Make sure we're using indexes during rollbacks

    Fixes #2083

    The `queryMinRefId` query uses
    ```sql
    SELECT id FROM <table> WHERE <field> >= $1 ORDER BY id ASC LIMIT 1.
    ```

    The planner sometimes picks a bad plan:

    ```sql
    Index Scan using tx_pkey on tx
      Filter: (block_id >= $1)
    ```

    the filter is not Index Cond, so this ends up in a sequential scan.
    The index refers to the primary key and is only used for sorting.

    Instead we use a simpler query without ORDER BY:

    SELECT id FROM <table> WHERE <field> >= $1 LIMIT 10000
    This forces the planner to use the field's index.
    The results are fetched and the minimum is found in Haskell.
    Near the tip this returns only a handful of rows.

    If there are more than 10000 matching rows (large rollback),
    we fall back to the original ORDER BY id ASC LIMIT 1 query.

Author: kderme

cardano-db/src/Cardano/Db/Operations/Query.hs

@@ -581,6 +581,45 @@ queryMinRefId ::
   field ->
   ReaderT SqlBackend m (Maybe (Key record))
 queryMinRefId txIdField txId = do
+  res <- select $ do
+    rec <- from $ table @record
+    where_ (rec ^. txIdField >=. val txId)
+    limit 10000
+    pure $ rec ^. persistIdField
+  if length res >= 10000
+    then queryMinRefIdFallback txIdField txId
+    else pure $ minimumMay $ map unValue res
+  where
+    minimumMay [] = Nothing
+    minimumMay xs = Just (minimum xs)
+
+queryMinRefIdNullable ::
+  forall m field record.
+  (MonadIO m, PersistEntity record, PersistField field) =>
+  EntityField record (Maybe field) ->
+  field ->
+  ReaderT SqlBackend m (Maybe (Key record))
+queryMinRefIdNullable txIdField txId = do
+  res <- select $ do
+    rec <- from $ table @record
+    where_ (isJust (rec ^. txIdField))
+    where_ (rec ^. txIdField >=. just (val txId))
+    limit 10000
+    pure $ rec ^. persistIdField
+  if length res >= 10000
+    then queryMinRefIdNullableFallback txIdField txId
+    else pure $ minimumMay $ map unValue res
+  where
+    minimumMay [] = Nothing
+    minimumMay xs = Just (minimum xs)
+
+queryMinRefIdFallback ::
+  forall m field record.
+  (MonadIO m, PersistEntity record, PersistField field) =>
+  EntityField record field ->
+  field ->
+  ReaderT SqlBackend m (Maybe (Key record))
+queryMinRefIdFallback txIdField txId = do
   res <- select $ do
     rec <- from $ table @record
     where_ (rec ^. txIdField >=. val txId)
@@ -589,13 +628,13 @@ queryMinRefId txIdField txId = do
     pure $ rec ^. persistIdField
   pure $ unValue <$> listToMaybe res
 
-queryMinRefIdNullable ::
+queryMinRefIdNullableFallback ::
   forall m field record.
   (MonadIO m, PersistEntity record, PersistField field) =>
   EntityField record (Maybe field) ->
   field ->
   ReaderT SqlBackend m (Maybe (Key record))
-queryMinRefIdNullable txIdField txId = do
+queryMinRefIdNullableFallback txIdField txId = do
   res <- select $ do
     rec <- from $ table @record
     where_ (isJust (rec ^. txIdField))