duckdb: disable function serialization

vortex-duckdb/cpp/table_function.cpp

@@ -208,6 +208,28 @@ duckdb_state register_table_function(DatabaseInstance &db, LogicalType parameter
     fn.get_partition_stats = get_partition_stats;
     fn.get_multi_file_reader = get_multi_file_reader;
 
+    /**
+     * duckdb's serialization is broken. If you don't set serialize/deserialize
+     * callbacks, duckdb serializes only the internal state which doesn't work
+     * for Vortex if you have filters pushed down. Worse, duckdb uses this
+     * information for CommonSubplanOptimizer which then merges different
+     * Vortex scans (with different filters pushed down) into one scan in tpcds.
+     *
+     * However, this is a regression on q15 and such where we do have
+     * completely equal scans which can't be merged. This is a reasonable price
+     * for correctness.
+     *
+     * Very unexpectedly verify_serialization doesn't do any verification but
+     * disables serialization at all.
+     */
+    fn.verify_serialization = false;
+    fn.serialize = [](auto &, auto, auto &) {
+        throw NotImplementedException("Can't serialize Vortex state");
+    };
+    fn.deserialize = [](auto &, auto &) -> unique_ptr<FunctionData> {
+        throw NotImplementedException("Can't deserialize Vortex state");
+    };
+
     try {
         auto &system_catalog = Catalog::GetSystemCatalog(db);
         auto data = CatalogTransaction::GetSystemTransaction(db);