五种提高SQL性能的方法(三)

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 This query is not only more efficient than the previous one, it's shorter. Temporary tables consume a lot of resources. If you only need the data to join to other queries, you might want to try using an inline view to conserve resources.
 Avoid LEFT JOINs and NULLs
 There are, of course, times when you need to perform a LEFT JOIN and use NULL values. But they are not a solution for all occasions. Changing the way you structure your SQL queries can mean the difference between a report that takes minutes to run and one that takes only seconds. Sometimes you have to morph the data in a query to look the way your application wants it to look. While the TABLE datatype reduces resource gluttony, there are still plenty of areas in a query that can be optimized. One valuable, commonly used feature of SQL is the LEFT JOIN. It can be used to retrieve all of the rows from a first table and all matching rows from a second table, plus all rows from the second table that do not match the first one. For example, if you wanted to return every Customer and their orders, a LEFT JOIN would show the Customers who did and did not have orders.
 This tool can be overused. LEFT JOINs are costly since they involve matching data against NULL (nonexistent) data. In some cases this is unavoidable, but the cost can be high. A LEFT JOIN is more costly than an INNER JOIN, so if you could rewrite a query so it doesn't use a LEFT JOIN, it could pay huge dividends (see the diagram in Figure 1).
 
 Figure 1 Query
 One technique to speed up a query that uses a LEFT JOIN involves creating a TABLE datatype and inserting all of the rows from the first table (the one on the left-hand side of the LEFT JOIN), then updating the TABLE datatype with the values from the second table. This technique is a two-step process, but could save a lot of time compared to a standard LEFT JOIN. A good rule is to try out different techniques and time each of them until you get the best performing query for your application.
 When you are testing your query's speed, it's important to run it several times and take an average. Your query (or stored procedure) could be stored in the procedure cache in SQL Server's memory and thus would appear to take longer the first time and shorter on all subsequent tries. In addition, other queries could be running against the same tables while your query runs. This could cause your query to stand in line while other queries lock and unlock tables. For example, if you are querying while someone is updating data in that table, your query may take longer to execute while the update commits.
 One of the easiest ways to avoid slowdowns with LEFT JOINs is to design the database around them as much as possible. For example, let's assume that a product may or may not have a category. If the product table stores the ID of its category and there was no category for a particular product, you could store a NULL value in the field. Then you would have to perform a LEFT JOIN to get all of the products and their categories. You could create a category with the value of "No Category" and thus specify the foreign key relationship to disallow NULL values. By doing this, you can now use an INNER JOIN to retrieve all products and their categories. While this may seem like a workaround with extra data, this can be a valuable technique as it can eliminate costly LEFT JOINs in SQL batches. Using this concept across the board in a database can save you lots of processing time. Remember, even a few seconds means a lot to your users, and those seconds really ad

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