Brad’s Sure Guide to SQL Server Maintenance Plans- P23 pptx

Chapter 7: Rebuild Index Task 111 For this example, we are going to keep things simple, so let's assume we want to perform the Rebuild Index task for all user databases. In this case, choose the option shown in Figure 7.2, and then click OK. Figure 7.2: To keep things simple, select the "All user databases" option. The Define Rebuild Index Task screen reappears, and the two drop-down boxes I referred to earlier are displayed below the Databases drop-down box, but they are grayed out, as shown in Figure 7.3. Figure 7.3: The "Object" and "Selection" drop down boxes are not available. Chapter 7: Rebuild Index Task 112 So what's going on? Why are these two options grayed out? The reason is that these two options are only available if you select one database on which to run the Rebuild Index task. Since we selected All user databases, they are not available. Selecting a Specific Database Although it does not apply to our example, let's take a look at what happens if you select only a single database, such as AdventureWorks, for the task. To do this, select These databases from the screen shown in Figure 7.2 and then check the checkbox for AdventureWorks. When you click OK, this section of the Define Rebuild Index Task screen will look as shown in Figure 7.4. Figure 7.4: When a single database is selected, then the "Object" drop-down box becomes available. Notice that Specific databases now appears in the Databases drop-down box, the Object box is now available, and the Selection box is still, for the time being, grayed out. What the Object and Selection options allow you to do is to selectively rebuild some of the indexes in your database, and not others. If you click on the Object drop-down box, you'll see the choices as shown in Figure 7.5. Figure 7.5: You must select either "Table" or "View." Notice that there are three choices for Object. If you leave the default option selected, Tables and views, then the Rebuild Index task will be applied to the indexes associated with all tables and all indexed views in the selected database. In other words, you haven't changed anything. In order to narrow the scope of the task to specific objects, you need to choose either Table or View. Having done this, the Selection drop-down box becomes available. For example, choose Table, and then click on "Select one or more" in the now available Selection drop-down box, as shown in Figure 7.6. Chapter 7: Rebuild Index Task 113 Figure 7.6: You can select which tables you want to rebuild with the Rebuild Index task. Now, you get the option of selecting specific tables within the AdventureWorks database, to which this task should apply. For example, you could choose to rebuild only the indexes associated with the dbo.ErrorLog table, or you could select some combination of tables, by checking each of the relevant checkboxes. Why would you want to rebuild the indexes for some tables and not others? Actually, there is a very good reason for this. In most databases, there are some tables that are virtually static; they rarely if ever change, and so there is no benefit in rebuilding their associated indexes as they don't, over time, develop wasted empty space or become logically fragmented. By selecting only those indexes that really need defragmenting, you can reduce the time it takes to perform the Rebuild Index task and, at the same time, reduce the resource overhead associated with this task. The problem I see is that most people who are using the Maintenance Wizard won't have the knowledge to determine which indexes are relatively static and which are subject to a lot of wasted space and logical fragmentation. If you are at the level where you know how to evaluate each index using the sys.dm_db_index_physical_stats DMF, in order to apply a selective rebuild process, then the chances are you are probably better off implementing this process using T-SQL or PowerShell scripts, and avoiding use of the Maintenance Plan Wizard in the first place. Before we move on, let's briefly consider the View option that is available in the Object drop- down box, as shown in Figure 7.7. Chapter 7: Rebuild Index Task 114 Figure 7.7: You can select which indexed views you want to rebuild with the Rebuild Index task. In this case, View doesn't refer to conventional views, but to indexed views. Indexed views are physical views, unlike regular views, which are only materialized when they are called by a query. Because indexed views are physical, they need rebuilding just like regular indexes. As per my advice with regard to the Table option, if you need this kind of granularity for the maintenance of your indexes, you shouldn't be using the Maintenance Plan Wizard for this task. Chapter 7: Rebuild Index Task 115 While I have taken a little time to explain what the Object and Selection drop-down boxes do, I am recommending that you don't use then, as they just make Maintenance Plans overly complicated, defeating the benefit of using them in the first place. Free space options We still have several more choices to make before we are done configuring this task. Note that the discussion of these options assumes that each of your tables has a clustered index, and is not a heap. A heap is a table without a clustered index. As a best practice, all tables should have a clustered index. The first two choices are listed under Free space options and include Reorganize pages with the default amount of free space and Change free space per page percentage to, as shown in Figure 7.8. You can choose one option or the other, but not both. Figure 7.8: These options can have a significant impact on the Rebuild Index task. The default option of Reorganize pages with the default amount of free space is a little confusing. First, it says reorganize, not rebuild. Remember, we are working on the Rebuild Index task, not the Reorganize Index task. Don't let this confuse you into thinking that selecting this option reorganizes indexes, rather than rebuilding them. It does the latter, and this is actually a mistake in the user interface. It really should say "rebuild," not "reorganize." The second part of this first option says "default amount of free space." What does that mean? When creating a SQL Server index, there is an option to create the index with a certain amount of free space on each data page. This setting is known as the fill factor. If an index is created without specifying a fill factor, then the default fill factor is used, which is 100 (actually 0, but 0 means the same thing as a 100% fill factor). This means that no free space is created for the data pages of an index. The potential problem with a fill factor of 100 arises when data is added to a table as a result of an INSERT or UPDATE, and a new row needs to be added to a data page. If there is no room for it, then SQL Server will reorganize the rows, moving some of the rows onto a new data page, and leaving some on the old data page. This is known as page splitting. While page splitting is a normal SQL Server activity, too much page splitting can cause performance . with a fill factor of 100 arises when data is added to a table as a result of an INSERT or UPDATE, and a new row needs to be added to a data page. If there is no room for it, then SQL Server will. mean? When creating a SQL Server index, there is an option to create the index with a certain amount of free space on each data page. This setting is known as the fill factor. If an index is. moving some of the rows onto a new data page, and leaving some on the old data page. This is known as page splitting. While page splitting is a normal SQL Server activity, too much page splitting