablet_flat/flat_part_dump.cpp
index f338d251f91..619e17526f9 100644
--- a/ydb/core/tablet_flat/flat_part_dump.cpp
+++ b/ydb/core/tablet_flat/flat_part_dump.cpp
@@ -99,13 +99,13 @@ namespace {
 
     void TDump::Index(const TPart &part, ui32 depth) noexcept
     {
-        if (!part.IndexPages.Groups) {
+        if (!part.IndexPages.HasFlat()) {
             return;
         }
 
         TVector<TCell> key(Reserve(part.Scheme->Groups[0].KeyTypes.size()));
 
-        auto indexPageId = part.IndexPages.Groups[0];
+        auto indexPageId = part.IndexPages.GetFlat({});
         auto indexPage = Env->TryGetPage(&part, indexPageId);
 
         if (!indexPage) {
@@ -172,8 +172,8 @@ namespace {
 
     void TDump::BTreeIndex(const TPart &part) noexcept
     {
-        if (part.IndexPages.BTreeGroups) {
-            auto meta = part.IndexPages.BTreeGroups.front();
+        if (part.IndexPages.HasBTree()) {
+            auto meta = part.IndexPages.GetBTree({});
             if (meta.LevelCount) {
                 BTreeIndexNode(part, meta);
             } else {
diff --git a/ydb/core/tablet_flat/flat_part_index_iter.h b/ydb/core/tablet_flat/flat_part_index_iter.h
index e2d43a5419e..2cf880e262d 100644
--- a/ydb/core/tablet_flat/flat_part_index_iter.h
+++ b/ydb/core/tablet_flat/flat_part_index_iter.h
@@ -171,7 +171,7 @@ private:
         if (Index) {
             return &*Index;
         }
-        auto pageId = GroupId.IsHistoric() ? Part->IndexPages.Historic[GroupId.Index] : Part->IndexPages.Groups[GroupId.Index];
+        auto pageId = Part->IndexPages.GetFlat(GroupId);
         auto page = Env->TryGetPage(Part, pageId);
         if (page) {
             Index = TIndex(*page);
diff --git a/ydb/core/tablet_flat/flat_part_index_iter_create.cpp b/ydb/core/tablet_flat/flat_part_index_iter_create.cpp
index 65a19e10666..a0f73299d51 100644
--- a/ydb/core/tablet_flat/flat_part_index_iter_create.cpp
+++ b/ydb/core/tablet_flat/flat_part_index_iter_create.cpp
@@ -5,7 +5,7 @@ namespace NKikimr::NTable {
 
 THolder<IIndexIter> CreateIndexIter(const TPart* part, IPages* env, NPage::TGroupId groupId)
 {
-    if (groupId.Index < (groupId.IsHistoric() ? part->IndexPages.BTreeHistoric : part->IndexPages.BTreeGroups).size()) {
+    if (part->IndexPages.HasBTree()) {
         return MakeHolder<TPartBtreeIndexIt>(part, env, groupId);
     } else {
         return MakeHolder<TPartIndexIt>(part, env, groupId);
diff --git a/ydb/core/tablet_flat/flat_part_loader.cpp b/ydb/core/tablet_flat/flat_part_loader.cpp
index 294e2be3151..dca805a4a70 100644
--- a/ydb/core/tablet_flat/flat_part_loader.cpp
+++ b/ydb/core/tablet_flat/flat_part_loader.cpp
@@ -58,7 +58,6 @@ void TLoader::StageParseMeta() noexcept
         const auto &layout = Root.GetLayout();
 
         SchemeId = layout.HasScheme() ? layout.GetScheme() : SchemeId;
-        IndexId = layout.HasIndex() ? layout.GetIndex() : IndexId;
         GlobsId = layout.HasGlobs() ? layout.GetGlobs() : GlobsId;
         LargeId = layout.HasLarge() ? layout.GetLarge() : LargeId;
         SmallId = layout.HasSmall() ? layout.GetSmall() : SmallId;
@@ -66,33 +65,42 @@ void TLoader::StageParseMeta() noexcept
         GarbageStatsId = layout.HasGarbageStats() ? layout.GetGarbageStats() : GarbageStatsId;
         TxIdStatsId = layout.HasTxIdStats() ? layout.GetTxIdStats() : TxIdStatsId;
 
-        GroupIndexesIds.clear();
+        FlatGroupIndexes.clear();
+        FlatHistoricIndexes.clear();
+        if (layout.HasIndex() && layout.GetIndex() != Max<TPageId>()) {
+            FlatGroupIndexes.push_back(layout.GetIndex());
+        }
         for (ui32 id : layout.GetGroupIndexes()) {
-            GroupIndexesIds.push_back(id);
+            FlatGroupIndexes.push_back(id);
         }
-
-        HistoricIndexesIds.clear();
         for (ui32 id : layout.GetHistoricIndexes()) {
-            HistoricIndexesIds.push_back(id);
+            FlatHistoricIndexes.push_back(id);
         }
 
         BTreeGroupIndexes.clear();
         BTreeHistoricIndexes.clear();
-        if (AppData()->FeatureFlags.GetEnableLocalDBBtreeIndex()) {
-            for (bool history : {false, true}) {
-                for (const auto &meta : history ? layout.GetBTreeHistoricIndexes() : layout.GetBTreeGroupIndexes()) {
-                    NPage::TBtreeIndexMeta converted{{
-                        meta.GetRootPageId(), 
-                        meta.GetRowCount(), 
-                        meta.GetDataSize(), 
-                        meta.GetErasedRowCount()}, 
-                        meta.GetLevelCount(), 
-                        meta.GetIndexSize()};
-                    (history ? BTreeHistoricIndexes : BTreeGroupIndexes).push_back(converted);
-                }
+        for (bool history : {false, true}) {
+            for (const auto &meta : history ? layout.GetBTreeHistoricIndexes() : layout.GetBTreeGroupIndexes()) {
+                NPage::TBtreeIndexMeta converted{{
+                    meta.GetRootPageId(), 
+                    meta.GetRowCount(), 
+                    meta.GetDataSize(), 
+                    meta.GetErasedRowCount()}, 
+                    meta.GetLevelCount(), 
+                    meta.GetIndexSize()};
+                (history ? BTreeHistoricIndexes : BTreeGroupIndexes).push_back(converted);
             }
         }
 
