@Override
  public void setSafelyDeletedSearchIndex(long safelyDeletedSearchIndex) {
    this.safelyDeletedSearchIndex = safelyDeletedSearchIndex;
  }
Snapshot or flush one memTable.

@return true if snapshot or flush is executed successfully
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/wal/node/WALNode.java