barcode vb.net 2008 Optimizing object retrieval in C#

Make USS Code 39 in C# Optimizing object retrieval

Optimizing object retrieval
Code 3/9 Generation In C#
Using Barcode maker for VS .NET Control to generate, create Code 3/9 image in Visual Studio .NET applications.
www.OnBarcode.com
ANSI/AIM Code 39 Scanner In Visual C#
Using Barcode reader for Visual Studio .NET Control to read, scan read, scan image in .NET framework applications.
www.OnBarcode.com
string query = @"select new MaxAmount( item.id, max(bid.Amount) ) from Item item join item.Bids bid" where item.Seller = :user group by item.id"; IList maxAmounts = session.CreateQuery(query) .SetEntity("user", user) .List();
Generating PDF417 In Visual C#.NET
Using Barcode drawer for .NET Control to generate, create PDF 417 image in Visual Studio .NET applications.
www.OnBarcode.com
Painting QR In Visual C#
Using Barcode creator for VS .NET Control to generate, create QR Code JIS X 0510 image in VS .NET applications.
www.OnBarcode.com
Unfortunately, this isn t a complete solution to the generic issue. In general, you may need to do more complex processing on the bids than merely calculating the maximum amount. It s better to do this processing in the .NET application. You can try enabling eager fetching at the level of the mapping document:
Code 3 Of 9 Printer In C#.NET
Using Barcode generation for .NET framework Control to generate, create Code 39 image in .NET framework applications.
www.OnBarcode.com
Generate UPC-A In C#.NET
Using Barcode drawer for Visual Studio .NET Control to generate, create GTIN - 12 image in Visual Studio .NET applications.
www.OnBarcode.com
<set name="Bids" lazy="false" inverse="true" outer-join="true">
Draw Barcode In C#.NET
Using Barcode printer for Visual Studio .NET Control to generate, create Barcode image in VS .NET applications.
www.OnBarcode.com
Code 11 Generation In C#.NET
Using Barcode generator for Visual Studio .NET Control to generate, create USD8 image in VS .NET applications.
www.OnBarcode.com
The outer-join attribute is available for collections and other associations. It forces NHibernate to load the association eagerly, using a SQL outer join. You may also use the fetch attribute; fetch="select" is equivalent to outer-join="false", and fetch= "join" is equivalent to outer-join="true". (Note that, as previously mentioned, HQL queries ignore the outer-join attribute; but you may be using a criteria query.) This mapping avoids the problem as far as this transaction is concerned; you re now able to load all bids in the initial select. Unfortunately, any other transaction that retrieves items using Get(), Load(), or a criteria query will also retrieve all the bids at once. Retrieving unnecessary data imposes extra load on both the database server and the application server and may also reduce the concurrency of the system, creating too many unnecessary read locks at the database level. Hence we consider eager fetching at the level of the mapping file to be almost always a bad approach. The outer-join attribute of collection mappings is arguably a misfeature of NHibernate (fortunately, it s disabled by default). Occasionally, it makes sense to enable outer-join for a <many-to-one> or <one-to-one> association (the default is auto; see section 4.4.6), but you d never do this in the case of a collection. Our recommended solution for this problem is to take advantage of NHibernate s support for runtime (code-level) declarations of association fetching strategies. The example can be implemented like this:
Print Code 39 Extended In None
Using Barcode drawer for Font Control to generate, create Code 39 Full ASCII image in Font applications.
www.OnBarcode.com
Decode Code 39 Full ASCII In VB.NET
Using Barcode scanner for Visual Studio .NET Control to read, scan read, scan image in .NET framework applications.