+        if (!AppData()->FeatureFlags.GetEnableLocalDBBtreeIndex() && FlatGroupIndexes) {
+            BTreeGroupIndexes.clear();
+            BTreeHistoricIndexes.clear();
+        }
+        if (!AppData()->FeatureFlags.GetEnableLocalDBFlatIndex() && BTreeGroupIndexes) {
+            FlatGroupIndexes.clear();
+            FlatHistoricIndexes.clear();
+        }
+
     } else { /* legacy page collection w/o layout data, (Evolution < 14) */
         do {
             pageId--;
@@ -100,7 +108,7 @@ void TLoader::StageParseMeta() noexcept
 
             switch (type) {
             case EPage::Scheme: SchemeId = pageId; break;
-            case EPage::Index: IndexId = pageId; break;
+            case EPage::Index: FlatGroupIndexes = {pageId}; break;
             /* All special pages have to be placed after the index
                 page, hack is required for legacy page collections without
                 topology data in metablob.
@@ -122,12 +130,14 @@ void TLoader::StageParseMeta() noexcept
 
     if (!HasBasics() || (Rooted && SchemeId != meta.TotalPages() - 1)
         || (LargeId == Max<TPageId>()) != (GlobsId == Max<TPageId>())
-        || (1 + GroupIndexesIds.size() + (SmallId == Max<TPageId>() ? 0 : 1)) != Packs.size())
+        || (Max(BTreeGroupIndexes.size(), FlatGroupIndexes.size()) + (SmallId == Max<TPageId>() ? 0 : 1)) != Packs.size())
     {
         Y_Fail("Part " << Packs[0]->PageCollection->Label() << " has"
             << " invalid layout : " << (Rooted ? "rooted" : "legacy")
             << " " << Packs.size() << "s " << meta.TotalPages() << "pg"
-            << ", Scheme " << SchemeId << ", Index " << IndexId
+            << ", Scheme " << SchemeId 
+            << ", FlatIndex " << (FlatGroupIndexes.size() ? FlatGroupIndexes[0] : Max<TPageId>())
+            << ", BTreeIndex " << (BTreeGroupIndexes.size() ? BTreeGroupIndexes[0].PageId : Max<TPageId>())
             << ", Blobs " << GlobsId << ", Small " << SmallId
             << ", Large " << LargeId << ", ByKey " << ByKeyId
             << ", Garbage " << GarbageStatsId
@@ -164,7 +174,7 @@ TAutoPtr<NPageCollection::TFetch> TLoader::StageCreatePartView() noexcept
         Y_ABORT("Scheme page is not loaded");
     } else if (ByKeyId != Max<TPageId>() && !byKey) {
         Y_ABORT("Filter page must be loaded if it exists");
-    } else if (small && Packs.size() != (1 + GroupIndexesIds.size() + 1)) {
+    } else if (small && Packs.size() != (1 + Max(BTreeGroupIndexes.size(), FlatGroupIndexes.size()))) {
         Y_Fail("TPart has small blobs, " << Packs.size() << " page collections");
     }
 
@@ -175,12 +185,6 @@ TAutoPtr<NPageCollection::TFetch> TLoader::StageCreatePartView() noexcept
     // Use epoch from metadata unless it has been provided to loader externally
     TEpoch epoch = Epoch != TEpoch::Max() ? Epoch : TEpoch(Root.GetEpoch());
 
-    TVector<TPageId> groupIndexesIds(Reserve(GroupIndexesIds.size() + 1));
-    groupIndexesIds.push_back(IndexId);
-    for (auto pageId : GroupIndexesIds) {
-        groupIndexesIds.push_back(pageId);
-    }
-
     // TODO: put index size to stat?
     // TODO: include history indexes bytes
     size_t indexesRawSize = 0;
@@ -190,7 +194,7 @@ TAutoPtr<NPageCollection::TFetch> TLoader::StageCreatePartView() noexcept
         }
         // Note: although we also have flat index, it shouldn't be loaded; so let's not count it here
     } else {
-        for (auto indexPage : groupIndexesIds) {
+        for (auto indexPage : FlatGroupIndexes) {
             indexesRawSize += GetPageSize(indexPage);
         }
     }
@@ -200,7 +204,7 @@ TAutoPtr<NPageCollection::TFetch> TLoader::StageCreatePartView() noexcept
         {
             epoch,
             TPartScheme::Parse(*scheme, Rooted),
-            { std::move(groupIndexesIds), HistoricIndexesIds, BTreeGroupIndexes, BTreeHistoricIndexes },
+            { FlatGroupIndexes, FlatHistoricIndexes, BTreeGroupIndexes, BTreeHistoricIndexes },
             blobs ? new NPage::TExtBlobs(*blobs, extra) : nullptr,
             byKey ? new NPage::TBloom(*byKey) : nullptr,
             large ? new NPage::TFrames(*large) : nullptr,
diff --git a/ydb/core/tablet_flat/flat_part_loader.h b/ydb/core/tablet_flat/flat_part_loader.h
index 8eef08d3c98..5f549fcf31c 100644
--- a/ydb/core/tablet_flat/flat_part_loader.h
+++ b/ydb/core/tablet_flat/flat_part_loader.h
@@ -4,7 +4,6 @@
 #include "flat_sausagecache.h"
 #include "shared_cache_events.h"
 #include "util_fmt_abort.h"
-#include "util_basics.h"
 #include <ydb/core/tablet_flat/protos/flat_table_part.pb.h>
 #include <ydb/core/util/pb.h>
 #include <util/generic/hash.h>
@@ -137,7 +136,8 @@ namespace NTable {
     private:
         bool HasBasics() const noexcept
         {
-            return SchemeId != Max<TPageId>() && IndexId != Max<TPageId>();
+            return SchemeId != Max<TPageId>() && 
+                (FlatGroupIndexes || BTreeGroupIndexes);
         }
 
