Selecting a fetch strategy eager fetch that shouldn t (Affordable web design)
Selecting a fetch strategy eager fetch that shouldn t be executed with an immediate second SELECT, but with a JOIN in the initial query. You can keep the FetchType.EAGER Java Persistence annotation but switch from join fetching to an immediate second select explicitly by adding a Hibernate extension annotation: @Entity public class Item { … @ManyToOne(fetch = FetchType.EAGER) @org.hibernate.annotations.Fetch( org.hibernate.annotations.FetchMode.SELECT ) private User seller; } If an Item instance is loaded, Hibernate will eagerly load the seller of this item with an immediate second SELECT. Finally, we have to introduce a global Hibernate configuration setting that you can use to control the maximum number of joined entity associations (not collections). Consider all many-to-one and one-to-one association mappings you ve set to fetch=”join” (or FetchType.EAGER) in your mapping metadata. Let s assume that Item has a successfulBid association, that Bid has a bidder, and that User has a shippingAddress. If all these associations are mapped with fetch=”join”, how many tables are joined and how much data is retrieved when you load an Item? The number of tables joined in this case depends on the global hibernate. max_fetch_depth configuration property. By default, no limit is set, so loading an Item also retrieves a Bid, a User, and an Address in a single select. Reasonable settings are small, usually between 1 and 5. You may even disable join fetching for many-to-one and one-to-one associations by setting the property to 0! (Note that some database dialects may preset this property: For example, MySQLDialect sets it to 2.) SQL queries also get more complex if inheritance or joined mappings are involved. You need to consider a few extra optimization options whenever second ary tables are mapped for a particular entity class. 13.2.4 Optimizing fetching for secondary tables If you query for objects that are of a class which is part of an inheritance hierarchy, the SQL statements get more complex:
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