public void unpinMemTable() throws MemTablePinException {
    if (walNode == null || memTableId < 0) {
      throw new MemTablePinException("Fail to pin memTable because of internal error.");
    }
    walNode.unpinMemTable(memTableId);
  }
Unpin the wal files of the given memory table.

@throws MemTablePinException If there aren't corresponding pin operations
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/wal/utils/WALEntryHandler.java