         const TSharedData* GetPage(TPageId page) noexcept
@@ -172,15 +172,14 @@ namespace NTable {
         EStage Stage = EStage::Meta;
         bool Rooted = false; /* Has full topology metablob */
         TPageId SchemeId = Max<TPageId>();
-        TPageId IndexId = Max<TPageId>();
         TPageId GlobsId = Max<TPageId>();
         TPageId LargeId = Max<TPageId>();
         TPageId SmallId = Max<TPageId>();
         TPageId ByKeyId = Max<TPageId>();
         TPageId GarbageStatsId = Max<TPageId>();
         TPageId TxIdStatsId = Max<TPageId>();
-        TVector<TPageId> GroupIndexesIds;
-        TVector<TPageId> HistoricIndexesIds;
+        TVector<TPageId> FlatGroupIndexes;
+        TVector<TPageId> FlatHistoricIndexes;
         TVector<NPage::TBtreeIndexMeta> BTreeGroupIndexes;
         TVector<NPage::TBtreeIndexMeta> BTreeHistoricIndexes;
         TRowVersion MinRowVersion;
diff --git a/ydb/core/tablet_flat/flat_part_writer.h b/ydb/core/tablet_flat/flat_part_writer.h
index 572f1e8d54f..4c7b9e22683 100644
--- a/ydb/core/tablet_flat/flat_part_writer.h
+++ b/ydb/core/tablet_flat/flat_part_writer.h
@@ -5,18 +5,15 @@
 #include "flat_page_conf.h"
 #include "flat_page_gstat.h"
 #include "flat_page_txidstat.h"
-#include "flat_page_txstatus.h"
 #include "flat_page_writer.h"
 #include "flat_page_other.h"
 #include "flat_part_iface.h"
 #include "flat_part_overlay.h"
-#include "flat_part_screen.h"
 #include "flat_part_slice.h"
 #include "flat_part_laid.h"
 #include "flat_row_state.h"
 #include "flat_bloom_writer.h"
 #include "util_fmt_abort.h"
-#include "util_deref.h"
 
 #include <ydb/core/tablet_flat/protos/flat_table_part.pb.h>
 #include <ydb/core/util/intrusive_heap.h>
@@ -43,6 +40,7 @@ namespace NTable {
             : Final(conf.Final)
             , CutIndexKeys(conf.CutIndexKeys)
             , WriteBTreeIndex(conf.WriteBTreeIndex)
+            , WriteFlatIndex(conf.WriteFlatIndex || !conf.WriteBTreeIndex)
             , SmallEdge(conf.SmallEdge)
             , LargeEdge(conf.LargeEdge)
             , MaxLargeBlob(conf.MaxLargeBlob)
@@ -189,7 +187,7 @@ namespace NTable {
                 // N.B. non-main groups have no key
                 TCellsRef groupKey = groupIdx == 0 ? KeyState.Key : TCellsRef{ };
                 g.NextDataSize = g.Data.CalcSize(groupKey, row, KeyState.Final, TRowVersion::Min(), TRowVersion::Max(), txId);
-                g.NextIndexSize = g.Index.CalcSize(groupKey);
+                g.NextIndexSize = WriteFlatIndex ? g.Index.CalcSize(groupKey) : 0;
                 g.NextBTreeIndexSize = WriteBTreeIndex ? g.BTreeIndex.CalcSize(groupKey) : 0;
                 overheadBytes += (
                     g.NextDataSize.DataPageSize +
@@ -255,7 +253,7 @@ namespace NTable {
                 // N.B. non-main groups have no key
                 TCellsRef groupKey = groupIdx == 0 ? KeyState.Key : TCellsRef{ };
                 g.NextDataSize = g.Data.CalcSize(groupKey, row, KeyState.Final, minVersion, maxVersion, /* txId */ 0);
-                g.NextIndexSize = g.Index.CalcSize(groupKey);
+                g.NextIndexSize = WriteFlatIndex ? g.Index.CalcSize(groupKey) : 0;
                 g.NextBTreeIndexSize = WriteBTreeIndex ? g.BTreeIndex.CalcSize(groupKey) : 0;
 
                 // FIXME: not each row produces index row so overhead bytes shouldn't add index size
@@ -368,7 +366,7 @@ namespace NTable {
                 // N.B. non-main groups have no key
                 TCellsRef groupKey = groupIdx == 0 ? syntheticKey : TCellsRef{ };
                 g.NextDataSize = g.Data.CalcSize(groupKey, row, KeyState.Final, TRowVersion::Min(), maxVersion, /* txId */ 0);
-                g.NextIndexSize = g.Index.CalcSize(groupKey);
+                g.NextIndexSize = WriteFlatIndex ? g.Index.CalcSize(groupKey) : 0;
                 g.NextBTreeIndexSize = WriteBTreeIndex ? g.BTreeIndex.CalcSize(groupKey) : 0;
 
                 // FIXME: not each row produces index row so overhead bytes shouldn't add index size
@@ -442,7 +440,9 @@ namespace NTable {
                     ui32 smallRefs = 0;
                     ui32 largeRefs = 0;
                     for (auto& g : Groups) {
-                        indexSize += g.Index.BytesUsed() + g.FirstKeyIndexSize;
+                        if (WriteFlatIndex) {
+                            indexSize += g.Index.BytesUsed() + g.FirstKeyIndexSize;
+                        }
                         if (WriteBTreeIndex) {
                             indexSize += g.BTreeIndex.EstimateBytesUsed() + g.FirstKeyBTreeIndexSize;
                         }
@@ -506,24 +506,27 @@ namespace NTable {
                 WriteStats.HiddenRows += Current.HiddenRows;
                 WriteStats.HiddenDrops += Current.HiddenDrops;
 
