Add aggregate functions (#2925)

The aggregate functions SUM(), MAX(), MIN(), AVG(), and COUNT() are now supported.

src/PostgREST/Error.hs

@@ -61,6 +61,7 @@ class (JSON.ToJSON a) => PgrstError a where
     responseLBS (status err) (baseHeader : headers err) $ errorPayload err
 
 instance PgrstError ApiRequestError where
+  status AggregatesNotAllowed{}  = HTTP.status400
   status AmbiguousRelBetween{}   = HTTP.status300
   status AmbiguousRpc{}          = HTTP.status300
   status BinaryFieldError{}      = HTTP.status406
@@ -198,6 +199,9 @@ instance JSON.ToJSON ApiRequestError where
     (Just $ JSON.String $ T.decodeUtf8 ("Invalid preferences: " <> BS.intercalate ", " prefs))
     Nothing
 
+  toJSON AggregatesNotAllowed = toJsonPgrstError
+    ApiRequestErrorCode23 "Use of aggregate functions is not allowed" Nothing Nothing
+
   toJSON (NoRelBetween parent child embedHint schema allRels) = toJsonPgrstError
     SchemaCacheErrorCode00
     ("Could not find a relationship between '" <> parent <> "' and '" <> child <> "' in the schema cache")
@@ -604,6 +608,7 @@ data ErrorCode
   | ApiRequestErrorCode20
   | ApiRequestErrorCode21
   | ApiRequestErrorCode22
+  | ApiRequestErrorCode23
   -- Schema Cache errors
   | SchemaCacheErrorCode00
   | SchemaCacheErrorCode01
@@ -652,6 +657,7 @@ buildErrorCode code = "PGRST" <> case code of
   ApiRequestErrorCode20  -> "120"
   ApiRequestErrorCode21  -> "121"
   ApiRequestErrorCode22  -> "122"
+  ApiRequestErrorCode23  -> "123"
 
   SchemaCacheErrorCode00 -> "200"
   SchemaCacheErrorCode01 -> "201"