Checks if given blobs exist.

def CheckBlobsExist(self, blob_ids, cursor=None):
    """Checks if given blobs exist."""
    if not blob_ids:
      return {}

    exists = {blob_id: False for blob_id in blob_ids}
    query = ("SELECT blob_id "
             "FROM blobs "
             "FORCE INDEX (PRIMARY) "
             "WHERE blob_id IN {}".format(
                 mysql_utils.Placeholders(len(blob_ids))))
    cursor.execute(query, [blob_id.AsBytes() for blob_id in blob_ids])
    for blob_id, in cursor.fetchall():
      exists[rdf_objects.BlobID.FromBytes(blob_id)] = True
    return exists