-                if (Current.HistoryWritten > 0) {
-                    Current.HistoricIndexes.clear();
-                    Current.HistoricIndexes.reserve(Histories.size());
-                    for (auto& g : Histories) {
-                        Current.HistoricIndexes.push_back(WritePage(g.Index.Flush(), EPage::Index));
+                Current.HistoricIndexes.clear();
+                Current.GroupIndexes.clear();
+                Current.Index = Max<TPageId>();
+                if (WriteFlatIndex) {
+                    if (Current.HistoryWritten > 0) {
+                        Current.HistoricIndexes.reserve(Histories.size());
+                        for (auto& g : Histories) {
+                            Current.HistoricIndexes.push_back(WritePage(g.Index.Flush(), EPage::Index));
+                        }
                     }
-                }
 
-                if (Groups.size() > 1) {
-                    Current.GroupIndexes.clear();
-                    Current.GroupIndexes.reserve(Groups.size() - 1);
-                    for (ui32 group : xrange(ui32(1), ui32(Groups.size()))) {
-                        Current.GroupIndexes.push_back(WritePage(Groups[group].Index.Flush(), EPage::Index));
+                    if (Groups.size() > 1) {
+                        Current.GroupIndexes.reserve(Groups.size() - 1);
+                        for (ui32 group : xrange(ui32(1), ui32(Groups.size()))) {
+                            Current.GroupIndexes.push_back(WritePage(Groups[group].Index.Flush(), EPage::Index));
+                        }
                     }
-                }
-
-                Current.Index = WritePage(Groups[0].Index.Flush(), EPage::Index);
 
+                    Current.Index = WritePage(Groups[0].Index.Flush(), EPage::Index);
+                }
+                
                 Current.BTreeGroupIndexes.clear();
                 Current.BTreeHistoricIndexes.clear();
                 if (WriteBTreeIndex) {
@@ -618,7 +621,7 @@ namespace NTable {
                 if (!last || WriteStats.Parts > 0)
                     head = Max(head, ui32(20) /* Multiple part outputs */);
 
-                if (!Current.GroupIndexes.empty())
+                if (!Current.GroupIndexes.empty() || Current.BTreeGroupIndexes.size() > 1)
                     head = Max(head, ui32(26) /* Multiple column groups */);
 
                 if (Current.Versioned)
@@ -644,8 +647,9 @@ namespace NTable {
 
             if (auto *lay = proto.MutableLayout()) {
                 lay->SetScheme(Current.Scheme);
-                lay->SetIndex(Current.Index);
 
+                if (Current.Index != Max<TPageId>())
+                    lay->SetIndex(Current.Index);
                 if (Current.Globs != Max<TPageId>())
                     lay->SetGlobs(Current.Globs);
                 if (Current.Large != Max<TPageId>())
@@ -751,7 +755,7 @@ namespace NTable {
                  */
 
                 Y_ABORT_UNLESS(dataPage->Count, "Invalid EPage::DataPage blob");
-                TPgSize keySize = g.FirstKeyIndexSize;
+                TPgSize flatKeyIndexSize = g.FirstKeyIndexSize;
 
                 if (groupId.IsMain()) {
                     Y_DEBUG_ABORT_UNLESS(NextSliceFirstRowId != Max<TRowId>());
@@ -760,7 +764,7 @@ namespace NTable {
 
                     if (CutIndexKeys) {
                         CutKey(groupId);
-                        keySize = g.Index.CalcSize(Key);
+                        flatKeyIndexSize = WriteFlatIndex ? g.Index.CalcSize(Key) : 0;
                     }
                 } else if (groupId.Index == 0) {
                     InitKey(Key, dataPage->Record(0), groupId);
@@ -777,7 +781,7 @@ namespace NTable {
                 Current.Bytes += raw.size(); /* before encoding */
 
                 if (g.Codec == NPage::ECodec::Plain) {
-                    /* Ecoding was not enabled, keep as is */
+                    /* Encoding was not enabled, keep as is */
                 } else if (keep = Encode(raw, g.Codec, g.ForceCompression)) {
                     std::swap(raw, keep);
                 }
@@ -786,10 +790,12 @@ namespace NTable {
 
                 auto page = WritePage(raw, EPage::DataPage, groupId.Index);
 
-                // N.B. non-main groups have no key
-                Y_DEBUG_ABORT_UNLESS(g.Index.CalcSize(Key) == keySize);
+                if (WriteFlatIndex) {
+                    // N.B. non-main groups have no key
+                    Y_DEBUG_ABORT_UNLESS(g.Index.CalcSize(Key) == flatKeyIndexSize);
 
-                g.Index.Add(keySize, Key, dataPage.BaseRow(), page);
+                    g.Index.Add(flatKeyIndexSize, Key, dataPage.BaseRow(), page);
+                }
 
                 if (WriteBTreeIndex) {
                     if (dataPage.BaseRow()) {
@@ -815,8 +821,10 @@ namespace NTable {
                     SaveSlice(lastRowId, TSerializedCellVec(Key));
 
