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Paul White: Page Free Space

A technical SQL Server blog from New Zealand.

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  • So…is it a Seek or a Scan?

    You’re probably most familiar with the terms ‘Seek’ and ‘Scan’ from the graphical plans produced by SQL Server Management Studio (SSMS). The image to the left shows the most common ones, with the three types of scan at the top, followed by four types of seek. You might look to the SSMS tool-tip descriptions to explain the differences between them: Not hugely helpful are they? Both mention scans and ranges (nothing about seeks) and the Index Seek description implies that it will not scan the index entirely (which isn’t necessarily true). Recall also yesterday’s post where we saw two Clustered Index Seek operations doing very different things. The first Seek performed 63 single-row seeking operations; and the second performed a ‘Range Scan’ (more on those later in this post). I hope you agree that those were two very different operations, and perhaps you are wondering why there aren’t different graphical plan icons for Range Scans and Seeks? I have often wondered about that, and the first person to mention it after yesterday’s post was Erin Stellato (twitter | blog): Read More...
  • A Tale of Two Index Hints

    If you look up Table Hints in Books Online, you’ll find the following statement: If a clustered index exists, INDEX(0) forces a clustered index scan and INDEX(1) forces a clustered index scan or seek. If no clustered index exists, INDEX(0) forces a table scan and INDEX(1) is interpreted as an error. The interesting thing there is that both hints can result in a scan. If that is the case, you might wonder if there is any effective difference between the two. This blog entry explores that question, and highlights an optimizer quirk that can result in a much less efficient query plan when using INDEX(0). I’ll also cover some stuff about ordering guarantees. Read More...
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