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


May 21, 2014 by Kenneth Fisher

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?)

5 thoughts on “DBA Myths: A table with a primary key is not a heap

  1. seb says:


    I think the word you’re looking for is “hepicity”. Or possibly “heapitude” 😉

  2. Joe Celko says:

    Wait until you work with other SQLs. Teradata and larger products use perfect hashing to enforce uniqueness because they have no indexes.

    • Fair enough. The only other DBMS I work with these days is DB2 and I don’t even know it well enough to be certain how it handles uniqueness. I should probably make it more clear that I’m talking specifically about MSSQL.

      Thanks for the comment!

  3. […] want to point out a few things about my tests. I’m using code to create/modify a PRIMARY KEY. PRIMARY KEYs are not necessarily backed by CLUSTERED INDEXes but they are by default and that’s what I’m using here. I could just as easily have […]

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