@Override
    public Void visit(ParamExpression<?> param, Void context) {
        constants.add(param);
        serializeConstant(constants.size(), null);
        if (constantPaths.size() < constants.size()) {
            constantPaths.add(null);
        }
        return null;
    }
Return a list of expressions that can be used to uniquely define the query sources

@param joins
@return identifier columns
querydsl-sql/src/main/java/com/querydsl/sql/SQLSerializer.java