An extender for a computer-implemented relational database system is disclosed for storing, querying, and retrieving structured documents. The extender provides a new abstract data type and includes a plurality of user defined functions for storing, querying, and retrieving structured documents internally, as character-based large objects (CLOB), or externally, in flat files or URLs, for example. A method and apparatus using an extender having a “conditional select” functionality is further disclosed for querying structured documents. The “conditional select” functionality is based on user defined functions residing within the extender being applied to a projection in a structured query in the database system. A method and apparatus using an extender having a set of user defined functions is further disclosed for creating one or more indexes for structured documents stored in the relational database system. The method uses the set of functions to create the indexes and uses already created B+ tree index structures implemented in the database system to support the indexes. A method and apparatus using an extender is further disclosed for creating and supporting structure indexes. The method includes a tag counting system for indexing structured documents and for implementing the structure indexes within the relational database.