code: public final UnresolvedReferenceExpression[] mergeOperands() {
  		UnresolvedReferenceExpression[] aggBuffers = aggBufferAttributes();
  		UnresolvedReferenceExpression[] ret = new UnresolvedReferenceExpression[aggBuffers.length];
  		for (int i = 0; i < aggBuffers.length; i++) {
  			String name = String.valueOf(i);
  			validateOperandName(name);
  			ret[i] = new UnresolvedReferenceExpression(name);
  		}
  		return ret;
  	}
docs: Merge inputs of {@link #mergeExpressions()}, these inputs are agg buffer generated by user definition.