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13.2.5 Optimization guidelines
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By default, Hibernate never loads data that you didn t ask for, which reduces the memory consumption of your persistence context. However, it also exposes you to the so-called n+1 selects problem. If every association and collection is
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Selecting a fetch strategy
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initialized only on demand, and you have no other strategy configured, a particular procedure may well execute dozens or even hundreds of queries to get all the data you require. You need the right strategy to avoid executing too many SQL statements. If you switch from the default strategy to queries that eagerly fetch data with joins, you may run into another problem, the Cartesian product issue. Instead of executing too many SQL statements, you may now (often as a side effect) create statements that retrieve too much data. You need to find the middle ground between the two extremes: the correct fetching strategy for each procedure and use case in your application. You need to know which global fetch plan and strategy you should set in your mapping metadata, and which fetching strategy you apply only for a particular query (with HQL or Criteria). We now introduce the basic problems of too many selects and Cartesian products and then walk you through optimization step by step. The n+1 selects problem The n+1 selects problem is easy to understand with some example code. Let s assume that you don t configure any fetch plan or fetching strategy in your mapping metadata: Everything is lazy and loaded on demand. The following example code tries to find the highest Bids for all Items (there are many other ways to do this more easily, of course):
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List<Item> allItems = session.createQuery("from Item").list(); // List<Item> allItems = session.createCriteria(Item.class).list(); Map<Item, Bid> highestBids = new HashMap<Item, Bid>(); for (Item item : allItems) { Bid highestBid = null; for (Bid bid : item.getBids() ) { // Initialize the collection if (highestBid == null) highestBid = bid; if (bid.getAmount() > highestBid.getAmount()) highestBid = bid; } highestBids.put(item, highestBid); }
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First you retrieve all Item instances; there is no difference between HQL and Criteria queries. This query triggers one SQL SELECT that retrieves all rows of the ITEM table and returns n persistent objects. Next, you iterate through this result and access each Item object.
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Optimizing fetching and caching
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What you access is the bids collection of each Item. This collection isn t initialized so far, the Bid objects for each item have to be loaded with an additional query. This whole code snippet therefore produces n+1 selects. You always want to avoid n+1 selects. A first solution could be a change of your global mapping metadata for the collection, enabling prefetching in batches:
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<set name="bids" inverse="true" batch-size="10"> <key column="ITEM_ID"/> <one-to-many class="Bid"/> </set>
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Instead of n+1 selects, you now see n/10+1 selects to retrieve the required collections into memory. This optimization seems reasonable for an auction application: Only load the bids for an item when they re needed, on demand. But if one collection of bids must be loaded for a particular item, assume that other item objects in the persistence context also need their bids collections initialized. Do this in batches, because it s somewhat likely that not all item objects need their bids. With a subselect-based prefetch, you can reduce the number of selects to exactly two:
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<set name="bids" inverse="true" fetch="subselect"> <key column="ITEM_ID"/> <one-to-many class="Bid"/> </set>
The first query in the procedure now executes a single SQL SELECT to retrieve all Item instances. Hibernate remembers this statement and applies it again when you hit the first uninitialized collection. All collections are initialized with the second query. The reasoning for this optimization is slightly different: Only load the bids for an item when they re needed, on demand. But if one collection of bids must be loaded, for a particular item, assume that all other item objects in the persistence context also need their bids collection initialized. Finally, you can effectively turn off lazy loading of the bids collection and switch to an eager fetching strategy that results in only a single SQL SELECT:
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