public final String getExpressionString() {
    if (expressionStringCache == null) {
      expressionStringCache = getExpressionStringInternal();
    }
    return expressionStringCache;
  }
Get the representation of the expression in string. Compared to output symbol, it does not
consider logical views.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/expression/Expression.java