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Microsoft Windows key generator. Hey, any key to PyCharm pro Thanks Man. Now, each application simply has to run a much simpler query referencing the view, as shown in Listing 3. However, when looking at the execution plan Figure 1 we can see that SQL Server still performs index scans against each of the five underlying tables. Although the use of the view made writing the query easier, it had no impact on query performance.
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. In other words, when we query a simple view, the optimizer still has to access all of the underlying tables and perform the necessary JOIN s and aggregations.
It derives cardinality estimations, and hence the query plan, from statistics associated with those tables. Before we start, I should mention that there are a host of requirements attached to the creation of indexed views, in any SQL Server Edition.
In order to turn our normal Sales. 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. 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. This means faster, more efficient queries. Use the techniques and tips in this article to ensure your views are optimal! When we re-run the query from Listing 3, we get the same result set, but the execution plan, shown in Figure 4, looks very different.
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. Any query that the Optimizer determines the view could satisfy can use the indexed view rather than underlying tables, a process termed view matching.
Try re-running Listing 1, which references the base tables rather than our indexed view. Here, the execution plan shows that the Optimizer chose to use the clustered index on the view, rather than indexes on the base tables.
Indexed views can really come into their own when we have many applications that need to perform complex aggregations , and other calculations, on the same set of base tables. Rather than force SQL Server to perform these aggregations and calculations every time, upon query execution, we can encapsulate them in an indexed view. This can significantly reduce the amount of IO SQL Server must perform to retrieve the necessary data, and CPU time required to perform the calculations, and so can provide tremendous performance boosts.
This query produces an execution plan with several index scans and joins, shown in Figure 9. It also requires aggregation. Its execution plan is similar in nature to the one we saw in Figure 4, but with additional operations and a higher cost, of 7.
Listing 7 creates an indexed view, vSalesSummaryCustomerProduct , to help reduce the cost of this and similar queries.
It is there for the internal maintenance of indexed views — it maintains a count of the rows per group in the indexed view. We can see that SQL Server has created a statistics object for this clustered index, as shown in Figure What is going on? The difference lies in when and how SQL Server creates automatic statistics, and when it uses them. Simply put, if we do not use the WITH NOEXPAND hint when querying an indexed view, the query optimizer will not use statistics created on the indexed view and neither will it create or update statistics automatically i.
Without automatically created or updated statistics, there can be a slight or even drastic difference between the numbers of rows the optimizer estimates a query will return, and the actual number of rows returned. Pay attention to statistics warnings if you see them! What is the lesson to be learned here? Unfortunately, there may still be occasions when the query optimizer decides not to use an indexed view, even though it seems that it could satisfy a query. In fact, SQL Server may refuse to use the clustered index or any non-clustered indexes on a view, even if we reference the view directly in the query.
The execution plan, in Figure 13, shows that the plan references the underlying tables and ignores our view and its index. The query cost is. Now, the execution plan shows a clustered index seek, as shown in Figure 14, and the query cost is.
We can add non-clustered indexes to boost query performance. Once again, exercise care. SQL Server has to maintain every index we add to a view, every time someone updates one of the contributing base tables. Indexed views work better for relatively static base tables. However, once the clustered index exists, we can easily add useful non-clustered indexes, just as we can for any normal table. When we run the query in Listing 12 again, the execution plan is as shown in Figure The query cost has gone down, to 1.
This time the optimizer chose a seek operation on the new non-clustered index, which is what we wanted. However, it also needed to perform a key lookup to return the additional columns contained in the SELECT clause but not included in the non-clustered index.
To make this index more effective, we can make it a covering index for this query by including all of the columns the query references, as shown in Listing When we run the query again, we see an optimized index seek on the non-clustered index.
We also see a significantly reduced query cost, down to. In SQL Server Standard Edition, we can still create indexed views, but the optimizer will not automatically consider its indexes when formulating an execution plan for a query; it will simply access all of the underlying tables. We also noted occasions, even when using SQL Server Enterprise Edition, when we may need to use this hint to get the plan we expect.
Bear in mind also that if you write queries in stored procedures, your applications, or reports that use WITH NOEXPAND and then drop the index on the view at a later point in time, the queries that reference that index will fail. In short, the same proviso applies here as applies to the use of any table index , join, or query hints: use them cautiously and sparingly, and document them.
Another, implied but not discussed directly, is that the indexed view definition can only reference tables, not other views. Learn more. System Center System Center offers the latest innovation and security in enterprise-class datacenter management. Help your team flourish in a hybrid work world. The pandemic is fundamentally transforming employee and customer expectations. Get the guide. Learn how Microsoft products help companies run their business.
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SQL Serial | PDF | Data Management Software | Computing - Windows Server 2022
This can affect write performance. In addition, they also have the potential to cause other issues. This article will start from the basics of creating indexed views, and the underlying requirements in order to do so, and then discuss their advantages and the situations in which they can offer a significant boost to query performance.
Nobody sets out to write overly complex queries. Unfortunately, however, applications grow more complex as the users demand new features, and so the accompanying queries grow more complex also. Standard SQL Server views can help. When we encapsulate complex multi-table query logic in a view, any application that needs that data is then able to issue a much simpler query against the view, rather than a complex multi- JOIN query against the underlying tables.
Views bring other advantages too. We can grant users SELECT permissions on the view, rather than the underlying tables, and use the view to restrict the columns and rows that are accessible to the user. We can use views to aggregate data in a meaningful way.
The query in Listing 1 joins five tables to get information such as the client name, the order number and date, the products and quantities ordered. Notice that we use two-part naming for all tables. To make it easier for our application to consume this data, we can create a view. This option stipulates that we cannot delete any of the base tables for the view, or ALTER any of the columns in those tables. In order to make one of these changes, we would have to drop the view, change the table, and then recreate the view and any indexes on the view.
