public String getValidTxnList() {
    return validTxnList;
  }
Gets {@link ValidTxnWriteIdList} of the MV source tables when the MV was created/last rebuilt
@return {@link String} representation of {@link ValidTxnWriteIdList} or null if all source tables are non-native.
storage-api/src/java/org/apache/hadoop/hive/common/MaterializationSnapshot.java