                     if (Phase == 1) {
-                        Y_DEBUG_ABORT_UNLESS(g.Index.CalcSize(Key) == g.LastKeyIndexSize);
-                        g.Index.Add(g.LastKeyIndexSize, Key, lastRowId, page);
+                        if (WriteFlatIndex) {
+                            Y_DEBUG_ABORT_UNLESS(g.Index.CalcSize(Key) == g.LastKeyIndexSize);
+                            g.Index.Add(g.LastKeyIndexSize, Key, lastRowId, page);
+                        }
                         Y_ABORT_UNLESS(std::exchange(Phase, 2) == 1);
                         PrevPageLastKey.clear(); // new index will be started
                         PrevPageData = { };
@@ -1024,6 +1032,7 @@ namespace NTable {
         const bool Final = false;
         const bool CutIndexKeys;
         const bool WriteBTreeIndex;
+        const bool WriteFlatIndex;
         const ui32 SmallEdge;
         const ui32 LargeEdge;
         const ui32 MaxLargeBlob;
diff --git a/ydb/core/tablet_flat/flat_table_part.h b/ydb/core/tablet_flat/flat_table_part.h
index 6ba390d99af..770f8d47177 100644
--- a/ydb/core/tablet_flat/flat_table_part.h
+++ b/ydb/core/tablet_flat/flat_table_part.h
@@ -49,11 +49,28 @@ namespace NTable {
 
         struct TIndexPages {
             using TPageId = NPage::TPageId;
+            using TGroupId = NPage::TGroupId;
             using TBtreeIndexMeta = NPage::TBtreeIndexMeta;
-            const TVector<TPageId> Groups;
-            const TVector<TPageId> Historic;
+            const TVector<TPageId> FlatGroups;
+            const TVector<TPageId> FlatHistoric;
             const TVector<TBtreeIndexMeta> BTreeGroups;
             const TVector<TBtreeIndexMeta> BTreeHistoric;
+
+            bool HasBTree() const noexcept {
+                return !BTreeGroups.empty();
+            }
+
+            bool HasFlat() const noexcept {
+                return !FlatGroups.empty();
+            }
+
+            const TBtreeIndexMeta& GetBTree(TGroupId groupId) const noexcept {
+                return groupId.IsHistoric() ? BTreeHistoric[groupId.Index] : BTreeGroups[groupId.Index];
+            }
+
+            TPageId GetFlat(TGroupId groupId) const noexcept {
+                return groupId.IsHistoric() ? FlatHistoric[groupId.Index] : FlatGroups[groupId.Index];
+            }
         };
 
         struct TParams {
@@ -92,8 +109,8 @@ namespace NTable {
             , GarbageStats(std::move(params.GarbageStats))
             , TxIdStats(std::move(params.TxIdStats))
             , Stat(stat)
-            , GroupsCount(IndexPages.Groups.size())
-            , HistoricGroupsCount(IndexPages.Historic.size())
+            , GroupsCount(Max(IndexPages.FlatGroups.size(), IndexPages.BTreeGroups.size()))
+            , HistoricGroupsCount(Max(IndexPages.FlatHistoric.size(), IndexPages.BTreeHistoric.size()))
             , IndexesRawSize(params.IndexesRawSize)
             , MinRowVersion(params.MinRowVersion)
             , MaxRowVersion(params.MaxRowVersion)
diff --git a/ydb/core/tablet_flat/ut/ut_charge.cpp b/ydb/core/tablet_flat/ut/ut_charge.cpp
index 76f9ccf1648..28d1648d8c1 100644
--- a/ydb/core/tablet_flat/ut/ut_charge.cpp
+++ b/ydb/core/tablet_flat/ut/ut_charge.cpp
@@ -322,10 +322,10 @@ namespace {
             auto &pages = Eggs.Lone()->IndexPages;
             TGroupId mainGroupId{};
             
-            for (auto x : pages.Groups) {
+            for (auto x : pages.FlatGroups) {
                 result.insert({mainGroupId, x});
             }
-            for (auto x : pages.Historic) {
+            for (auto x : pages.FlatHistoric) {
                 result.insert({mainGroupId, x});
             }
 
@@ -988,17 +988,17 @@ Y_UNIT_TEST_SUITE(Charge) {
 
             // no index => touch index
             me.To(100).CheckIndex(6, 22, 0, TMap<TGroupId, TArr>{
-                {TGroupId{0}, {pages.Groups[0]}},
+                {TGroupId{0}, {pages.FlatGroups[0]}},
             }, TSet<TPageId> {
 
             });
 
             // index => touch pages + index
             me.To(101).CheckIndex(6, 22, 0, TMap<TGroupId, TArr>{
-                {TGroupId{0}, {1, 2, 3, 4, 5, 6, pages.Groups[0]}}
+                {TGroupId{0}, {1, 2, 3, 4, 5, 6, pages.FlatGroups[0]}}
             },
             TSet<TPageId> {
-                pages.Groups[0]
+                pages.FlatGroups[0]
             });
         }
 
@@ -1008,7 +1008,7 @@ Y_UNIT_TEST_SUITE(Charge) {
 
             // no index => touch index
             me.To(200).CheckIndex(6, 22, 0, TMap<TGroupId, TArr>{
-                {TGroupId{0}, {pages.Groups[0]}},
+                {TGroupId{0}, {pages.FlatGroups[0]}},
                 {TGroupId{0, true}, {}}
             }, TSet<TPageId> {
 
@@ -1016,20 +1016,20 @@ Y_UNIT_TEST_SUITE(Charge) {
 
             // no history index => touch main pages + index + history index
             me.To(201).CheckIndex(6, 22, 0, TMap<TGroupId, TArr>{
-                {TGroupId{0}, {1, 2, 3, 4, 5, 6, pages.Groups[0], pages.Historic[0]}},
+                {TGroupId{0}, {1, 2, 3, 4, 5, 6, pages.FlatGroups[0], pages.FlatHistoric[0]}},
                 {TGroupId{0, true}, {}}
             },
             TSet<TPageId> {
-                pages.Groups[0]
+                pages.FlatGroups[0]
             });
 
             // history index => touch main pages + history pages + index + history index
             me.To(202).CheckIndex(6, 22, 0, TMap<TGroupId, TArr>{
-                {TGroupId{0}, {1, 2, 3, 4, 5, 6, pages.Groups[0], pages.Historic[0]}},
+                {TGroupId{0}, {1, 2, 3, 4, 5, 6, pages.FlatGroups[0], pages.FlatHistoric[0]}},
                 {TGroupId{0, true}, {1, 2, 3, 4}}
             },
             TSet<TPageId> {
-                pages.Groups[0], pages.Historic[0]
+                pages.FlatGroups[0], pages.FlatHistoric[0]
             });
         }
         
