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

, 2014-05-21

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





Related content

Database Mirroring FAQ: Can a 2008 SQL instance be used as the witness for a 2005 database mirroring setup?

Question: Can a 2008 SQL instance be used as the witness for a 2005 database mirroring setup? This question was sent to me via email. My reply follows. Can a 2008 SQL instance be used as the witness for a 2005 database mirroring setup? Databases to be mirrored are currently running on 2005 SQL instances but will be upgraded to 2008 SQL in the near future.


1,567 reads

Networking - Part 4

You may want to read Part 1 , Part 2 , and Part 3 before continuing. This time around I'd like to talk about social networking. We'll start with social networking. Facebook, MySpace, and Twitter are all good examples of using technology to let...


1,530 reads

Speaking at Community Events - More Thoughts

Last week I posted Speaking at Community Events - Time to Raise the Bar?, a first cut at talking about to what degree we should require experience for speakers at events like SQLSaturday as well as when it might be appropriate to add additional focus/limitations on the presentations that are accepted. I've got a few more thoughts on the topic this week, and I look forward to your comments.


360 reads