diff --git a/bench/micro/Bench/Database/LSMTree/Internal/Merge.hs b/bench/micro/Bench/Database/LSMTree/Internal/Merge.hs
index 23a3e4adc..99286465f 100644
--- a/bench/micro/Bench/Database/LSMTree/Internal/Merge.hs
+++ b/bench/micro/Bench/Database/LSMTree/Internal/Merge.hs
@@ -18,6 +18,7 @@ import           Database.LSMTree.Extras.RunData
 import           Database.LSMTree.Extras.UTxO
 import           Database.LSMTree.Internal.Entry
 import qualified Database.LSMTree.Internal.Merge as Merge
+import           Database.LSMTree.Internal.Merge (MergeType (..))
 import           Database.LSMTree.Internal.Paths (RunFsPaths (..),
                      pathsForRunFiles, runChecksumsPath)
 import           Database.LSMTree.Internal.Run (Run)
@@ -106,7 +107,7 @@ benchmarks = bgroup "Bench.Database.LSMTree.Internal.Merge" [
         { name         = "word64-delete-x4-lastlevel"
         , nentries     = totalEntries `splitInto` 4
         , fdeletes     = 1
-        , mergeLevel   = Merge.LastLevel
+        , mergeType    = MergeLastLevel
         }
       -- different key and value sizes
     , benchMerge configWord64
@@ -157,21 +158,21 @@ benchmarks = bgroup "Bench.Database.LSMTree.Internal.Merge" [
         , nentries     = totalEntries `splitInto` 4
         , finserts     = 1
         , fdeletes     = 1
-        , mergeLevel   = Merge.LastLevel
+        , mergeType    = MergeLastLevel
         }
     , benchMerge configUTxO
         { name         = "utxo-x4+1-min-skewed-lastlevel"  -- live levelling merge
         , nentries     = totalEntries `distributed` [1, 1, 1, 1, 4]
         , finserts     = 1
         , fdeletes     = 1
-        , mergeLevel   = Merge.LastLevel
+        , mergeType    = MergeLastLevel
         }
     , benchMerge configUTxO
         { name         = "utxo-x4+1-max-skewed-lastlevel"  -- live levelling merge
         , nentries     = totalEntries `distributed` [1, 1, 1, 1, 16]
         , finserts     = 1
         , fdeletes     = 1
-        , mergeLevel   = Merge.LastLevel
+        , mergeType    = MergeLastLevel
         }
     ]
   where
@@ -233,7 +234,8 @@ merge ::
 merge fs hbio Config {..} targetPaths runs = do
     let f = fromMaybe const mergeMappend
     m <- fromMaybe (error "empty inputs, no merge created") <$>
-      Merge.new fs hbio Run.CacheRunData (RunAllocFixed 10) mergeLevel f targetPaths runs
+      Merge.new fs hbio Run.CacheRunData (RunAllocFixed 10)
+                mergeType f targetPaths runs
     Merge.stepsToCompletion m stepSize
 
 outputRunPaths :: Run.RunFsPaths
@@ -271,7 +273,7 @@ data Config = Config {
   , randomKey    :: Rnd SerialisedKey
   , randomValue  :: Rnd SerialisedValue
   , randomBlob   :: Rnd SerialisedBlob
-  , mergeLevel   :: !Merge.Level
+  , mergeType    :: !MergeType
     -- | Needs to be defined when generating mupserts.
   , mergeMappend :: !(Maybe Mappend)
     -- | Merging is done in chunks of @stepSize@ entries.
@@ -291,7 +293,7 @@ defaultConfig = Config {
   , randomKey    = error "randomKey not implemented"
   , randomValue  = error "randomValue not implemented"
   , randomBlob   = error "randomBlob not implemented"
-  , mergeLevel   = Merge.MidLevel
+  , mergeType    = MergeMidLevel
   , mergeMappend = Nothing
   , stepSize     = maxBound  -- by default, just do in one go
   }
diff --git a/src-extras/Database/LSMTree/Extras/Generators.hs b/src-extras/Database/LSMTree/Extras/Generators.hs
index 67f5cff85..232e55043 100644
--- a/src-extras/Database/LSMTree/Extras/Generators.hs
+++ b/src-extras/Database/LSMTree/Extras/Generators.hs
@@ -53,7 +53,7 @@ import           Database.LSMTree.Extras.Index (Append (..))
 import           Database.LSMTree.Extras.Orphans ()
 import           Database.LSMTree.Internal.BlobRef (BlobSpan (..))
 import           Database.LSMTree.Internal.Entry (Entry (..), NumEntries (..))
-import qualified Database.LSMTree.Internal.Merge as Merge
+import           Database.LSMTree.Internal.Merge (MergeType (..))
 import           Database.LSMTree.Internal.Page (PageNo (..))
 import           Database.LSMTree.Internal.RawBytes as RB
 import           Database.LSMTree.Internal.Serialise
@@ -555,7 +555,8 @@ instance Arbitrary BlobSpan where
   Merge
 -------------------------------------------------------------------------------}
 
