@Override
    public List<Variable> visitGroup_by(@NotNull SiddhiQLParser.Group_byContext ctx) {

        List<Variable> variableList = new ArrayList<Variable>(ctx.attribute_reference().size());
        for (SiddhiQLParser.Attribute_referenceContext attributeReferenceContext : ctx.attribute_reference()) {
            variableList.add((Variable) visit(attributeReferenceContext));
        }
        return variableList;
    }
{@inheritDoc}
<p>The default implementation returns the result of calling
{@link #visitChildren} on {@code ctx}.</p>

@param ctx
modules/siddhi-query-compiler/src/main/java/io/siddhi/query/compiler/internal/SiddhiQLBaseVisitorImpl.java