@@ -1039,7 +1039,7 @@ Y_UNIT_TEST_SUITE(Charge) {
 
             // no index => touch index
             me.To(300).CheckIndex(6, 22, 0, TMap<TGroupId, TArr>{
-                {TGroupId{0}, {pages.Groups[0]}},
+                {TGroupId{0}, {pages.FlatGroups[0]}},
                 {TGroupId{1}, {}}
             }, TSet<TPageId> {
 
@@ -1047,20 +1047,20 @@ Y_UNIT_TEST_SUITE(Charge) {
 
             // no groups index => touch main pages + index + all groups indexes
             me.To(301).CheckIndex(6, 22, 0, TMap<TGroupId, TArr>{
-                {TGroupId{0}, {1, 2, 3, 4, 5, 6, pages.Groups[0], pages.Groups[1], pages.Groups[2], pages.Groups[3]}},
+                {TGroupId{0}, {1, 2, 3, 4, 5, 6, pages.FlatGroups[0], pages.FlatGroups[1], pages.FlatGroups[2], pages.FlatGroups[3]}},
                 {TGroupId{1}, {}}
             },
             TSet<TPageId> {
-                pages.Groups[0]
+                pages.FlatGroups[0]
             });
 
             // groups index => touch all pages + index + all groups indexes
             me.To(302).CheckIndex(6, 22, 0, TMap<TGroupId, TArr>{
-                {TGroupId{0}, {1, 2, 3, 4, 5, 6, pages.Groups[0], pages.Groups[1], pages.Groups[2], pages.Groups[3]}},
+                {TGroupId{0}, {1, 2, 3, 4, 5, 6, pages.FlatGroups[0], pages.FlatGroups[1], pages.FlatGroups[2], pages.FlatGroups[3]}},
                 {TGroupId{1}, {3, 4, 5, 6, 7}}
             },
             TSet<TPageId> {
-                pages.Groups[0], pages.Groups[1]
+                pages.FlatGroups[0], pages.FlatGroups[1]
             });
         }
 
@@ -1070,7 +1070,7 @@ Y_UNIT_TEST_SUITE(Charge) {
 
             // no index => touch index
             me.To(400).CheckIndex(6, 22, 0, TMap<TGroupId, TArr>{
-                {TGroupId{0}, {pages.Groups[0]}},
+                {TGroupId{0}, {pages.FlatGroups[0]}},
                 {TGroupId{0, true}, {}},
                 {TGroupId{1}, {}},
                 {TGroupId{1, true}, {}},
@@ -1082,7 +1082,7 @@ Y_UNIT_TEST_SUITE(Charge) {
 
             // only index => touch main pages + index + all groups indexes + history index
             me.To(401).CheckIndex(6, 22, 0, TMap<TGroupId, TArr>{
-                {TGroupId{0}, {1, 2, 3, 4, 5, 6, pages.Groups[0], pages.Historic[0], pages.Groups[1], pages.Groups[2], pages.Groups[3]}},
+                {TGroupId{0}, {1, 2, 3, 4, 5, 6, pages.FlatGroups[0], pages.FlatHistoric[0], pages.FlatGroups[1], pages.FlatGroups[2], pages.FlatGroups[3]}},
                 {TGroupId{0, true}, {}},
                 {TGroupId{1}, {}},
                 {TGroupId{1, true}, {}},
@@ -1090,13 +1090,13 @@ Y_UNIT_TEST_SUITE(Charge) {
                 {TGroupId{2, true}, {}}
             },
             TSet<TPageId> {
-                pages.Groups[0]
+                pages.FlatGroups[0]
             });
 
             // history index => touch main pages + index + all groups indexes + main history pages
             me.To(402).CheckIndex(6, 22, 0, TMap<TGroupId, TArr>{
-                {TGroupId{0}, {1, 2, 3, 4, 5, 6, pages.Groups[0], 
-                    pages.Historic[0], pages.Historic[1], pages.Historic[2], pages.Historic[3], pages.Groups[1], pages.Groups[2], pages.Groups[3]}},
+                {TGroupId{0}, {1, 2, 3, 4, 5, 6, pages.FlatGroups[0], 
+                    pages.FlatHistoric[0], pages.FlatHistoric[1], pages.FlatHistoric[2], pages.FlatHistoric[3], pages.FlatGroups[1], pages.FlatGroups[2], pages.FlatGroups[3]}},
                 {TGroupId{0, true}, {1, 2, 3, 4}},
                 {TGroupId{1}, {}},
                 {TGroupId{1, true}, {}},
@@ -1104,13 +1104,13 @@ Y_UNIT_TEST_SUITE(Charge) {
                 {TGroupId{2, true}, {}}
             },
             TSet<TPageId> {
-                pages.Groups[0], pages.Historic[0]
+                pages.FlatGroups[0], pages.FlatHistoric[0]
             });
 
             // main history and history => touch main pages + index + all groups indexes + history pages + history groups pages
             me.To(403).CheckIndex(6, 22, 0, TMap<TGroupId, TArr>{
-                {TGroupId{0}, {1, 2, 3, 4, 5, 6, pages.Groups[0], 
-                    pages.Historic[0], pages.Historic[1], pages.Historic[2], pages.Historic[3], pages.Groups[1], pages.Groups[2], pages.Groups[3]}},
+                {TGroupId{0}, {1, 2, 3, 4, 5, 6, pages.FlatGroups[0], 
+                    pages.FlatHistoric[0], pages.FlatHistoric[1], pages.FlatHistoric[2], pages.FlatHistoric[3], pages.FlatGroups[1], pages.FlatGroups[2], pages.FlatGroups[3]}},
                 {TGroupId{0, true}, {1, 2, 3, 4}},
                 {TGroupId{1}, {}},
                 {TGroupId{1, true}, {3, 4, 5}},
@@ -1118,13 +1118,13 @@ Y_UNIT_TEST_SUITE(Charge) {
                 {TGroupId{2, true}, {}}
             },
             TSet<TPageId> {
-                pages.Groups[0], pages.Historic[0], pages.Historic[1] 
+                pages.FlatGroups[0], pages.FlatHistoric[0], pages.FlatHistoric[1] 
             });
 