-instance Arbitrary Merge.Level where
-  arbitrary = QC.elements [Merge.MidLevel, Merge.LastLevel]
-  shrink Merge.LastLevel = [Merge.MidLevel]
-  shrink Merge.MidLevel  = []
+instance Arbitrary MergeType where
+  arbitrary = QC.elements [MergeMidLevel, MergeLastLevel, MergeUnion]
+  shrink MergeMidLevel  = []
+  shrink MergeLastLevel = [MergeMidLevel]
+  shrink MergeUnion     = [MergeLastLevel]
diff --git a/src-extras/Database/LSMTree/Extras/NoThunks.hs b/src-extras/Database/LSMTree/Extras/NoThunks.hs
index b0ff43c18..f158edd7d 100644
--- a/src-extras/Database/LSMTree/Extras/NoThunks.hs
+++ b/src-extras/Database/LSMTree/Extras/NoThunks.hs
@@ -42,7 +42,7 @@ import           Database.LSMTree.Internal.CRC32C
 import           Database.LSMTree.Internal.Entry
 import           Database.LSMTree.Internal.Index.Compact
 import           Database.LSMTree.Internal.Index.CompactAcc
-import           Database.LSMTree.Internal.Merge hiding (Level)
+import           Database.LSMTree.Internal.Merge
 import qualified Database.LSMTree.Internal.Merge as Merge
 import           Database.LSMTree.Internal.MergeSchedule
 import           Database.LSMTree.Internal.MergingRun
@@ -377,8 +377,8 @@ deriving stock instance Generic (Merge m h)
 deriving anyclass instance (Typeable m, Typeable (PrimState m), Typeable h)
                         => NoThunks (Merge m h)
 
-deriving stock instance Generic Merge.Level
-deriving anyclass instance NoThunks Merge.Level
+deriving stock instance Generic MergeType
+deriving anyclass instance NoThunks MergeType
 
 deriving stock instance Generic Merge.StepResult
 deriving anyclass instance NoThunks Merge.StepResult
diff --git a/src/Database/LSMTree/Internal/Merge.hs b/src/Database/LSMTree/Internal/Merge.hs
index b779e5b6d..6b751648f 100644
--- a/src/Database/LSMTree/Internal/Merge.hs
+++ b/src/Database/LSMTree/Internal/Merge.hs
@@ -3,7 +3,7 @@
 -- concurrency primitive, such as an 'MVar'.
 module Database.LSMTree.Internal.Merge (
     Merge (..)
-  , Level (..)
+  , MergeType (..)
   , Mappend
   , MergeState (..)
   , new
@@ -46,7 +46,7 @@ import           System.FS.BlockIO.API (HasBlockIO)
 -- Since we always resolve all entries of the same key in one go, there is no
 -- need to store incompletely-resolved entries.
 data Merge m h = Merge {
-      mergeLevel      :: !Level
+      mergeType       :: !MergeType
     , mergeMappend    :: !Mappend
     , mergeReaders    :: {-# UNPACK #-} !(Readers m h)
     , mergeBuilder    :: !(RunBuilder m h)
@@ -72,12 +72,25 @@ data MergeState =
     -- | The merge was closed before it was completed.
   | Closed
 
-data Level = MidLevel | LastLevel
+-- | Merges can either exist on a level of the LSM, or be a union merge of two
+-- tables.
+--
+-- A last level merge behaves differently from a mid-level merge: last level
+-- merges can actually remove delete operations, whereas mid-level merges must
+-- preserve them.
+--
+-- Union merges follow the semantics of @Data.Map.unionWith (<>)@. Since the
+-- input runs are semantically treated like @Data.Map@s, deletes are ignored
+-- and inserts act like mupserts, so they need to be merged monoidally using
+-- 'resolveValue'.
+--
+data MergeType = MergeMidLevel | MergeLastLevel | MergeUnion
   deriving stock (Eq, Show)
 
