message: ombine union chunks
diff: @@ -660,6 +660,7 @@ Status Union(CylonContext *ctx,
  // create final table
  std::shared_ptr<arrow::Table> table = arrow::Table::Make(ltab->schema(), final_data_arrays);
  + table->CombineChunks(ctx->GetMemoryPool(), table);
  PutTable(dest_id, table);
  return Status::OK();
  }
  