www.OnBarcode.com
IList<Item> results = session.CreateCriteria(typeof(Item)) .Add( Expression.Eq("item.Seller", user) ) .SetFetchMode("Bids", FetchMode.Eager) .List<Item>(); // Make results distinct ISet<Item> distinctResults = new HashedSet<Item>(results); IList maxAmounts = new ArrayList(); foreach ( Item item in distinctResults ) { double maxAmount = 0; foreach ( Bid bid in item.Bids ) { if( maxAmount < bid.Amount ) maxAmount = bid.Amount; } maxAmounts.Add( new MaxAmount( item.Id, maxAmount ) ); }
EAN / UCC - 13 Decoder In None
Using Barcode reader for Software Control to read, scan read, scan image in Software applications.
www.OnBarcode.com
Code39 Creator In VS .NET
Using Barcode drawer for ASP.NET Control to generate, create Code 39 image in ASP.NET applications.
www.OnBarcode.com
Retrieving objects efficiently
2D Encoder In Visual Studio .NET
Using Barcode generation for ASP.NET Control to generate, create Matrix image in ASP.NET applications.
www.OnBarcode.com
Universal Product Code Version A Printer In None
Using Barcode drawer for Software Control to generate, create UPC A image in Software applications.
www.OnBarcode.com
You disabled batch fetching and eager fetching at the mapping level; the collection is lazy by default. Instead, you enable eager fetching for this query alone by calling SetFetchMode(). As discussed earlier in this chapter, this is equivalent to a fetch join in the from clause of an HQL query. The previous code example has one extra complication: the result list returned by the NHibernate criteria query isn t guaranteed to be distinct. In the case of a query that fetches a collection by outer join, it will contain duplicate items. It s the application s responsibility to make the results distinct if that is required. You implement this by adding the results to a HashedSet (from the library Iesi.Collections) and then iterating the set. You ve now established a general solution to the n+1 selects problem. Rather than retrieving just the top-level objects in the initial query and then fetching needed associations as the application navigates the object graph, you follow a two-step process:
QR-Code Decoder In VB.NET
Using Barcode scanner for VS .NET Control to read, scan read, scan image in .NET applications.
www.OnBarcode.com
PDF 417 Drawer In .NET
Using Barcode generator for .NET framework Control to generate, create PDF 417 image in VS .NET applications.
www.OnBarcode.com
Fetch all needed data in the initial query by specifying exactly which associations will be accessed in the following unit of work. Navigate the object graph, which consists entirely of objects that have already been fetched from the database.
Matrix Barcode Creator In Visual Studio .NET
Using Barcode generator for .NET framework Control to generate, create Matrix 2D Barcode image in .NET applications.
www.OnBarcode.com
Barcode Encoder In Java
Using Barcode creation for Android Control to generate, create Barcode image in Android applications.
www.OnBarcode.com
This is the only true solution to the mismatch between the object-oriented world, where data is accessed by navigation, and the relational world, where data is accessed by joining. Another efficient solution, for deep graphs of objects, is to issue one query per level and let NHibernate resolve the references between the objects. For example, you can query categories, asking NHibernate to fetch their items. Then you can query these items, asking NHibernate to fetch their bids. Finally, there is one further solution to the n+1 selects problem. For some classes or collections with a sufficiently small number of instances, it s possible to keep all instances in the second-level cache, avoiding the need for database access. Obviously, this solution is preferred where and when it s possible (it isn t possible in the case of the Bids of an Item, because you wouldn t enable caching for this kind of data). The n+1 selects problem may appear whenever you use the List() method of IQuery to retrieve the result. As we mentioned earlier, this issue can be hidden in more complex logic; we highly recommend the optimization strategies mentioned in section 4.4.7 to find such scenarios. It s also possible to generate too many selects by using Find(), the shortcut for queries on the ISession API, or Load() and Get(). Next, we examine a third query API method. It s extremely important to understand when it s applicable, because it produces n+1 selects!
EAN-13 Supplement 5 Creation In Visual Basic .NET
Using Barcode printer for .NET Control to generate, create EAN / UCC - 13 image in VS .NET applications.
www.OnBarcode.com
PDF 417 Printer In None
Using Barcode generator for Excel Control to generate, create PDF417 image in Office Excel applications.
www.OnBarcode.com
Copyright © OnBarcode.com . All rights reserved.