@Override
    public Boolean visitFieldAccess(RexFieldAccess fieldAccess) {
      // "<expr>.FIELD" is constant iff "<expr>" is constant.
      return fieldAccess.getReferenceExpr().accept(this);
    }
Walks over an expression and determines whether it is constant.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/HiveCalciteUtil.java