public PlanBuilder getSubPlan() {
      return subPlan;
    }
Creates a projection with any additional coercions by identity of the provided expressions.

@return the new subplan and a mapping of each expression to the symbol representing the
    coercion or an existing symbol if a coercion wasn't needed
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/relational/planner/QueryPlanner.java