XQuery

Performance

When querying large documents or databases, it is important to tune queries to optimize performance. Implementations vary significantly in their ability to take clues from the query in order to optimize its evaluation. This section provides some general tips for improving query performance. For more specific tuning information for your XQuery processor, consult the documentation.

Avoid Reevaluating the Same or Similar Expressions

A let clause can be used to evaluate an expression once and bind the value to a variable that can be referenced many times. This can be much more efficient than evaluating the expression many times. For example, suppose you want to add a bargain-bin element to your results, but only if there are products whose price is less than 30. You first need to check whether any bargain products exist, and if so, construct a bargain-bin element and list the products in it. Example 15-3 shows an example of this.

Example 15-3. Avoid re-evaluating the same expression

Less efficient query
if (doc("prices.xml")/prices/priceList/prod[price < 30])
then <bargain-bin>{
       doc("prices.xml")/*/priceList/prod[price < 30]
     }</bargain-bin>
else ( )
More efficient query
let $bargains := doc("prices.xml")/prices/priceList/prod[price < 30]
return if ($bargains)
       then <bargain-bin>{$bargains}</bargain-bin>
      else ( )

In the first query, similar path expressions appear in the if expression and in the bargain-bin element constructor. In the second query, the expression is evaluated ...