Question

Defining a column to be a primary in table on SQL Server – will this make inserts slower?

I ask because I understand this is the case for indexes.

The table has millions of records.

Answer 1

No, not necessarily! Sounds counter-intuitive, but read this quote from Kim Tripp’s blog post:

Inserts are faster in a clustered
table (but only in the “right”
clustered table) than compared to a
heap. The primary problem here is that
lookups in the IAM/PFS to determine
the insert location in a heap are
slower than in a clustered table
(where insert location is known,
defined by the clustered key). Inserts
are faster when inserted into a table
where order is defined (CL) and where
that order is ever-increasing.

So actually, having a good clustered index (e.g. on a INT IDENTITY column, if ever possible) does speed things up – even insert, updates and deletes!