public void setMatch() {
      hasRows = true;
      isSingleRow = KeyRef.getIsSingleFlag(refWord);

      // We must set the position since equalKey does not leave us positioned correctly.
      hashMapStore.writeBuffers.setReadPoint(
          keyAbsoluteOffset + keyLength, readPos);

      // Save first value absolute offset...
      firstValueAbsoluteOffset = hashMapStore.writeBuffers.getReadPoint(readPos);

      // Position to beginning.
      readIndex = 0;
      isNextEof = false;
      setJoinResult(JoinResult.MATCH);
    }
Mark the key matched with equalKey as a match and set up for reading the values.
Afterward, methods isSingleRow, cappedCount, first, next, etc may be called.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/mapjoin/fast/VectorMapJoinFastBytesHashMapStore.java