change keys database to use IKey type with more efficient serialization

This breaks any existing keys database!

IKey serializes more efficiently than SKey, although this limits the
use of its Read/Show instances.

This makes the keys database use less disk space, and so should be a win.

Updated benchmark:

benchmarking keys database/getAssociatedFiles from 1000 (hit)
time                 64.04 μs   (63.95 μs .. 64.13 μs)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 64.02 μs   (63.96 μs .. 64.08 μs)
std dev              218.2 ns   (172.5 ns .. 299.3 ns)

benchmarking keys database/getAssociatedFiles from 1000 (miss)
time                 52.53 μs   (52.18 μs .. 53.21 μs)
                     0.999 R²   (0.998 R² .. 1.000 R²)
mean                 52.31 μs   (52.18 μs .. 52.91 μs)
std dev              734.6 ns   (206.2 ns .. 1.623 μs)

benchmarking keys database/getAssociatedKey from 1000 (hit)
time                 64.60 μs   (64.46 μs .. 64.77 μs)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 64.74 μs   (64.57 μs .. 65.20 μs)
std dev              900.2 ns   (389.7 ns .. 1.733 μs)

benchmarking keys database/getAssociatedKey from 1000 (miss)
time                 52.46 μs   (52.29 μs .. 52.68 μs)
                     1.000 R²   (0.999 R² .. 1.000 R²)
mean                 52.63 μs   (52.35 μs .. 53.37 μs)
std dev              1.362 μs   (562.7 ns .. 2.608 μs)
variance introduced by outliers: 24% (moderately inflated)

benchmarking keys database/addAssociatedFile to 1000 (old)
time                 487.3 μs   (484.7 μs .. 490.1 μs)
                     1.000 R²   (0.999 R² .. 1.000 R²)
mean                 490.9 μs   (487.8 μs .. 496.5 μs)
std dev              13.95 μs   (6.841 μs .. 22.03 μs)
variance introduced by outliers: 20% (moderately inflated)

benchmarking keys database/addAssociatedFile to 1000 (new)
time                 6.633 ms   (5.741 ms .. 7.751 ms)
                     0.905 R²   (0.850 R² .. 0.965 R²)
mean                 8.252 ms   (7.803 ms .. 8.602 ms)
std dev              1.126 ms   (900.3 μs .. 1.430 ms)
variance introduced by outliers: 72% (severely inflated)

benchmarking keys database/getAssociatedFiles from 10000 (hit)
time                 65.36 μs   (64.71 μs .. 66.37 μs)
                     0.998 R²   (0.995 R² .. 1.000 R²)
mean                 65.28 μs   (64.72 μs .. 66.45 μs)
std dev              2.576 μs   (920.8 ns .. 4.122 μs)
variance introduced by outliers: 42% (moderately inflated)

benchmarking keys database/getAssociatedFiles from 10000 (miss)
time                 52.34 μs   (52.25 μs .. 52.45 μs)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 52.49 μs   (52.42 μs .. 52.59 μs)
std dev              255.4 ns   (205.8 ns .. 312.9 ns)

benchmarking keys database/getAssociatedKey from 10000 (hit)
time                 64.76 μs   (64.67 μs .. 64.84 μs)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 64.67 μs   (64.62 μs .. 64.72 μs)
std dev              177.3 ns   (148.1 ns .. 217.1 ns)

benchmarking keys database/getAssociatedKey from 10000 (miss)
time                 52.75 μs   (52.66 μs .. 52.82 μs)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 52.69 μs   (52.63 μs .. 52.75 μs)
std dev              210.6 ns   (173.7 ns .. 265.9 ns)

benchmarking keys database/addAssociatedFile to 10000 (old)
time                 489.7 μs   (488.7 μs .. 490.7 μs)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 490.4 μs   (489.6 μs .. 492.2 μs)
std dev              3.990 μs   (2.435 μs .. 7.604 μs)

benchmarking keys database/addAssociatedFile to 10000 (new)
time                 9.994 ms   (9.186 ms .. 10.74 ms)
                     0.959 R²   (0.928 R² .. 0.979 R²)
mean                 9.906 ms   (9.343 ms .. 10.40 ms)
std dev              1.384 ms   (1.051 ms .. 2.100 ms)
variance introduced by outliers: 69% (severely inflated)