-instance NFData Level where
-  rnf MidLevel  = ()
-  rnf LastLevel = ()
+instance NFData MergeType where
+  rnf MergeMidLevel  = ()
+  rnf MergeLastLevel = ()
+  rnf MergeUnion     = ()
 
 type Mappend = SerialisedValue -> SerialisedValue -> SerialisedValue
 
@@ -86,7 +99,7 @@ type Mappend = SerialisedValue -> SerialisedValue -> SerialisedValue
   -> HasBlockIO IO h
   -> RunDataCaching
   -> RunBloomFilterAlloc
-  -> Level
+  -> MergeType
   -> Mappend
   -> Run.RunFsPaths
   -> V.Vector (Ref (Run IO h))
@@ -99,12 +112,12 @@ new ::
   -> HasBlockIO m h
   -> RunDataCaching
   -> RunBloomFilterAlloc
-  -> Level
+  -> MergeType
   -> Mappend
   -> Run.RunFsPaths
   -> V.Vector (Ref (Run m h))
   -> m (Maybe (Merge m h))
-new fs hbio mergeCaching alloc mergeLevel mergeMappend targetPaths runs = do
+new fs hbio mergeCaching alloc mergeType mergeMappend targetPaths runs = do
     -- no offset, no write buffer
     mreaders <- Readers.new Readers.NoOffsetKey Nothing runs
     for mreaders $ \mergeReaders -> do
@@ -265,7 +278,7 @@ steps Merge {..} requestedSteps = assertStepsInvariant <$> do
               handleEntry (n + 1) key entry
             Readers.Drained -> do
               -- no future entries, no previous entry to resolve, just write!
-              writeReaderEntry mergeLevel mergeBuilder key entry
+              writeReaderEntry mergeType mergeBuilder key entry
               writeMutVar mergeState $! MergingDone
               pure (n + 1, MergeDone)
 
@@ -277,7 +290,7 @@ steps Merge {..} requestedSteps = assertStepsInvariant <$> do
         handleMupdate n key (Reader.appendOverflow len overflowPages v)
     handleEntry !n !key entry = do
         -- otherwise, we can just drop all following entries of same key
-        writeReaderEntry mergeLevel mergeBuilder key entry
+        writeReaderEntry mergeType mergeBuilder key entry
         dropRemaining n key
 
     -- the value is from a mupsert, complete (not just a prefix)
@@ -286,7 +299,7 @@ steps Merge {..} requestedSteps = assertStepsInvariant <$> do
         if nextKey /= key
           then do
             -- resolved all entries for this key, write it
-            writeSerialisedEntry mergeLevel mergeBuilder key (Mupdate v)
+            writeSerialisedEntry mergeType mergeBuilder key (Mupdate v)
             go n
           else do
             (_, nextEntry, hasMore) <- Readers.pop mergeReaders
@@ -301,10 +314,10 @@ steps Merge {..} requestedSteps = assertStepsInvariant <$> do
                   handleMupdate (n + 1) key v'
                 _ -> do
                   -- done with this key, now the remaining entries are obsolete
-                  writeSerialisedEntry mergeLevel mergeBuilder key resolved
+                  writeSerialisedEntry mergeType mergeBuilder key resolved
                   dropRemaining (n + 1) key
               Readers.Drained -> do
-                writeSerialisedEntry mergeLevel mergeBuilder key resolved
+                writeSerialisedEntry mergeType mergeBuilder key resolved
                 writeMutVar mergeState $! MergingDone
                 pure (n + 1, MergeDone)
 
@@ -317,19 +330,19 @@ steps Merge {..} requestedSteps = assertStepsInvariant <$> do
             pure (n + dropped, MergeDone)
 