             // groups index => touch main pages + index + history index + all groups indexes + groups pages
             me.To(404).CheckIndex(6, 22, 0, TMap<TGroupId, TArr>{
-                {TGroupId{0}, {1, 2, 3, 4, 5, 6, pages.Groups[0], 
-                    pages.Historic[0], pages.Groups[1], pages.Groups[2], pages.Groups[3]}},
+                {TGroupId{0}, {1, 2, 3, 4, 5, 6, pages.FlatGroups[0], 
+                    pages.FlatHistoric[0], pages.FlatGroups[1], pages.FlatGroups[2], pages.FlatGroups[3]}},
                 {TGroupId{0, true}, {}},
                 {TGroupId{1}, {3, 4, 5, 6, 7}},
                 {TGroupId{1, true}, {}},
@@ -1132,13 +1132,13 @@ Y_UNIT_TEST_SUITE(Charge) {
                 {TGroupId{2, true}, {}}
             },
             TSet<TPageId> {
-                pages.Groups[0], pages.Groups[1]
+                pages.FlatGroups[0], pages.FlatGroups[1]
             });
 
             // main history and groups => touch main pages + index + all groups indexes + groups pages + history main pages
             me.To(405).CheckIndex(6, 22, 0, TMap<TGroupId, TArr>{
-                {TGroupId{0}, {1, 2, 3, 4, 5, 6, pages.Groups[0], 
-                    pages.Historic[0], pages.Historic[1], pages.Historic[2], pages.Historic[3], pages.Groups[1], pages.Groups[2], pages.Groups[3]}},
+                {TGroupId{0}, {1, 2, 3, 4, 5, 6, pages.FlatGroups[0], 
+                    pages.FlatHistoric[0], pages.FlatHistoric[1], pages.FlatHistoric[2], pages.FlatHistoric[3], pages.FlatGroups[1], pages.FlatGroups[2], pages.FlatGroups[3]}},
                 {TGroupId{0, true}, {1, 2, 3, 4}},
                 {TGroupId{1}, {3, 4, 5, 6, 7}},
                 {TGroupId{1, true}, {}},
@@ -1146,13 +1146,13 @@ Y_UNIT_TEST_SUITE(Charge) {
                 {TGroupId{2, true}, {}}
             },
             TSet<TPageId> {
-                pages.Groups[0], pages.Historic[0], pages.Groups[1]
+                pages.FlatGroups[0], pages.FlatHistoric[0], pages.FlatGroups[1]
             });
 
             // all indexes
             me.To(406).CheckIndex(6, 22, 0, TMap<TGroupId, TArr>{
-                {TGroupId{0}, {1, 2, 3, 4, 5, 6, pages.Groups[0], 
-                    pages.Historic[0], pages.Historic[1], pages.Historic[2], pages.Historic[3], pages.Groups[1], pages.Groups[2], pages.Groups[3]}},
+                {TGroupId{0}, {1, 2, 3, 4, 5, 6, pages.FlatGroups[0], 
+                    pages.FlatHistoric[0], pages.FlatHistoric[1], pages.FlatHistoric[2], pages.FlatHistoric[3], pages.FlatGroups[1], pages.FlatGroups[2], pages.FlatGroups[3]}},
                 {TGroupId{0, true}, {1, 2, 3, 4}},
                 {TGroupId{1}, {3, 4, 5, 6, 7}},
                 {TGroupId{1, true}, {3, 4, 5}},
@@ -1160,7 +1160,7 @@ Y_UNIT_TEST_SUITE(Charge) {
                 {TGroupId{2, true}, {}}
             },
             TSet<TPageId> {
-                pages.Groups[0], pages.Historic[0], pages.Historic[1], pages.Groups[1]
+                pages.FlatGroups[0], pages.FlatHistoric[0], pages.FlatHistoric[1], pages.FlatGroups[1]
             });
         }
     }
diff --git a/ydb/core/tablet_flat/ut/ut_part.cpp b/ydb/core/tablet_flat/ut/ut_part.cpp
index 51203cb2b0f..e62df79fc9e 100644
--- a/ydb/core/tablet_flat/ut/ut_part.cpp
+++ b/ydb/core/tablet_flat/ut/ut_part.cpp
@@ -402,7 +402,7 @@ Y_UNIT_TEST_SUITE(TPart) {
         /*_ Ensure that produced part has enough pages for code coverage and
             index grow algorithm in data pages writer has been triggered. */
 
-        UNIT_ASSERT(pages > 100 && part.GetPageSize(part.IndexPages.Groups[0], { }) >= minIndex);
+        UNIT_ASSERT(pages > 100 && part.GetPageSize(part.IndexPages.GetFlat({}), { }) >= minIndex);
 
         { /*_ Ensure that part has some external blobs written to room 1 */
             auto one = Eggs0().Lone()->Blobs->Total();
diff --git a/ydb/core/tablet_flat/ut_large/ut_btree_index_large.cpp b/ydb/core/tablet_flat/ut_large/ut_btree_index_large.cpp
index 3af2bfb805a..c38cb94fe3d 100644
--- a/ydb/core/tablet_flat/ut_large/ut_btree_index_large.cpp
+++ b/ydb/core/tablet_flat/ut_large/ut_btree_index_large.cpp
@@ -34,7 +34,7 @@ Y_UNIT_TEST_SUITE(TBtreeIndexTPartLarge) {
         const auto part = eggs.Lone();
 