3 files changed, 48 insertions, 30 deletions

diff --git a/Database/Keys.hs b/Database/Keys.hs
index 89410e741..fdba05312 100644
--- a/Database/Keys.hs
+++ b/Database/Keys.hs
@@ -133,20 +133,20 @@ openDb createdb _ = withExclusiveLock gitAnnexKeysDbLock $ do
 	open db = liftIO $ DbOpen <$> H.openDbQueue db SQL.containedTable
 
 addAssociatedFile :: Key -> TopFilePath -> Annex ()
-addAssociatedFile k f = runWriterIO $ SQL.addAssociatedFile (toSKey k) f
+addAssociatedFile k f = runWriterIO $ SQL.addAssociatedFile (toIKey k) f
 
 {- Note that the files returned were once associated with the key, but
  - some of them may not be any longer. -}
 getAssociatedFiles :: Key -> Annex [TopFilePath]
-getAssociatedFiles = runReaderIO . SQL.getAssociatedFiles . toSKey
+getAssociatedFiles = runReaderIO . SQL.getAssociatedFiles . toIKey
 
 {- Gets any keys that are on record as having a particular associated file.
  - (Should be one or none but the database doesn't enforce that.) -}
 getAssociatedKey :: TopFilePath -> Annex [Key]
-getAssociatedKey = map fromSKey <$$> runReaderIO . SQL.getAssociatedKey
+getAssociatedKey = map fromIKey <$$> runReaderIO . SQL.getAssociatedKey
 
 removeAssociatedFile :: Key -> TopFilePath -> Annex ()
-removeAssociatedFile k = runWriterIO . SQL.removeAssociatedFile (toSKey k)
+removeAssociatedFile k = runWriterIO . SQL.removeAssociatedFile (toIKey k)
 
 {- Find all unlocked associated files. This is expensive, and so normally
  - the associated files are updated incrementally when changes are noticed. -}
@@ -168,7 +168,7 @@ scanAssociatedFiles = whenM (isJust <$> inRepo Git.Branch.current) $
 	isregfile i = Git.Types.toBlobType (Git.LsTree.mode i) == Just Git.Types.FileBlob
 	add h i k = liftIO $ flip SQL.queueDb h $ 
 		void $ insertUnique $ SQL.Associated
-			(toSKey k)
+			(toIKey k)
 			(getTopFilePath $ Git.LsTree.file i)
 
 {- Stats the files, and stores their InodeCaches. -}
@@ -177,12 +177,12 @@ storeInodeCaches k fs = withTSDelta $ \d ->
 	addInodeCaches k . catMaybes =<< liftIO (mapM (`genInodeCache` d) fs)
 
 addInodeCaches :: Key -> [InodeCache] -> Annex ()
-addInodeCaches k is = runWriterIO $ SQL.addInodeCaches (toSKey k) is
+addInodeCaches k is = runWriterIO $ SQL.addInodeCaches (toIKey k) is
 
 {- A key may have multiple InodeCaches; one for the annex object, and one
  - for each pointer file that is a copy of it. -}
 getInodeCaches :: Key -> Annex [InodeCache]
-getInodeCaches = runReaderIO . SQL.getInodeCaches . toSKey
+getInodeCaches = runReaderIO . SQL.getInodeCaches . toIKey
 
 removeInodeCaches :: Key -> Annex ()
-removeInodeCaches = runWriterIO . SQL.removeInodeCaches . toSKey
+removeInodeCaches = runWriterIO . SQL.removeInodeCaches . toIKey
diff --git a/Database/Keys/SQL.hs b/Database/Keys/SQL.hs
index 79230b60d..456b48e46 100644
--- a/Database/Keys/SQL.hs
+++ b/Database/Keys/SQL.hs
@@ -25,12 +25,12 @@ import Control.Monad
 
 share [mkPersist sqlSettings, mkMigrate "migrateKeysDb"] [persistLowerCase|
 Associated
-  key SKey
+  key IKey
   file FilePath
   KeyFileIndex key file
   FileKeyIndex file key
 Content
-  key SKey
+  key IKey
   cache SInodeCache
   KeyCacheIndex key cache
 |]
@@ -58,51 +58,51 @@ queueDb a (WriteHandle h) = H.queueDb h checkcommit a
 			now <- getCurrentTime
 			return $ diffUTCTime lastcommittime now > 300
 
-addAssociatedFile :: SKey -> TopFilePath -> WriteHandle -> IO ()
-addAssociatedFile sk f = queueDb $ do
+addAssociatedFile :: IKey -> TopFilePath -> WriteHandle -> IO ()
+addAssociatedFile ik f = queueDb $ do
 	-- If the same file was associated with a different key before,
 	-- remove that.
 	delete $ from $ \r -> do
-		where_ (r ^. AssociatedFile ==. val (getTopFilePath f) &&. not_ (r ^. AssociatedKey ==. val sk))
-	void $ insertUnique $ Associated sk (getTopFilePath f)
+		where_ (r ^. AssociatedFile ==. val (getTopFilePath f) &&. not_ (r ^. AssociatedKey ==. val ik))
+	void $ insertUnique $ Associated ik (getTopFilePath f)
 