 {-# SPECIALISE writeReaderEntry ::
-     Level
+     MergeType
   -> RunBuilder IO h
   -> SerialisedKey
   -> Reader.Entry IO h
   -> IO () #-}
 writeReaderEntry ::
      (MonadSTM m, MonadST m, MonadThrow m)
-  => Level
+  => MergeType
   -> RunBuilder m h
   -> SerialisedKey
   -> Reader.Entry m h
   -> m ()
-writeReaderEntry level builder key (Reader.Entry entryFull) =
+writeReaderEntry mergeType builder key (Reader.Entry entryFull) =
       -- Small entry.
       -- Note that this small entry could be the only one on the page. We only
       -- care about it being small, not single-entry, since it could still end
@@ -339,10 +352,10 @@ writeReaderEntry level builder key (Reader.Entry entryFull) =
       -- entry of a page (which would for example happen a lot if most entries
       -- have 2k-4k bytes). In that case we could have copied the RawPage
       -- (but we find out too late to easily exploit it).
-      writeSerialisedEntry level builder key entryFull
-writeReaderEntry level builder key entry@(Reader.EntryOverflow prefix page _ overflowPages)
+      writeSerialisedEntry mergeType builder key entryFull
+writeReaderEntry mergeType builder key entry@(Reader.EntryOverflow prefix page _ overflowPages)
   | InsertWithBlob {} <- prefix =
-      assert (shouldWriteEntry level prefix) $ do -- large, can't be delete
+      assert (shouldWriteEntry prefix mergeType) $ do -- large, can't be delete
         -- has blob, we can't just copy the first page, fall back
         -- we simply append the overflow pages to the value
         Builder.addKeyOp builder key (Reader.toFullEntry entry)
@@ -352,30 +365,30 @@ writeReaderEntry level builder key entry@(Reader.EntryOverflow prefix page _ ove
         -- 2. write a RawPage + SerialisedBlob + [RawOverflowPage], rewriting
         --      the raw page's blob offset (slightly faster, but a bit hacky)
   | otherwise =
-      assert (shouldWriteEntry level prefix) $  -- large, can't be delete
+      assert (shouldWriteEntry prefix mergeType) $  -- large, can't be delete
         -- no blob, directly copy all pages as they are
         Builder.addLargeSerialisedKeyOp builder key page overflowPages
 
 {-# SPECIALISE writeSerialisedEntry ::
-     Level
+     MergeType
   -> RunBuilder IO h
   -> SerialisedKey
   -> Entry SerialisedValue (RawBlobRef IO h)
   -> IO () #-}
 writeSerialisedEntry ::
      (MonadSTM m, MonadST m, MonadThrow m)
-  => Level
+  => MergeType
   -> RunBuilder m h
   -> SerialisedKey
   -> Entry SerialisedValue (RawBlobRef m h)
   -> m ()
-writeSerialisedEntry level builder key entry =
-    when (shouldWriteEntry level entry) $
+writeSerialisedEntry mergeType builder key entry =
+    when (shouldWriteEntry entry mergeType) $
       Builder.addKeyOp builder key entry
 
