public Optional<Expression> getCorrelatedPredicates() {
      if (correlatedPredicates.isEmpty()) {
        return Optional.empty();
      }
      return Optional.of(and(correlatedPredicates));
    }
@return constant symbols from a perspective of a subquery
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/relational/planner/optimizations/PlanNodeDecorrelator.java