docs: def ReadBlobs(self, blob_ids, cursor=None):
      """Reads given blobs."""
      if not blob_ids:
        return {}
  
      query = ("SELECT blob_id, blob_chunk "
               "FROM blobs "
               "FORCE INDEX (PRIMARY) "
               "WHERE blob_id IN {} "
               "ORDER BY blob_id, chunk_index ASC").format(
                   mysql_utils.Placeholders(len(blob_ids)))
      cursor.execute(query, [blob_id.AsBytes() for blob_id in blob_ids])
      results = {blob_id: None for blob_id in blob_ids}
      for blob_id_bytes, blob in cursor.fetchall():
        blob_id = rdf_objects.BlobID.FromBytes(blob_id_bytes)
        if results[blob_id] is None:
          results[blob_id] = blob
        else:
          results[blob_id] += blob
      return results