@Override
    public String toString() {
      return "Path: " + baseDirPath + "; writeId: " + writeId + "; visibilityTxnId: " + visibilityTxnId;
    }
Since version 3 but prior to version 4, format of a base is "base_X" where X is a writeId.
If this base was produced by a compactor, X is the highest writeId that the compactor included.
If this base is produced by Insert Overwrite stmt, X is a writeId of the transaction that
executed the insert.
Since Hive Version 4.0, the format of a base produced by a compactor is
base_X_vY.  X is like before, i.e. the highest writeId compactor included and Y is the
visibilityTxnId of the transaction in which the compactor ran.
(v(isibility) is a literal to help parsing).
ql/src/java/org/apache/hadoop/hive/ql/io/AcidUtils.java