 -- One the last level we could also turn Mupdate into Insert,
 -- but no need to complicate things.
-shouldWriteEntry :: Level -> Entry v b -> Bool
-shouldWriteEntry level = \case
-    Delete -> level == MidLevel
-    _      -> True
+shouldWriteEntry :: Entry v b -> MergeType -> Bool
+shouldWriteEntry Delete MergeLastLevel = False
+shouldWriteEntry Delete MergeUnion     = False
+shouldWriteEntry _      _              = True
diff --git a/src/Database/LSMTree/Internal/MergeSchedule.hs b/src/Database/LSMTree/Internal/MergeSchedule.hs
index f20d23dd2..33eeeb58f 100644
--- a/src/Database/LSMTree/Internal/MergeSchedule.hs
+++ b/src/Database/LSMTree/Internal/MergeSchedule.hs
@@ -46,7 +46,7 @@ import           Database.LSMTree.Internal.Entry (Entry, NumEntries (..),
                      unNumEntries)
 import           Database.LSMTree.Internal.Index.Compact (IndexCompact)
 import           Database.LSMTree.Internal.Lookup (ResolveSerialisedValue)
-import qualified Database.LSMTree.Internal.Merge as Merge
+import           Database.LSMTree.Internal.Merge (MergeType (..))
 import           Database.LSMTree.Internal.MergingRun (MergePolicyForLevel (..),
                      MergingRun, NumRuns (..))
 import qualified Database.LSMTree.Internal.MergingRun as MR
@@ -92,7 +92,7 @@ data MergeTrace =
       RunDataCaching
       RunBloomFilterAlloc
       MergePolicyForLevel
-      Merge.Level
+      MergeType
   | TraceCompletedMerge  -- TODO: currently not traced for Incremental merges
       NumEntries -- ^ Size of output run
       RunNumber
@@ -572,7 +572,7 @@ addRunToLevels tr conf@TableConfig{..} resolve hfs hbio root uc r0 reg levels =
         traceWith tr $ AtLevel ln TraceAddLevel
         -- Make a new level
         let policyForLevel = mergePolicyForLevel confMergePolicy ln V.empty
-        ir <- newMerge policyForLevel Merge.LastLevel ln rs
+        ir <- newMerge policyForLevel MergeLastLevel ln rs
         return $! V.singleton $ Level ir V.empty
     go !ln rs' (V.uncons -> Just (Level ir rs, ls)) = do
         r <- expectCompletedMerge ln ir
@@ -588,7 +588,7 @@ addRunToLevels tr conf@TableConfig{..} resolve hfs hbio root uc r0 reg levels =
           -- as a bundle and move them down to the level below. We start a merge
           -- for the new incoming runs. This level is otherwise empty.
           LevelTiering | levelIsFull confSizeRatio rs -> do
-            ir' <- newMerge LevelTiering Merge.MidLevel ln rs'
+            ir' <- newMerge LevelTiering MergeMidLevel ln rs'
             ls' <- go (succ ln) (r `V.cons` rs) ls
             pure $! Level ir' V.empty `V.cons` ls'
           -- This tiering level is not yet full. We move the completed merged run
@@ -607,13 +607,13 @@ addRunToLevels tr conf@TableConfig{..} resolve hfs hbio root uc r0 reg levels =
           -- empty) level .
           LevelLevelling | Run.size r > maxRunSize' conf LevelLevelling ln -> do
             assert (V.null rs && V.null ls) $ pure ()
-            ir' <- newMerge LevelTiering Merge.MidLevel ln rs'
+            ir' <- newMerge LevelTiering MergeMidLevel ln rs'
             ls' <- go (succ ln) (V.singleton r) V.empty
             pure $! Level ir' V.empty `V.cons` ls'
           -- Otherwise we start merging the incoming runs into the run.
           LevelLevelling -> do
             assert (V.null rs && V.null ls) $ pure ()
-            ir' <- newMerge LevelLevelling Merge.LastLevel ln (rs' `V.snoc` r)
+            ir' <- newMerge LevelLevelling MergeLastLevel ln (rs' `V.snoc` r)
             pure $! Level ir' V.empty `V.cons` V.empty
 
     -- Releases the incoming run.
@@ -631,11 +631,11 @@ addRunToLevels tr conf@TableConfig{..} resolve hfs hbio root uc r0 reg levels =
 
     -- Releases the runs.
     newMerge :: MergePolicyForLevel
-             -> Merge.Level
+             -> MergeType
              -> LevelNo
              -> V.Vector (Ref (Run m h))
              -> m (IncomingRun m h)
-    newMerge mergePolicy mergeLevel ln rs
+    newMerge mergePolicy mergeType ln rs
       | Just (r, rest) <- V.uncons rs
       , V.null rest = do
           traceWith tr $ AtLevel ln $
@@ -654,11 +654,11 @@ addRunToLevels tr conf@TableConfig{..} resolve hfs hbio root uc r0 reg levels =
             !alloc = bloomFilterAllocForLevel conf ln
             !runPaths = Paths.runPath root (uniqueToRunNumber n)
         traceWith tr $ AtLevel ln $
-          TraceNewMerge (V.map Run.size rs) (runNumber runPaths) caching alloc mergePolicy mergeLevel
+          TraceNewMerge (V.map Run.size rs) (runNumber runPaths) caching alloc mergePolicy mergeType
         -- The runs will end up inside the merging run, with fresh references.
         -- The original references can be released (but only on the happy path).
         mr <- withRollback reg
-          (MR.new hfs hbio resolve caching alloc mergeLevel runPaths rs)
+          (MR.new hfs hbio resolve caching alloc mergeType runPaths rs)
           releaseRef
         V.forM_ rs $ \r -> delayedCommit reg (releaseRef r)
         case confMergeSchedule of
@@ -715,10 +715,10 @@ maxRunSize' :: TableConfig -> MergePolicyForLevel -> LevelNo -> NumEntries
 maxRunSize' config policy ln =
     maxRunSize (confSizeRatio config) (confWriteBufferAlloc config) policy ln
 