         Cerr << "DataBytes = " << part->Stat.Bytes << " DataPages = " << IndexTools::CountMainPages(*part) << Endl;
-        Cerr << "FlatIndexBytes = " << part->GetPageSize(part->IndexPages.Groups[0], {}) << " BTreeIndexBytes = " << part->IndexPages.BTreeGroups[0].IndexSize << Endl;
+        Cerr << "FlatIndexBytes = " << part->GetPageSize(part->IndexPages.FlatGroups[0], {}) << " BTreeIndexBytes = " << part->IndexPages.BTreeGroups[0].IndexSize << Endl;
 
         UNIT_ASSERT_GE(part->Stat.Bytes, 1ull*1024*1024*1024);
         UNIT_ASSERT_LE(part->Stat.Bytes, 1ull*1024*1024*1024 + 100*1024*1024);
@@ -64,7 +64,7 @@ Y_UNIT_TEST_SUITE(TBtreeIndexTPartLarge) {
         const auto part = eggs.Lone();
 
         Cerr << "DataBytes = " << part->Stat.Bytes << " DataPages = " << IndexTools::CountMainPages(*part) << Endl;
-        Cerr << "FlatIndexBytes = " << part->GetPageSize(part->IndexPages.Groups[0], {}) << " BTreeIndexBytes = " << part->IndexPages.BTreeGroups[0].IndexSize << Endl;
+        Cerr << "FlatIndexBytes = " << part->GetPageSize(part->IndexPages.FlatGroups[0], {}) << " BTreeIndexBytes = " << part->IndexPages.BTreeGroups[0].IndexSize << Endl;
 
         UNIT_ASSERT_GE(part->Stat.Bytes, 1ull*1024*1024*1024);
         UNIT_ASSERT_LE(part->Stat.Bytes, 1ull*1024*1024*1024 + 100*1024*1024);
@@ -94,7 +94,7 @@ Y_UNIT_TEST_SUITE(TBtreeIndexTPartLarge) {
         const auto part = eggs.Lone();
 
         Cerr << "DataBytes = " << part->Stat.Bytes << " DataPages = " << IndexTools::CountMainPages(*part) << Endl;
-        Cerr << "FlatIndexBytes = " << part->GetPageSize(part->IndexPages.Groups[0], {}) << " BTreeIndexBytes = " << part->IndexPages.BTreeGroups[0].IndexSize << Endl;
+        Cerr << "FlatIndexBytes = " << part->GetPageSize(part->IndexPages.FlatGroups[0], {}) << " BTreeIndexBytes = " << part->IndexPages.BTreeGroups[0].IndexSize << Endl;
 
         UNIT_ASSERT_GE(part->Stat.Bytes, 1ull*1024*1024*1024);
         UNIT_ASSERT_LE(part->Stat.Bytes, 1ull*1024*1024*1024 + 100*1024*1024);
@@ -126,7 +126,7 @@ Y_UNIT_TEST_SUITE(TBtreeIndexTPartLarge) {
         const auto part = eggs.Lone();
 
         Cerr << "DataBytes = " << part->Stat.Bytes << " DataPages = " << IndexTools::CountMainPages(*part) << Endl;
-        Cerr << "FlatIndexBytes = " << part->GetPageSize(part->IndexPages.Groups[0], {}) << " BTreeIndexBytes = " << part->IndexPages.BTreeGroups[0].IndexSize << Endl;
+        Cerr << "FlatIndexBytes = " << part->GetPageSize(part->IndexPages.FlatGroups[0], {}) << " BTreeIndexBytes = " << part->IndexPages.BTreeGroups[0].IndexSize << Endl;
 
         UNIT_ASSERT_GE(part->Stat.Bytes, 1ull*1024*1024*1024);
         UNIT_ASSERT_LE(part->Stat.Bytes, 1ull*1024*1024*1024 + 100*1024*1024);
@@ -157,7 +157,7 @@ Y_UNIT_TEST_SUITE(TBtreeIndexTPartLarge) {
         const auto part = eggs.Lone();
 
         Cerr << "DataBytes = " << part->Stat.Bytes << " DataPages = " << IndexTools::CountMainPages(*part) << Endl;
-        Cerr << "FlatIndexBytes = " << part->GetPageSize(part->IndexPages.Groups[1], {}) << " BTreeIndexBytes = " << part->IndexPages.BTreeGroups[1].IndexSize << Endl;
+        Cerr << "FlatIndexBytes = " << part->GetPageSize(part->IndexPages.FlatGroups[1], {}) << " BTreeIndexBytes = " << part->IndexPages.BTreeGroups[1].IndexSize << Endl;
 
         UNIT_ASSERT_GE(part->Stat.Bytes, 1ull*1024*1024*1024);
         UNIT_ASSERT_LE(part->Stat.Bytes, 1ull*1024*1024*1024 + 100*1024*1024);
@@ -188,7 +188,7 @@ Y_UNIT_TEST_SUITE(TBtreeIndexTPartLarge) {
         const auto part = eggs.Lone();
 
         Cerr << "DataBytes = " << part->Stat.Bytes << " DataPages = " << IndexTools::CountMainPages(*part) << Endl;
-        Cerr << "FlatIndexBytes = " << part->GetPageSize(part->IndexPages.Historic[0], {}) << " BTreeIndexBytes = " << part->IndexPages.BTreeHistoric[0].IndexSize << Endl;
+        Cerr << "FlatIndexBytes = " << part->GetPageSize(part->IndexPages.FlatHistoric[0], {}) << " BTreeIndexBytes = " << part->IndexPages.BTreeHistoric[0].IndexSize << Endl;
 
         UNIT_ASSERT_GE(part->Stat.Bytes, 1ull*1024*1024*1024);
         UNIT_ASSERT_LE(part->Stat.Bytes, 1ull*1024*1024*1024 + 100*1024*1024);