Improvements to connUpsertSql

connUpsertSql currently has the following type:

connUpsertSql :: Maybe (EntityDef -> NonEmpty (HaskellName,DBName) -> Text -> Text)

1. I don't see why the HaskellName is passed to this function. The HaskellName of a row seems irrelevant for generating SQL. It appears unused by the Postgres code.
2. The third argument, Text is the SQL for updating columns. In the case where there are no updates, upsertBy must fallback to defaultUpsertBy. defaultUpsertBy does more SQL queries, and is also prone to race conditions. Instead, I believe connUpsertSql should allow for no updates itself, and in that case do ON CONFLICT DO NOTHING.

Unfortunately, I think fixing this would require a breaking change.

-- | The slow but generic 'upsertBy' implementation for any 'PersistUniqueRead'.
-- * Lookup corresponding entities (if any) 'getBy'.
-- * If the record exists, update using 'updateGet'.
-- * If it does not exist, insert using 'insertEntity'.
-- @since 2.11
defaultUpsertBy
    :: ( PersistEntityBackend record ~ BaseBackend backend
       , PersistEntity record
       , MonadIO m
       , PersistStoreWrite backend
       , PersistUniqueRead backend
       )
    => Unique record   -- ^ uniqueness constraint to find by
    -> record          -- ^ new record to insert
    -> [Update record] -- ^ updates to perform if the record already exists
    -> ReaderT backend m (Entity record) -- ^ the record in the database after the operation
defaultUpsertBy uniqueKey record updates = do
    mrecord <- getBy uniqueKey
    maybe (insertEntity record) (`updateGetEntity` updates) mrecord
  where
    updateGetEntity (Entity k _) upds =
        (Entity k) `liftM` (updateGet k upds)

upsertSql' :: EntityDef -> NonEmpty (HaskellName, DBName) -> Text -> Text
upsertSql' ent uniqs updateVal =
    T.concat
        [ "INSERT INTO "
        , escape (entityDB ent)
        , "("
        , T.intercalate "," fieldNames
        , ") VALUES ("
        , T.intercalate "," placeholders
        , ") ON CONFLICT ("
        , T.intercalate "," $ map (escape . snd) (NEL.toList uniqs)
        , ") DO UPDATE SET "
        , updateVal
        , " WHERE "
        , wher
        , " RETURNING ??"
        ]
  where
    (fieldNames, placeholders) = unzip (Util.mkInsertPlaceholders ent escape)

    wher = T.intercalate " AND " $ map (singleClause . snd) $ NEL.toList uniqs

    singleClause :: DBName -> Text
    singleClause field = escape (entityDB ent) <> "." <> (escape field) <> " =?"

Start by tracing connUpsertSql and its uses, then compare the backend behavior with defaultUpsertBy and upsertSql'. Check how the PostgreSQL implementation handles HaskellName and empty update SQL. Done means defining the intended breaking API change and supporting no-update upserts without the generic fallback.