-mergeLastForLevel :: Levels m h -> Merge.Level
+mergeLastForLevel :: Levels m h -> MergeType
 mergeLastForLevel levels
- | V.null levels = Merge.LastLevel
- | otherwise     = Merge.MidLevel
+ | V.null levels = MergeLastLevel
+ | otherwise     = MergeMidLevel
 
 levelIsFull :: SizeRatio -> V.Vector run -> Bool
 levelIsFull sr rs = V.length rs + 1 >= (sizeRatioInt sr)
diff --git a/src/Database/LSMTree/Internal/MergingRun.hs b/src/Database/LSMTree/Internal/MergingRun.hs
index 6e4dfac45..a2390f3e3 100644
--- a/src/Database/LSMTree/Internal/MergingRun.hs
+++ b/src/Database/LSMTree/Internal/MergingRun.hs
@@ -42,6 +42,7 @@ import           Database.LSMTree.Internal.Entry (NumEntries (..), unNumEntries)
 import           Database.LSMTree.Internal.Lookup (ResolveSerialisedValue)
 import           Database.LSMTree.Internal.Merge (Merge, StepResult (..))
 import qualified Database.LSMTree.Internal.Merge as Merge
+import           Database.LSMTree.Internal.Merge (MergeType (..))
 import           Database.LSMTree.Internal.Paths (RunFsPaths (..))
 import           Database.LSMTree.Internal.Run (Run)
 import qualified Database.LSMTree.Internal.Run as Run
@@ -116,7 +117,7 @@ instance NFData MergeKnownCompleted where
   -> ResolveSerialisedValue
   -> Run.RunDataCaching
   -> RunBloomFilterAlloc
-  -> Merge.Level
+  -> MergeType
   -> RunFsPaths
   -> V.Vector (Ref (Run IO h))
   -> IO (Ref (MergingRun IO h)) #-}
@@ -134,16 +135,16 @@ new ::
   -> ResolveSerialisedValue
   -> Run.RunDataCaching
   -> RunBloomFilterAlloc
-  -> Merge.Level
+  -> MergeType
   -> RunFsPaths
   -> V.Vector (Ref (Run m h))
   -> m (Ref (MergingRun m h))
-new hfs hbio resolve caching alloc mergeLevel runPaths inputRuns =
+new hfs hbio resolve caching alloc mergeType runPaths inputRuns =
     -- If creating the Merge fails, we must release the references again.
     withActionRegistry $ \reg -> do
       runs <- V.mapM (\r -> withRollback reg (dupRef r) releaseRef) inputRuns
       merge <- fromMaybe (error "newMerge: merges can not be empty")
-        <$> Merge.new hfs hbio caching alloc mergeLevel resolve runPaths runs
+        <$> Merge.new hfs hbio caching alloc mergeType resolve runPaths runs
       let numInputRuns = NumRuns $ V.length runs
       let numInputEntries = V.foldMap' Run.size runs
       spentCreditsVar <- SpentCreditsVar <$> newPrimVar 0
diff --git a/src/Database/LSMTree/Internal/Snapshot.hs b/src/Database/LSMTree/Internal/Snapshot.hs
index 1f68221db..24cb16ab1 100644
--- a/src/Database/LSMTree/Internal/Snapshot.hs
+++ b/src/Database/LSMTree/Internal/Snapshot.hs
@@ -44,6 +44,7 @@ import           Database.LSMTree.Internal.Config
 import           Database.LSMTree.Internal.Entry
 import           Database.LSMTree.Internal.Lookup (ResolveSerialisedValue)
 import qualified Database.LSMTree.Internal.Merge as Merge
+import           Database.LSMTree.Internal.Merge (MergeType (..))
 import           Database.LSMTree.Internal.MergeSchedule
 import           Database.LSMTree.Internal.MergingRun (NumRuns (..))
 import qualified Database.LSMTree.Internal.MergingRun as MR
@@ -155,7 +156,7 @@ newtype UnspentCredits = UnspentCredits { getUnspentCredits :: Int }
 
 data SnapMergingRunState r =
     SnapCompletedMerge !r
-  | SnapOngoingMerge !(V.Vector r) !SpentCredits !Merge.Level
+  | SnapOngoingMerge !(V.Vector r) !SpentCredits !MergeType
   deriving stock (Show, Eq, Functor, Foldable, Traversable)
 
 instance NFData r => NFData (SnapMergingRunState r) where
@@ -221,7 +222,7 @@ toSnapMergingRunState (MR.CompletedMerge r) = pure (SnapCompletedMerge r)
 -- work on snapshot load.
 toSnapMergingRunState (MR.OngoingMerge rs (MR.SpentCreditsVar spentCreditsVar) m) = do
     spentCredits <- readPrimVar spentCreditsVar
-    pure (SnapOngoingMerge rs (SpentCredits spentCredits) (Merge.mergeLevel m))
+    pure (SnapOngoingMerge rs (SpentCredits spentCredits) (Merge.mergeType m))
 
