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Disorderly Queries

Chad Callihan wants you to think about that ORDER BY clause:

I recently came across a scenario where an application process was not performing correctly on one database but was working fine on others. The process should have been completing in seconds but was taking minutes with no indication of activity. After some investigation, I found that the process was stuck waiting on a SELECT statement to complete. Even worse, it was holding an exclusive lock on a table which was then blocking new information from processing.

One part of the SELECT query that stood out was that it was ordering by a date field. Considering what the process was doing, there was no need to have the data ordered. Generally, it’s better to have the data sorted on the application side instead of SQL Server but in this case not even that was necessary.

There are definitely good cases where you need to use ORDER BY in a database—such as paging scenarios. But if you don’t need things in a particular order, Chad shows that you can potentially save a good deal on performance without an explicit ordering.

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