Continously growing usage of XML (eXtensible Markup Language) data demands for development of powerful query optimization systems. Optimization approaches for XML databases depend on database type. Relational based XML DBMS (DataBase Management System) decompose documents into conventional or special binary relations. XQuery clauses in such systems are translated to queries in SQL (Structured Query Language) like language and query processors employ traditional relational query optimization techniques. Several requirements for algebra suitable for efficient cost-based optimization are presented. It is shown that known XML algebras do not fully satisfy this requirements. A new algebra to satisfy better the requirements is introduced.