qr code generator c# Lesson 2: Creating Indexes in Visual C#

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Lesson 2: Creating Indexes
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You can query the sys.dm_db_index_usage_stats dynamic management view (DMV) to find index-usage information. The columns from this view that are particularly interesting are shown in Table 6-9; you can find documentation of the whole table at SQL Server Books Online at http://msdn.microsoft.com/en-us/library/ms188755.aspx.
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tabLe 6-9 Subset of the sys.dm_db_index_usage_stats DMV
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database_id object_id index_id user_seeks user_scans user_lookups user_updates last_user_seek last_user_scan last_user_lookup
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smallint int int bigint bigint bigint bigint datetime datetime datetime
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ID of the database on which the table or view is defined ID of the table or view on which the index is defined ID of the index Number of seeks by user queries Number of scans by user queries Number of lookups by user queries Number of updates by user queries Time of last user seek Time of last user scan Time of last user lookup
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You should typically query this view for indexes that have low values in the user_seeks or user_scans column. All values in the sys.dm_db_index_usage_stats view are reset whenever the SQL Server service is restarted. The values for a specific database are removed if the database is either detached or shut down. Note that indexes that have not been used since the sys.dm_db_index_usage_stats view was reset are not included in the view.
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Partitioning
Starting with SQL Server 2005, you can choose to partition tables and indexes horizontally (that is, by rows) into smaller chunks. The main use for this is improving import performance by reducing the work needed to be performed by SQL Server when importing data. Why would it reduce the work needed As shown earlier in this lesson, it is quite a lot of work for SQL Server to maintain indexes when rows are inserted, updated, or deleted. Of course, this is also true when performing an import. If you don t use partitioning, you have the choice of either importing your new data into a table and letting SQL Server automatically update the indexes. As you have seen, this causes a lot of fragmentation. If it caused enough fragmentation, you are likely to want to rebuild your indexes after the import. Considering that this happens, it is faster to drop all the indexes, import the data, and then re-create all the indexes again. That way, SQL Server doesn t waste resources maintaining the indexes during the import. This is where partitioning really excels. What if, instead of re-creating your indexes for the entire table, you just did it for the data being inserted The import performance would vastly improve. With partitioning, you
236 CHAPTER 6 Techniques to Improve Query Performance
could create a new table without any indexes on it, import the data into the table, create the indexes on this new table, and finally add it as a new partition to the main table. Partitioning can also help query performance, but query performance is best helped using indexes rather than partitioning. To be able to partition tables and indexes, you first need to create two objects: a partition function and a partition scheme. The partition function simply defines the points (or rather, values) where each partition ends. The partition scheme defines on which file group each partition goes; note that you can also define one file group to hold all partitions.
Partition Functions
Partition functions are created using the CREATE PARTITION FUNCTION statement. A partition function is simply a list of up to 999 values that define dividers between partitions. You can decide if the values that you supply are interpreted as less than or equal to (<=) or less than (<) by defining the partition function as either LEFT or RIGHT. The following code creates a partition function defined as LEFT. The resulting partitions are shown in Table 6-10:
CREATE PARTITION FUNCTION PF(INT) AS RANGE LEFT FOR VALUES (10, 20, 30);
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