 {- Note that the files returned were once associated with the key, but
  - some of them may not be any longer. -}
-getAssociatedFiles :: SKey -> ReadHandle -> IO [TopFilePath]
-getAssociatedFiles sk = readDb $ do
+getAssociatedFiles :: IKey -> ReadHandle -> IO [TopFilePath]
+getAssociatedFiles ik = readDb $ do
 	l <- select $ from $ \r -> do
-		where_ (r ^. AssociatedKey ==. val sk)
+		where_ (r ^. AssociatedKey ==. val ik)
 		return (r ^. AssociatedFile)
 	return $ map (asTopFilePath . unValue) l
 
 {- Gets any keys that are on record as having a particular associated file.
  - (Should be one or none but the database doesn't enforce that.) -}
-getAssociatedKey :: TopFilePath -> ReadHandle -> IO [SKey]
+getAssociatedKey :: TopFilePath -> ReadHandle -> IO [IKey]
 getAssociatedKey f = readDb $ do
 	l <- select $ from $ \r -> do
 		where_ (r ^. AssociatedFile ==. val (getTopFilePath f))
 		return (r ^. AssociatedKey)
 	return $ map unValue l
 
-removeAssociatedFile :: SKey -> TopFilePath -> WriteHandle -> IO ()
-removeAssociatedFile sk f = queueDb $ 
+removeAssociatedFile :: IKey -> TopFilePath -> WriteHandle -> IO ()
+removeAssociatedFile ik f = queueDb $ 
 	delete $ from $ \r -> do
-		where_ (r ^. AssociatedKey ==. val sk &&. r ^. AssociatedFile ==. val (getTopFilePath f))
+		where_ (r ^. AssociatedKey ==. val ik &&. r ^. AssociatedFile ==. val (getTopFilePath f))
 
-addInodeCaches :: SKey -> [InodeCache] -> WriteHandle -> IO ()
-addInodeCaches sk is = queueDb $
-	forM_ is $ \i -> insertUnique $ Content sk (toSInodeCache i)
+addInodeCaches :: IKey -> [InodeCache] -> WriteHandle -> IO ()
+addInodeCaches ik is = queueDb $
+	forM_ is $ \i -> insertUnique $ Content ik (toSInodeCache i)
 
 {- A key may have multiple InodeCaches; one for the annex object, and one
  - for each pointer file that is a copy of it. -}
-getInodeCaches :: SKey -> ReadHandle -> IO [InodeCache]
-getInodeCaches sk = readDb $ do
+getInodeCaches :: IKey -> ReadHandle -> IO [InodeCache]
+getInodeCaches ik = readDb $ do
 	l <- select $ from $ \r -> do
-		where_ (r ^. ContentKey ==. val sk)
+		where_ (r ^. ContentKey ==. val ik)
 		return (r ^. ContentCache)
 	return $ map (fromSInodeCache . unValue) l
 
-removeInodeCaches :: SKey -> WriteHandle -> IO ()
-removeInodeCaches sk = queueDb $ 
+removeInodeCaches :: IKey -> WriteHandle -> IO ()
+removeInodeCaches ik = queueDb $ 
 	delete $ from $ \r -> do
-		where_ (r ^. ContentKey ==. val sk)
+		where_ (r ^. ContentKey ==. val ik)
diff --git a/Database/Types.hs b/Database/Types.hs
index 1476a693a..6667bc343 100644
--- a/Database/Types.hs
+++ b/Database/Types.hs
@@ -27,6 +27,24 @@ fromSKey (SKey s) = fromMaybe (error $ "bad serialied Key " ++ s) (file2key s)
 
 derivePersistField "SKey"
 
+-- A Key index. More efficient than SKey, but its Read instance does not
+-- work when it's used in any kind of complex data structure.
+newtype IKey = IKey String
+
+instance Read IKey where
+	readsPrec _ s = [(IKey s, "")]
+
+instance Show IKey where
+	show (IKey s) = s
+
+toIKey :: Key -> IKey
+toIKey = IKey . key2file
+
+fromIKey :: IKey -> Key
+fromIKey (IKey s) = fromMaybe (error $ "bad serialied Key " ++ s) (file2key s)
+
+derivePersistField "IKey"
+
 -- A serialized InodeCache
 newtype SInodeCache = I String
 	deriving (Show, Read)