Is a Primary Key necessary in SQL Server?

Question

This may be a pretty naive and stupid question, but I\'m going to ask it anyway

I have a table with several fields, none of which are unique, and a primary key, whic

Answer 1

Do you have any foreign keys, do you ever join on the PK?

If the answer to this is no, and your app never retrieves an item from the table by its PK, and no query ever uses it in a where clause, therefore you just added an IDENTITY column to have a PK, then:

• the PK in itself adds no value, but does no damage either
• the fact that the PK is very likely the clustered index too is .. it depends.

If you have NC indexes, then the fact that you have a narrow artificial clustered key (the IDENTITY PK) is helpful in keeping those indexes narrow (the CDX key is reproduced in every NC leaf slots). So a PK, even if never used, is helpful if you have significant NC indexes.

On the other hand, if you have a prevalent access pattern, a certain query that outweighs all the other is frequency and importance, or which is part of a critical time code path (eg. is the query run on every page visit on your site, or every second by and app etc) then that query is a good candidate to dictate the clustered key order.

And finally, if the table is seldom queried but often written to then it may be a good candidate for a HEAP (no clustered key at all) since heaps are so much better at inserts. See Comparing Tables Organized with Clustered Indexes versus Heaps.