An Example

A Movie has a to-one relationship to a Studio. Suppose that you have a WOComponent that lets you query movies and display all the movies with their studios. Without using prefetching, every time the component displays a group of Movies, EOF makes a separate query to the database to get each Studio. You can use prefetching to ensure that every time the movies are fetched, all the appropriate studios are also fetched, significantly reducing the number of database queries.

How to Use Prefetching

Following are two different ways to enable prefetching:

• Use EOModeler's Prefetching tab to specify relationship key paths for a particular fetch specification.
• Send setPrefetchingRelationshipKeyPaths to the fetch-specification object. This method takes an array of strings designating relationship key paths.

When to Use Prefetching

You can use prefetching when you know that a particular relationship will always be accessed. It is more instructive, however, to know when not to use prefetching:

• Don't use prefetching when memory usage is more important then database performance.

By prefetching, you may fetch in more objects than you need, and thus waste memory.

• Don't use prefetching when you have fetch limits on a fetch.

Because prefetching uses the qualifier of the original fetch, it fetches all the related objects, regardless of any limit placed on the original fetch. If you issue a fetch that returns 10,000 records, but use fetch limit to only get 100 of them, the prefetching hint will cause all 10,000 relationships to be fetched.

• Don't use prefetching when performing multiple queries that return the same records.

Using the movie-query screen example described earlier, suppose that a user makes a query for all movies that have the letter "a" in their titles. This returns a large number of movies and causes several studios to be fetched. If the user does another query for all movies with the letter "e" in their titles, the resulting records will overlap those previously fetched and EOF will refetch several studios. In this case, batch-faulting is more appropriate; all of the movies with the letter "a" in their names have had their faults fired and EOF fetches only the studios for movies containing the letter "e" and not the letter "a" in their names.