c# itextsharp datamatrix barcode Keeping the Data in Order in Visual C#

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Keeping the Data in Order
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As previously stated, a clustered index actually keeps the data in the table in a specific order. When you specify a column (or multiple columns) as a clustered index, on inserting a record SQL Server will place that record in a physical position to keep the records in the correct ascending or descending order that corresponds to the order defined in the index. To explain this a bit further, if you have a clustered index on customer numbers, and the data currently has customer numbers 10, 6, 4, 7, 2, and 5, then SQL Server will physically store the data in the following order: 2, 4, 5, 6, 7, 10. If a process then adds in a customer number 9, it will be physically inserted between 7 and 10, which may mean that the record for customer number 10 needs to move physically. Therefore, if you have defined a clustered index on a column or a set of columns where data insertions cause the clustered index to be reordered, this is going to greatly affect your insert performance. SQL Server does provide a way to reduce the reordering impact by allowing a fill factor to be specified when an index is created.
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CHAPTER 6 CREATING INDEXES AND DATABASE DIAGRAMMING
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Determining What Makes a Bad Index
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Now that you know what makes a good index, let s investigate what makes a bad index. There are several gotchas to be aware of: Using unsuitable columns Choosing unsuitable data Including too many columns Including too few records in the table
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Using Unsuitable Columns
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If a column isn t used by a query to locate a row within a table, then there is a good chance that the column does not need to be indexed, unless it is combined with another column to create a covering index, as described earlier. If this is the case, the index will still add overhead to the data-modification operations but will not produce and performance benefit to the dataretrieval operations.
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Choosing Unsuitable Data
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Indexes work best when the data contained in the index columns is highly selective between rows. The optimal index is one created on a column that has a unique value for every row within a table, such as a primary key. If a query requests a row based on a value within this column, SQL Server can quickly navigate the index structure and identify the single row that matches the query predicate. However, if the selectivity of the data in the index columns is poor, the effectiveness of the index is reduced. For example, if an index is created on a column that contains only three distinct values, the index would be able to reduce the number of rows to just a third of the total before applying other methods to identify the exact row. In this instance, SQL Server would probably ignore the index anyway and find that reading the data table instead would be faster. Therefore, when deciding on appropriate index columns, you should examine the data selectivity to estimate the effectiveness of the index.
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Including Too Many Columns
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The more columns there are in an index, the more data writing has to take place when a process completes an update or an insertion of data. Although in SQL Server 2005 these updates to the index data take a very short amount of time, it can add up. Therefore, each index that is added to a table will incur extra processing overhead, so it is recommended that you create the minimum number of indexes needed to give your data-retrieval operations acceptable performance.
Including Too Few Records in the Table
There is also absolutely no need to place an index on a table that has only one row. SQL Server will find the record at the first request, without the need of an index.
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