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When the Enterprise Objects Framework (EOF) fetches an enterprise object, it creates EOFault objects for the enterprise object's relationships. Each time you access one of these EOFaults, EOF fetches the data from the database. This procedure works well and minimizes the amount of unneeded data fetching from the database. However, each time an EOFault is accessed, a separate database query is performed. Using a technique called "batch faulting" can reduce the number of database queries. If you know that you will be faulting all instances of a particular relationship, you can use a technique called "prefetching" to skip the EOFault creating stage and produce a single database query to load all related objects.
By using EOFetchSpecification's setPrefetchingRelationshipKeyPaths to specify the key paths of relationships, the Enterprise Objects Framework uses the qualifier of the fetch specification along with the related attributes to perform a joined query against the related objects' table, thereby causing all the appropriate objects to be loaded from the database at once.
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.
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:
By prefetching, you may fetch in more objects than you need, and thus waste memory.
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.
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.
2 July, 1998. Paul Haddad. First Draft.
17 November, 1998. Terry Donoghue. Second Draft.