code: func dbFetchFilterIdxEntry(dbTx database.Tx, key []byte, h *chainhash.Hash) ([]byte, error) {
  	idx := dbTx.Metadata().Bucket(cfIndexParentBucketKey).Bucket(key)
  	return idx.Get(h[:]), nil
  }
docs: // dbFetchFilterIdxEntry retrieves a data blob from the filter index database.
  // An entry's absence is not considered an error.