Now, each application simply has to run a much simpler query referencing the view, as shown in Listing 3. However, when looking at the execution plan Figure 1 we can see that SQL Server still performs index scans against each of the five underlying tables.
Although the use of the view made writing the query easier, it had no impact on query performance. 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. In other words, when we query a simple view, the optimizer still has to access all of the underlying tables and perform the necessary JOIN s and aggregations.
It derives cardinality estimations, and hence the query plan, from statistics associated with those tables. Before we start, I should mention that there are a host of requirements attached to the creation of indexed views, in any SQL Server Edition.
In order to turn our normal Sales. 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.
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. This means faster, more efficient queries. Use the techniques and tips in this article to ensure your views are optimal! When we re-run the query from Listing 3, we get the same result set, but the execution plan, shown in Figure 4, looks very different. 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.
Any query that the Optimizer determines the view could satisfy can use the indexed view rather than underlying tables, a process termed view matching.
Try re-running Listing 1, which references the base tables rather than our indexed view. Here, the execution plan shows that the Optimizer chose to use the clustered index on the view, rather than indexes on the base tables. Indexed views can really come into their own when we have many applications that need to perform complex aggregations , and other calculations, on the same set of base tables.
Rather than force SQL Server to perform these aggregations and calculations every time, upon query execution, we can encapsulate them in an indexed view. This can significantly reduce the amount of IO SQL Server must perform to retrieve the necessary data, and CPU time required to perform the calculations, and so can provide tremendous performance boosts.
This query produces an execution plan with several index scans and joins, shown in Figure 9. It also requires aggregation. Its execution plan is similar in nature to the one we saw in Figure 4, but with additional operations and a higher cost, of 7.
Listing 7 creates an indexed view, vSalesSummaryCustomerProduct , to help reduce the cost of this and similar queries. It is there for the internal maintenance of indexed views — it maintains a count of the rows per group in the indexed view.
We can see that SQL Server has created a statistics object for this clustered index, as shown in Figure What is going on? The difference lies in when and how SQL Server creates automatic statistics, and when it uses them.
Simply put, if we do not use the WITH NOEXPAND hint when querying an indexed view, the query optimizer will not use statistics created on the indexed view and neither will it create or update statistics automatically i. Without automatically created or updated statistics, there can be a slight or even drastic difference between the numbers of rows the optimizer estimates a query will return, and the actual number of rows returned.
Pay attention to statistics warnings if you see them! What is the lesson to be learned here? Unfortunately, there may still be occasions when the query optimizer decides not to use an indexed view, even though it seems that it could satisfy a query.
In fact, SQL Server may refuse to use the clustered index or any non-clustered indexes on a view, even if we reference the view directly in the query.
The execution plan, in Figure 13, shows that the plan references the underlying tables and ignores our view and its index. The query cost is. Now, the execution plan shows a clustered index seek, as shown in Figure 14, and the query cost is.
We can add non-clustered indexes to boost query performance. Once again, exercise care. SQL Server has to maintain every index we add to a view, every time someone updates one of the contributing base tables. Indexed views work better for relatively static base tables. However, once the clustered index exists, we can easily add useful non-clustered indexes, just as we can for any normal table. When we run the query in Listing 12 again, the execution plan is as shown in Figure The query cost has gone down, to 1.
This time the optimizer chose a seek operation on the new non-clustered index, which is what we wanted. However, it also needed to perform a key lookup to return the additional columns contained in the SELECT clause but not included in the non-clustered index. To make this index more effective, we can make it a covering index for this query by including all of the columns the query references, as shown in Listing When we run the query again, we see an optimized index seek on the non-clustered index.
We also see a significantly reduced query cost, down to. In SQL Server Standard Edition, we can still create indexed views, but the optimizer will not automatically consider its indexes when formulating an execution plan for a query; it will simply access all of the underlying tables. We also noted occasions, even when using SQL Server Enterprise Edition, when we may need to use this hint to get the plan we expect.
Bear in mind also that if you write queries in stored procedures, your applications, or reports that use WITH NOEXPAND and then drop the index on the view at a later point in time, the queries that reference that index will fail. In short, the same proviso applies here as applies to the use of any table index , join, or query hints: use them cautiously and sparingly, and document them.
Another, implied but not discussed directly, is that the indexed view definition can only reference tables, not other views. SQL Server has to guarantee that it can return a consistent result set regardless of whether a query accesses a view or the underlying tables, so it will automatically maintain indexes in response to data modifications on the base tables.
We can see this in action if we update one of the base tables that make up our vSalesSummaryCustomerProduct view. The execution plan includes many operators, including an update of the vSalesSummaryCustomerProduct clustered index. SQL Server must always ensure that the data in the index and base tables is synchronized, so we need to be careful when adding indexes to views.
Every time an underlying column has a new row added or deleted, or is updated, SQL Server must maintain every clustered and non-clustered index, whether on the base table or the referenced indexed view. This will lead to additional writes, which can decrease performance. With no indexed view referenced by the table, both inserts will succeed. However, add an indexed view and the behavior will change.
The first insert will have to modify a row in the table, and it will have to modify the index. It will hold locks on both objects. Until that has completed, the second operation will not be able to complete because it also needs locks on both objects. A great demo of this is available from Alex Kuznetsov in his article, Be ready to drop your indexed view.
Can we, and should we, insert into, update, or delete from an indexed view, directly? The conditions for directly modifying data in an indexed view are the same as for a regular view, namely:. Windows 11 Enterprise is designed for hybrid work, offering features and enhancements focused on productivity, collaboration and security.
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