@Override public RexNode visitCall(RexCall call) {
      if (isSimpleCorrelatedExpression(call, correlationId)) {
        correlations.add(call);
        return call;
      } else {
        return super.visitCall(call);
      }
    }
A collector of simply correlated row expressions.

The shuttle traverses the tree and collects all calls and field accesses that are classified
as simply correlated expressions. Multiple nodes in a call hierarchy may satisfy the criteria
of a simple correlation so we peek the expressions closest to the root.

@see SimpleCorrelationDetector
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/rules/CorrelateProjectExtractor.java