- Determining what version of SQL Server is right for your firm
Looking for:
- Deciding Between Editions of SQL Server for Production |
Explained: SQL Web vs Standard vs Enterprise
- Microsoft sql server 2014 standard vs enterprise free
No is the only option when you click on the drop-down list under the Readable Secondary column in the Specify Replicas page of the New Availability Group Wizard. While there are limitations, there are also advantages when you compare them with Database Mirroring in Standard Edition. Understanding the benefits and limitations of Basic Availability Groups can give you enough information to decide whether you would like to still use Database Mirroring regardless of its deprecation status or upgrade to Enterprise Edition to take advantage of all the Availability Group features.
However, both options still put you at a disadvantage — the former being at the risk of using an unmaintained feature, the latter being more expensive.
This feature is not new and has been around since SQL Server 6. Highlighted below are the differences between the two that can help you make the right decision in choosing the appropriate high availability solution. While the comparisons apply to traditional Availability Groups as well, the focus will be on Basic Availability Groups. A database is an object that resides within an instance.
Understanding the difference between an instance and a database can help make decisions in implementing the appropriate high availability solution in terms of operational efficiency.
In an FCI, the entire instance is protected. If the primary node becomes unavailable, the entire instance is moved to the standby node. These instance-level objects are stored in the system databases which are physically stored in shared storage. In an Availability Group — be it the traditional or basic — only the databases in the group are protected. System databases cannot be added to an Availability Group — only user databases are allowed.
If all the dependent system objects are not replicated on all replicas, the database may end up becoming inaccessible to the application; as in the case of missing SQL Server logins or partially functional as in the case of missing certificates for Always Encrypted SQL Server Service Pack 1 made Always Encrypted available in Standard Edition.
If you are more concerned with instance-level protection to minimize possible human error during change management processes, then, an FCI is the way to go. An FCI requires some form of shared storage. The shared storage is accessible to all of the nodes in the failover cluster but only the current primary node has ownership at any given point in time.
The system and user databases are stored on the shared storage. When a failover occurs, ownership of the shared storage moves from the current primary to the standby, making the databases available to the new primary node.
From a capacity point-of-view, you only need to provision disk space based on the sizes of the databases. However, from an availability point-of-view, the shared storage becomes a single point of failure. The FCI will remain offline if the shared storage becomes unavailable, regardless of the number of nodes in the failover cluster.
An Availability Group does not require shared storage. Each replica has its own local storage independent of the Availability Group. If the primary replica becomes unavailable and a failover occurs, any secondary replica can take over without having to rely on the availability of the system and user databases from the primary replica.
From a capacity point-of-view, you need to provision disk space based on the sizes of the databases and the number of secondary replicas, significantly increasing the cost per gigabyte. But from an availability point-of-view, because the Availability Group replicas do not rely on a single storage source, the SQL Server instance hosting the replicas remains online regardless of what happens to the primary replica. Each Availability Group replica has its own copy of the system databases, thus, it is always online regardless of whether it is functioning as a primary or a secondary replica.
The complexities of a shared storage subsystem like a storage area network SAN require the proper administration to guarantee high availability and resiliency because the storage becomes a single point of failure. 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:.
Those are the official rules, but there are a host of other things to think about, as well. For example, when inserting into a table via a view, if there are NOT NULL columns defined in the table, but are not in the view, your insert will fail, as demonstrated with this example. It is much easier to deal directly with the base tables. Views are a powerful tool in SQL Server to help you write queries more efficiently and provide a layer of additional security. They can make complex aggregations more efficient and we can even apply additional non-clustered indexes to help satisfy further queries.
As with any tool, we must use it with care. This can affect write performance and it can sometimes lead to increased lock contention and blocking during data modifications. Learn the benefits and drawbacks of indexed views and test them in your environment. They are another tool to have in your T-SQL toolbox. As well as offering advanced IntelliSense-style code completion, full formatting options, object renaming, and other productivity features, SQL Prompt also offers fast and comprehensive code analysis as you type.
Try it free. Fortnightly newsletters help sharpen your skills and keep you ahead, with articles, ebooks and opinion to keep you informed. Her favorite topics are administration, automation, and performance. She frequently presents at user groups, SQL Saturdays, and other community events. She is also an avid runner and chef. View all articles by Jes Borland.
From Views to Indexed Views Nobody sets out to write overly complex queries. CustomerID ,. FirstName ,. Amit Banerjee. Business continuity is a key requirement for planning, designing, and implementing any business-critical system. When you bring data into the mix, business continuity becomes mandatory. You get mission-critical high availability and disaster recovery features that allow you to implement various topologies to meet your business SLAs.
With these new benefits, Software Assurance customers will be able to implement hybrid disaster recovery plans with SQL Server using our features like Always On Availability Groups without incurring additional licensing costs for the passive replicas.
The new benefit offers passive replica benefits running on an Azure Virtual Machine. Now a customer would need to only license 12 cores of SQL Server running on-premises as long as the disaster recovery criteria for the passive replica on Azure Virtual Machine is met.
Comments
Post a Comment