 {-------------------------------------------------------------------------------
   Write Buffer
@@ -452,10 +453,10 @@ fromSnapLevels reg hfs hbio conf@TableConfig{..} uc resolve dir (SnapLevels leve
               SnapCompletedMerge run ->
                 withRollback reg (MR.newCompleted nr ne run) releaseRef
 
-              SnapOngoingMerge runs spentCredits lvl -> do
+              SnapOngoingMerge runs spentCredits mt -> do
                 rn <- uniqueToRunNumber <$> incrUniqCounter uc
                 mr <- withRollback reg
-                  (MR.new hfs hbio resolve caching alloc lvl (mkPath rn) runs)
+                  (MR.new hfs hbio resolve caching alloc mt (mkPath rn) runs)
                   releaseRef
                 -- When a snapshot is created, merge progress is lost, so we
                 -- have to redo merging work here. UnspentCredits and
diff --git a/src/Database/LSMTree/Internal/Snapshot/Codec.hs b/src/Database/LSMTree/Internal/Snapshot/Codec.hs
index b44ae2be3..bc178a446 100644
--- a/src/Database/LSMTree/Internal/Snapshot/Codec.hs
+++ b/src/Database/LSMTree/Internal/Snapshot/Codec.hs
@@ -31,7 +31,7 @@ import           Database.LSMTree.Internal.Config
 import           Database.LSMTree.Internal.CRC32C
 import qualified Database.LSMTree.Internal.CRC32C as FS
 import           Database.LSMTree.Internal.Entry
-import qualified Database.LSMTree.Internal.Merge as Merge
+import           Database.LSMTree.Internal.Merge (MergeType (..))
 import           Database.LSMTree.Internal.MergeSchedule
 import           Database.LSMTree.Internal.MergingRun (NumRuns (..))
 import           Database.LSMTree.Internal.Run (ChecksumError (..),
@@ -547,16 +547,18 @@ instance Encode SpentCredits where
 instance DecodeVersioned SpentCredits where
   decodeVersioned V0 = SpentCredits <$> decodeInt
 
-  -- Merge.Level
+-- MergeType
 
-instance Encode Merge.Level where
-  encode Merge.MidLevel  = encodeWord 0
-  encode Merge.LastLevel = encodeWord 1
+instance Encode MergeType  where
+  encode MergeMidLevel  = encodeWord 0
+  encode MergeLastLevel = encodeWord 1
+  encode MergeUnion     = encodeWord 2
 
-instance DecodeVersioned Merge.Level where
+instance DecodeVersioned MergeType where
   decodeVersioned V0 = do
       tag <- decodeWord
       case tag of
-        0 -> pure Merge.MidLevel
-        1 -> pure Merge.LastLevel
-        _ -> fail ("[Merge.Level] Unexpected tag: " <> show tag)
+        0 -> pure MergeMidLevel
+        1 -> pure MergeLastLevel
+        2 -> pure MergeUnion
+        _ -> fail ("[MergeType] Unexpected tag: " <> show tag)
diff --git a/test/Test/Database/LSMTree/Internal/Merge.hs b/test/Test/Database/LSMTree/Internal/Merge.hs
index e388001e4..92e411da0 100644
--- a/test/Test/Database/LSMTree/Internal/Merge.hs
+++ b/test/Test/Database/LSMTree/Internal/Merge.hs
@@ -17,6 +17,7 @@ import           Database.LSMTree.Extras.RunData
 import qualified Database.LSMTree.Internal.BlobFile as BlobFile
 import qualified Database.LSMTree.Internal.Entry as Entry
 import qualified Database.LSMTree.Internal.Merge as Merge
+import           Database.LSMTree.Internal.Merge (MergeType (..))
 import           Database.LSMTree.Internal.PageAcc (entryWouldFitInPage)
 import           Database.LSMTree.Internal.Paths (RunFsPaths (..),
                      pathsForRunFiles)
@@ -65,7 +66,7 @@ tests = testGroup "Test.Database.LSMTree.Internal.Merge"
 prop_MergeDistributes ::
      FS.HasFS IO h ->
      FS.HasBlockIO IO h ->
-     Merge.Level ->
+     MergeType ->
      StepSize ->
      SmallList (RunData KeyForIndexCompact SerialisedValue SerialisedBlob) ->
      IO Property
@@ -137,11 +138,11 @@ prop_MergeDistributes fs hbio level stepSize (SmallList rds) =
 prop_AbortMerge ::
      FS.HasFS IO h ->
      FS.HasBlockIO IO h ->
-     Merge.Level ->
+     MergeType ->
      StepSize ->
      SmallList (RunData KeyForIndexCompact SerialisedValue SerialisedBlob) ->
      IO Property
-prop_AbortMerge fs hbio level (Positive stepSize) (SmallList wbs) =
+prop_AbortMerge fs hbio mergeType (Positive stepSize) (SmallList wbs) =
     withRuns fs hbio (V.fromList (zip (simplePaths [10..]) wbs')) $ \runs -> do
       let path0 = simplePath 0
       mergeToClose <- makeInProgressMerge path0 runs
@@ -156,7 +157,8 @@ prop_AbortMerge fs hbio level (Positive stepSize) (SmallList wbs) =
     wbs' = fmap serialiseRunData wbs
 
