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Indexing a View
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Views can be indexed just as tables can be indexed. Rules in choosing columns to make indexes on a view are similar to those for a table. There are also some major requirements you need to meet before you can index a view. I will show you these first so that you are aware of which views can be indexed and what you have to do with your view. When building indexes on views, the first index to be created must be a unique clustered index. Once such an index has been built, additional nonclustered indexes on this view can then be created. This can also be taken further, in that if we have a view with subsequent indexes on it, and we drop the unique clustered index, then all of the other indexes will automatically be dropped. Also, if we drop the view, as we would expect, the indexes are also dropped. The view that the index is to build on must only contain tables and cannot contain views. The tables must all come from one database, and the view must also reside in that database and have been built with the SCHEMABINDING option. As you saw when creating our database, certain options can be switched on or off. The following options must be set to ON while creating an index. These options need only be set to ON for that session and therefore would precede the CREATE INDEX statement. SET SET SET SET SET SET ANSI_NULLS ON ANSI_PADDING ON ANSI_WARNINGS ON CONCAT_NULL_YIELDS_NULL ON ARITHABORT ON QUOTED_IDENTIFIER ON On top of this, the NUMERIC_ROUNDABORT option must be set to OFF. SET NUMERIC_ROUNDABORT OFF Finally, the view itself cannot have text, ntext, or image columns defined in it. In 11, we ll look at how to group data through a clause called GROUP BY. If you have grouping within your view, then the columns used to group data are the only columns that can be in the first index. Although these seem like they could be quite restrictive requirements, the upside is that indexing views also comes with major speed implications. If a view remains without an index, every time that
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CHAPTER 9 BUILDING A VIEW
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the view is executed, the data behind the view, including any joins, is rebuilt and executed. However, as the first index is a clustered index, this is similar to a clustered table index, and the data will be retrieved at index-creation time and stored in that order. Also, like table indexes, when the data is modified, then the index will receive the updates as well. Therefore, if SQL Server can use the clustered index, there will be no need to run the query again. SQL Server will use any indexes that you have on the tables when building the views. Indexing a view is most beneficial when the data in the underlying tables is not changing frequently and when the view is executed often. Keep in mind that a view is taking information from other tables and is not a table itself, and therefore any updates to the underlying tables will not be reflected in the view until the view is rerun. By placing an index on a view, the columns named within the index are stored within the database, as are all of the columns defined for the view, along with the data rows. Therefore, any changes to the raw data within the native tables will also be reflected in the data stored for the view. Keep in mind the performance issues with this. Every data change in the tables used in the views requires SQL Server to evaluate the effect the change has on the view. This requires more processing by SQL Server, causing a slowdown in performance. Temper this perceived gain of using an index with the downside of the extra processing required to keep the data up to date in two places for the table and two places for the index for those columns involved in the view. Now that you are aware of the pros and cons of building indexes on views, and how they differ from indexes for tables, it is time to build an index on our view. The aim of this index is to locate a record in the view quickly. We want to be able to find all the products for a customer based on his or her account number. Notice that we are not using CustomerId here. First of all, that column is not within the view, so it is unavailable for selection anyway, but we have to cater to when a customer phones up and supplies the account number. This customer will be unaware of his or her ApressFinancial internal CustomerId. Building the index is very quick and very simple, especially since you already know the basics from building indexes earlier in the book.
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