[轉] Transact-SQL Optimization Tips

N_chow 2002-07-30 03:44:32

Here are fourteen little known tips that you can use to ensure your Transact-SQL queries are performing in the most efficient manner possible.

1. Try to restrict the queries result set by using the WHERE clause.

This can result in a performance benefit, as SQL Server will return to the client only particular rows, not all rows from the table(s). This can reduce network traffic and boost the overall performance of the query.

2. Try to restrict the queries result set by returning only the particular columns from the table, not all the table's columns.

This can result in a performance benefit as well, because SQL Server will return to the client only particular columns, not all the table's columns. This can reduce network traffic and boost the overall performance of the query.

3. Use views and stored procedures instead of heavy-duty queries.

This can reduce network traffic as your client will send to the server only stored procedures or view name (perhaps with some parameters) instead of large heavy-duty queries text. This can be used to facilitate permission management also, because you can restrict user access to table columns they should not see.

4. Whenever possible, try to avoid using SQL Server cursors.

SQL Server cursors can result in some performance degradation in comparison with select statements. Try to use correlated subquery or derived tables, if you need to perform row-by-row operations.

5. If you need to return the total table's row count, you can use an alternative way instead of the SELECT COUNT(*) statement.

Because the SELECT COUNT(*) statement makes a full table scan to return the total table's row count, it can take an extremely long time for large tables. There is another way to determine the total row count in a table. In this case, you can use the sysindexes system table. There is a ROWS column in the sysindexes table. This column contains the total row count for each table in your database. So, you can use the following select statement instead of SELECT COUNT(*):

SELECT rows FROM sysindexes WHERE id = OBJECT_ID('table_name') AND indid < 2

This way, you can improve the speed of such queries by several times. See this article for more details: Alternative way to get the table's row count.

6. Try to use constraints instead of triggers, whenever possible.
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Constraints are much more efficient than triggers and can boost performance. So, whenever possible, you should use constraints instead of triggers.

7. Use table variables instead of temporary tables.
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Table variables require fewer locking and logging resources than temporary tables, so table variables should be used whenever possible. The table variables are available in SQL Server 2000 only.

8. Try to avoid the HAVING clause, whenever possible.

The HAVING clause is used to restrict the result set returned by the GROUP BY clause. When you use GROUP BY with the HAVING clause, the GROUP BY clause divides the rows into sets of grouped rows and aggregates their values, and then the HAVING clause eliminates undesired aggregated groups. In many cases, you can write your select statement so that they will contain only WHERE and GROUP BY clauses without the HAVING clause. This can improve the performance of your query.

9. Whenever possible, try to avoid using the DISTINCT clause.

Because using the DISTINCT clause will result in some performance degradation, you should use this clause only when it is absolutely necessary.

10. Include SET NOCOUNT ON statement into your stored procedures to stop the message indicating the number of rows affected by a T-SQL statement.

This can reduce network traffic, as your client will not receive the message indicating the number of rows affected by a T-SQL statement.

11. Use select statements with the TOP keyword or the SET ROWCOUNT statement if you need to return only the first n rows.

This can improve performance of your queries, as a smaller result set will be returned. This can also reduce the traffic between the server and the clients.

12. Use the FAST number_rows table hint if you need to quickly return 'number_rows' rows.

You can quickly get the n rows and can work with them when the query continues execution and produces its full result set.

13. Try to use UNION ALL statement instead of UNION, whenever possible.
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The UNION ALL statement is much faster than UNION, because UNION ALL statement does not look for duplicate rows, while the UNION statement does look for duplicate rows, whether they exist or not.

14. Do not use optimizer hints in your queries.
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Because the SQL Server query optimizer is very clever, it is highly unlikely that you can optimize your query by using optimizer hints; more often than not, this will hurt performance.

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