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Rather than several index scans with joins, the optimizer now determines that the optimal way to satisfy the query is to scan the clustered index of our view.
Figure 4 The optimizer now reads all the pages required from one index, rather than five, and output reveals that this results in a 27% reduction in the number of logical reads the engine must perform in order to return the data, from 2,172 to 1,590.
A simple view is just a virtual table, generated from a saved query.
It does not have its own physical page structure to use, so it reads the pages of its underlying tables.
Views are a valuable tool for the SQL Server Developer, because they hide complexity and allow for a readable style of SQL expression.
They aren't there for reasons of performance, and so indexed views are designed to remedy this shortcoming.
Indexed views can be a powerful tool, but they are not a ‘free lunch’ and we need to use them with care.
Once we create an indexed view, every time we modify data in the underlying tables then not only must SQL Server maintain the index entries on those tables, but also the index entries on the view. In addition, they also have the potential to cause other issues.
Now, each application simply has to run a much simpler query referencing the view, as shown in Listing 3.A well-crafted indexed view can write fewer pages to disk than the underlying tables, meaning fewer pages queries need to read fewer pages to return results. Use the techniques and tips in this article to ensure your views are optimal!Let’s see the impact of our indexed view on query performance.Any aggregations defined by the indexed view are now pre-computed, and any joins pre-joined, so the engine no longer has to do this work at execution time.SQL Server creates statistics for the indexed view, different from those of the underlying tables, to optimize cardinality estimations.