@Override
  public SamzaSqlRelRecord getRecordKey(KV<SamzaSqlRelRecord, SamzaSqlRelMessage> record) {
    return record.getKey();
  }
This class joins incoming {@link SamzaSqlRelMessage} with records {@link KV}&lt;{@link SamzaSqlRelRecord}, {@link SamzaSqlRelMessage}&gt;}
from local table with the join key being {@link SamzaSqlRelRecord}.
samza-sql/src/main/java/org/apache/samza/sql/translator/SamzaSqlLocalTableJoinFunction.java