c# itextsharp datamatrix barcode Determining What Makes a Good Index in C#.NET

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Determining What Makes a Good Index
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To create an index on a table, you have to specify which columns are contained within the index. Columns in an index do not have to all be of the same data type. You should be aware that there is a limit of 16 columns on an index, and the total amount of data for the index columns within a row cannot be more than 900 bytes. To be honest, if you get to an index that contains more than four or five columns, you should stand back and re-evaluate the index definition. Sometimes you ll have more than five columns, but you really should double-check. It is possible to get around this restriction and have an index that does include columns that are not part of the key: the columns are tagged onto the end of the index. This will mean that the index takes up more space, but if it means that SQL Server can retrieve all of the data from an index search, then it will be faster. However, to reiterate, if you are going down this route for indexes, then perhaps you need to look at your design. In the sections that follow, we ll examine some of factors that can determine if an index is good: Using low-maintenance columns Using primary and foreign keys Being able to find a specific record Using covering indexes Looking for a range of information Keeping the data in order
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Using Low-Maintenance Columns
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As I ve indicated, for nonclustered indexes the actual index data is separate from the table data, although both can be stored in the same area or in different areas (e.g., on different hard drives). To reiterate, this means that when you insert a record into a table, the information from the columns included in the index is copied and inserted into the index area. So, if you alter data in a column within a table, and that column has been defined as making up an index, SQL Server
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CHAPTER 6 CREATING INDEXES AND DATABASE DIAGRAMMING
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also has to alter the data in the index. Instead of only one update being completed, two will be completed. If the table has more than one index, and in more than one of those indexes is a column that is to be updated a great deal, then there may be several disk writes to perform when updating just one record. While this will result in a performance reduction for datamodification operations, appropriate indexing will balance this out by greatly increasing the performance of data-retrieval operations. Therefore, data that is low maintenance namely, columns that are not heavily updated could become an index and would make a good index. The fewer disk writes that SQL Server has to do, the faster the database will be, as well as every other database within that SQL Server instance. Don t let this statement put you off. If you feel that data within a table is retrieved more often than it is modified, or if the performance of the retrieval is more critical than the performance of the modification, then do look at including the column within the index. In the example application we re building, each month we need to update a customer s bank balance with any interest gained or charged. However, we have a nightly job that wants to check for clients who have between $10,000 and $50,000, as the bank can get a higher rate of deposit with the Federal Reserve on those sorts of amounts. A client s bank balance will be constantly updated, but an index on this sort of column could speed up the overnight deposit check program. Before the index in this example is created, we need to determine if the slight performance degradation in the updating of the balances is justified by the improvement of performance of the deposit check program.
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