Provided is a path index lookup method, i.e., an extended markup language (XML) indexing method by which a relational database can stably process a query regardless of how a user defines the query. In the path index lookup method, a regular path expression with a certain length can be processed through a single joining operation using a PathLookup table and an ExtendedEdge table. Thus, a regular path expression query, which is a core type of XML user query, can be stably and quickly processed regardless of how a user describes the query.