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Cluster, non-cluster index, pk, unique...

Posted on 2014-12-21
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At 7:11 min. into this video, at the screenshot below, the explanation is about having cluster, non-cluster index, pk, unique:

https://www.youtube.com/watch?v=ITcOiLSfVJQ

Question: Could you please expand on this discussion and also explain why database safety issue raised here?

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Question by:Mike Eghtebas
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by:ste5an
ste5an earned 150 total points
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Imho "saftey" refers to carelessness when creating indices. So a primary key is per default a clustered index in shortened syntax.

See Heaps.
In most cases it's better to have a clustered index on the primary key, cause when it's used in foreign key relations it may avoid additional sorting.

But that's just a guess.
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Vitor Montalvão earned 350 total points
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Only if safety means a way to guarantee the uniqueness of a row (no duplicate rows).
The examples shows the different ways to do that, either using PK (always Unique but can be Clustered or non-clustered) or creating indexes with Unique Constraints (can be also Clustered or non-clustered).
Remember that you can only have one clustered index by table because it's the index that says how your records will be physically ordered.
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