message: only update column_indices once.
  * only update column_indices once.
  * Update bigquery_lib.h
  * Update bigquery_lib.h
  correct condition.
diff: @@ -237,8 +237,7 @@ class BigQueryReaderArrowDatasetIterator
  out_tensors->clear();
  out_tensors->reserve(columns.size());
  - if (this->current_row_index_ == 0) {
  - this->column_indices_.clear();
  + if (this->current_row_index_ == 0 && this->column_indices_.empty()) {
  this->column_indices_.resize(columns.size());
  for (size_t i = 0; i < columns.size(); ++i) {
  DataType output_type = output_types[i];
  