Delete data automatically when using the --disable-ledger flag

This is need when the database is restored from a snapshot and then
`db-sync` is run with `--disable-ledger`.

cardano-db-sync/app/cardano-db-sync.hs

@@ -103,7 +103,7 @@ pPGPassSource =
 pExtended :: Parser Bool
 pExtended =
   Opt.flag True False
-    ( Opt.long "no-epoch-table"
+    ( Opt.long "disable-epoch"
     <> Opt.help "Makes epoch table remain empty"
     )
 

cardano-db-sync/src/Cardano/DbSync/Sync.hs

@@ -132,6 +132,9 @@ runSyncNode metricsSetters trce iomgr aop snEveryFollowing snEveryLagging dbConn
       -- If the DB is empty it will be inserted, otherwise it will be validated (to make
       -- sure we are on the right chain).
       lift $ Db.runIohkLogging trce $ withPostgresqlConn dbConnString $ \backend -> do
+        liftIO $ unless (enpHasLedger enp) $ do
+          logInfo trce "Migrating to a no ledger schema"
+          Db.noLedgerMigrations backend trce
         lift $ orDie renderSyncNodeError $ insertValidateGenesisDist trce backend (dncNetworkName enc) genCfg (useShelleyInit enc)
         liftIO $ epochStartup (enpExtended enp) trce backend
 

cardano-db/src/Cardano/Db/Migration.hs

@@ -16,6 +16,7 @@ module Cardano.Db.Migration
   , validateMigrations
   , hashMigrations
   , renderMigrationValidateError
+  , noLedgerMigrations
   ) where
 
 import           Control.Exception (SomeException, handle)
@@ -43,6 +44,7 @@ import           Data.Time.Format (defaultTimeLocale, formatTime, iso8601DateFor
 import           Database.Persist.Sql (Single (..), SqlBackend, SqlPersistT, entityVal,
                    getMigration, rawExecute, rawSql, selectFirst)
 
+import           Cardano.BM.Trace (Trace)
 import           Cardano.Crypto.Hash (Blake2b_256, ByteString, Hash, hashToStringAsHex, hashWith)
 import           Cardano.Db.Migration.Haskell
 import           Cardano.Db.Migration.Version
@@ -283,4 +285,11 @@ isOfficialMigrationFile fn =
       case takeWhile isDigit . drop 1 $ dropWhile (/= '-') str of
         stage -> fromMaybe 0 $ readMaybe stage
 
-
+noLedgerMigrations :: SqlBackend -> Trace IO Text -> IO ()
+noLedgerMigrations backend trce = do
+  void $ runDbIohkLogging backend trce $ do
+    rawExecute "update redeemer set fee = null" []
+    rawExecute "delete from reward" []
+    rawExecute "delete from epoch_stake" []
+    rawExecute "delete from ada_pots" []
+    rawExecute "delete from epoch_param" []

doc/configuration.md

@@ -40,8 +40,14 @@ delete from ada_pots;
 delete from epoch_param;
 ```
 
-### --disable-cache
+### --disable-cache : Experimental
 
 This disables the application level caches of db-sync. It reduces memory usage by a bit, but
 increases sync time. Usually this flag is not worth it, unless there are some pretty big memory
 issues.
+
+## --disable-epoch : Experimental
+
+With this option the epoch table is left empty. Mostly left for historical reasons, since it
+provides a negligible improvement in sync time.
+