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DBA Myths: A table with a primary key is not a heap

Typically when you see a heading like this you know the answer is “No” or “False” but in this case it’s more of a “hu?” You see a primary key and a table being a heap have nothing to do with each other. Well very little anyway.

  • A “Primary Key” is a special case of a unique constraint (enforced by an index) that will not allow NULL values. There can be only one Primary Key.
  • A “Heap” on the other hand is a table without a clustered index.

Note the important terms here are “unique key” and “clustered index”. I should probably point out that a unique key/primary key can be clustered or non-clustered. Thus a table with a primary key can be a heap or not.

So why the confusion? Usually the default when you create a primary key is to create a unique clustered index to enforce it. Thus by default creating a primary key does in fact stop a table from being a heap. However you can override this (and in some cases clustered isn’t even the default) and create a non-clustered unique index to support the constraint. And a non-clustered primary key does not affect a table’s “heapness” (is that even a word?)

Filed under: Index, Microsoft SQL Server, SQLServerPedia Syndication Tagged: code language, index, language sql, microsoft sql server, T-SQL


My name is Kenneth Fisher and I am Senior DBA for a large (multi-national) insurance company. I have been working with databases for over 20 years starting with Clarion and Foxpro. I’ve been working with SQL Server for 12 years but have only really started “studying” the subject for the last 3. I don’t have any real "specialities" but I enjoy trouble shooting and teaching. Thus far I’ve earned by MCITP Database Administrator 2008, MCTS Database Administrator 2005, and MCTS Database Developer 2008. I’m currently studying for my MCITP Database Developer 2008 and should start in on the 2012 exams next year. My blog is at www.sqlstudies.com.


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