     makeInProgressMerge path runs =
-      Merge.new fs hbio Run.CacheRunData (RunAllocFixed 10) level mappendValues path runs >>= \case
+      Merge.new fs hbio Run.CacheRunData (RunAllocFixed 10)
+               mergeType mappendValues path runs >>= \case
         Nothing -> return Nothing  -- not in progress
         Just merge -> do
           -- just do a few steps once, ideally not completing the merge
@@ -176,13 +178,14 @@ type StepSize = Positive Int
 mergeRuns ::
      FS.HasFS IO h ->
      FS.HasBlockIO IO h ->
-     Merge.Level ->
+     MergeType ->
      RunNumber ->
      V.Vector (Ref (Run.Run IO h)) ->
      StepSize ->
      IO (Int, Ref (Run.Run IO h))
-mergeRuns fs hbio level runNumber runs (Positive stepSize) = do
-    Merge.new fs hbio Run.CacheRunData (RunAllocFixed 10) level mappendValues
+mergeRuns fs hbio mergeType runNumber runs (Positive stepSize) = do
+    Merge.new fs hbio Run.CacheRunData (RunAllocFixed 10)
+              mergeType mappendValues
               (RunFsPaths (FS.mkFsPath []) runNumber) runs >>= \case
       Nothing -> (,) 0 <$> unsafeFlushAsWriteBuffer fs hbio
                              (RunFsPaths (FS.mkFsPath []) runNumber) (RunData Map.empty)
@@ -190,11 +193,11 @@ mergeRuns fs hbio level runNumber runs (Positive stepSize) = do
 
 type SerialisedEntry = Entry.Entry SerialisedValue SerialisedBlob
 
-mergeWriteBuffers :: Merge.Level
+mergeWriteBuffers :: MergeType
                   -> [Map SerialisedKey SerialisedEntry]
                   ->  Map SerialisedKey SerialisedEntry
-mergeWriteBuffers level =
-        (if level == Merge.LastLevel then Map.filter (not . isDelete) else id)
+mergeWriteBuffers mergeType =
+        (if mergeType == MergeLastLevel then Map.filter (not . isDelete) else id)
       . Map.unionsWith (Entry.combine mappendValues)
   where
     isDelete Entry.Delete = True
diff --git a/test/Test/Database/LSMTree/Internal/Snapshot/Codec.hs b/test/Test/Database/LSMTree/Internal/Snapshot/Codec.hs
index e0d1c93fb..dd05faec7 100644
--- a/test/Test/Database/LSMTree/Internal/Snapshot/Codec.hs
+++ b/test/Test/Database/LSMTree/Internal/Snapshot/Codec.hs
@@ -14,7 +14,7 @@ import           Data.Typeable
 import qualified Data.Vector as V
 import           Database.LSMTree.Internal.Config
 import           Database.LSMTree.Internal.Entry
-import qualified Database.LSMTree.Internal.Merge as Merge
+import           Database.LSMTree.Internal.Merge (MergeType (..))
 import           Database.LSMTree.Internal.MergeSchedule
 import           Database.LSMTree.Internal.MergingRun
 import           Database.LSMTree.Internal.RunNumber
@@ -174,7 +174,7 @@ testAll test = [
     , test (Proxy @UnspentCredits)
     , test (Proxy @(SnapMergingRunState RunNumber))
     , test (Proxy @SpentCredits)
-    , test (Proxy @Merge.Level)
+    , test (Proxy @MergeType)
     ]
 
 {-------------------------------------------------------------------------------