XSLT, 2nd Edition

XSLT’s Key Facility

Now that we’ve covered the id() function in great detail, we’ll move on to XSLT’s key() function and the <xsl:key> element. Each <xsl:key> element effectively creates an index of the document. You can then use that index to find all elements that have a particular property. Once the key is created, we can use the key() function to retrieve parts of the document.

For example, if you have a database of (U.S. postal) addresses, you might want to index that database by the people’s last names, by the states in which they live, by their zip codes, etc. Each index takes a certain amount of time to build, but it saves processing time later. (Be aware that it can take a significant amount of memory to create a key, particularly for very large documents.) If you want to find all the people who live in the state of Idaho, you can use the index to find all those people directly; you don’t have to search the entire database.

We’ll discuss the details of how the key facility works, and then we’ll compare it to the id() function.