The XPath Query Engine, which is located between the user interface and the database server, is the core component of the XISS/R system. The query engine accepts XPath queries and generates SQL statements to send to the database server. Query results from the database server are then formated and forwarded to the user interface by the query engine.

Upon receiving an XPath query, the Query Parser first parses the query and translates it into an intermediate tree-structured format. For example, the query A[B]/C will be translated into a tree with node A as the root, and B and C as two children of A.

This tree-structured intermediary is then simplified by the Query Transformer based on current cache information in the Cache Directory. The Cache Directory stores result information from previous queries. Each entry in the directory consists of a query pattern in tree-structured intermediate format, and a pointer to it's associated result set.

The Query Transformer matches the current query tree against patterns in the cache directory. If a match is found, the matched part in the query tree will be replaced by a super node. This super node is similar to a normal node except that when parsed into SQL, the SQL Generator will evaluate it based on cached results. For example, if the result of the query A[B] is in the cache, the subtree representing A[B] can be replaced by a super node representing cached results. The query is thus simplified into a two-node query tree. In addition, other optimizations can be implemented inside the Query Transformer.

The simplified query tree is translated into SQL statements by the SQL Translator and communicated to the database server. For example, the query media[@media-type="image"] will be translated into the following SQL statement when using Schema A:

SELECT
    et0.DID as Document, et0.NID as Node_ID
FROM
    elem_tab et0, attr_tab at0
WHERE
    et0.NAME = 'media' and
    at0.NAME = 'media-type' and
    at0.VALUE = 'image' and
    at0.DID = et0.DID and
    at0.PARENT_ID = et0.NID

The database server processes the SQL query and returns the result to the XPath Query Engine, whose Result Constructor formats the result and sends it to the user interface for display. The result is also sent to the Cache Manager, which makes a caching decision based on information such as result set and data size, query pattern or sub-pattern frequency, query processing time, etc.