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Composite Indexes and Included Columns
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You can specify a column for a clustered index and then specify included columns to create a composite index. If you specify two or more column names, you create a composite index on the combined values in the specified columns. In the CREATE INDEX Transact-SQL statement, you should list the columns to be included in the composite index, in sort-priority order, inside the parentheses after table_or_view_name. You can combine up to 16 columns into a single composite index key. All the columns in a composite index key must be in the same table or view. The maximum allowable size of the combined index values is 900 bytes.
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Normalization
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The logical design and optimization of a database including the design of tables, indexes, views, constraints, and table joins is part of a process known as normalization. A good logical database design can lay the foundation for optimal database and application performance. Normalizing a logical database design involves using formal methods to separate the data into multiple, related tables. Several narrow tables with fewer columns are characteristic of a normalized database. A few wide tables with more columns are characteristic of a non-normalized database. Reasonable normalization frequently improves performance. When useful indexes are available, the SQL Server query optimizer is efficient at selecting rapid, efficient joins between tables.
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Using Covering Indexes
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If you use a nonclustered index and you know that your application will be performing the same query many times on the same table, you should consider creating a covering index on the table. A covering index includes all the columns referenced in the SELECT, JOIN, and WHERE clauses of a query. As a result, the index contains the data for which you are looking and your query does not need to access the table to obtain the data you require. The rows of the covering index contain a subset of the
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columns in the table; thus, there are more rows per page. As a result, SQL Server needs to perform fewer paging operations to retrieve the query data, boosting performance. If, on the other hand, the covering index covers too many columns, disk I/O might be increased and performance degraded. You should use covering indexes only if the query or queries are run frequently. Otherwise, the overhead of maintaining the covering index might outweigh the benefits it provides. The covering index should not add significantly to the size of the key and must include all columns found in the SELECT list, JOIN clause, and WHERE clause. If you want to create a covering index, try if possible to incorporate already existing table indexes. For example, if you need a covering index for the Price and Description columns, and you already have an index on the Price column, you should change the index on the Price column to a composite index on Price and Description instead of creating a new covering index. In general, try not to create more than one index on the same column. An alternative to creating covering indexes is to let SQL Server 2005 create the covering indexes for you. The query optimizer can perform index intersection. The optimizer considers multiple indexes from a table, builds a hash table based on the multiple indexes, and then uses the hash table to reduce the I/O the query requires. In effect, the hash table becomes a covering index for the query. If you create single column, nonclustered indexes on all the columns in a table that will be queried frequently, the query optimizer will be provided with the data it needs to create covering indexes as required.
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A SQL Server 2005 database contains a table named Products.Electrical that has more than 30 columns. Typically, queries against that table are of the following form:
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