Adding a Primary Key and Altering a Column to DatetTime2

Question

I have a table that tracks statuses that a particular file goes through as it is checked over by our system. It looks like this:
FileID int
Status tinyint
TouchedBy varchar(50)
TouchedWhen datetime

There is currently NO primary key on this table however there is a clustered index on Status and TouchedWhen

As the table has continued to grow and performance decrease in querying against it, one thought I've had is to do add a PrimaryKey so that I get off the heap lookups -- a primary key on FileID, Status and TouchedWhen

The problem I'm running into si that TouchedWhen, due to it's rounding issues, has, on occasion, 2 entries with the exact same datetime.

So then I started researching what it takes to convert that to a datetime2(7) and alter those that are duplicate at that time. My table would then look like:
FileID int
Status tinyint
TouchedBy varchar(50)
TouchedWhen datetime2(7)

And a primarykey on FileID, Status and TouchedWhen

My question is this -- what is the best way to go through and add a millisecond to the existing tables if there are duplicates? How can I do this to a table that needs to remain online?

In advance, thanks,
Brent

Answer 1

You shouldn't need to add a primary key to make queries faster - just adding an index on FileID, Status, TouchedWhen will have just as much of a performance impact as adding a primary key. The main benefit of defining a primary key is for record identity and referential integrity, which could be accomplished with a auto-increment primary key.

(I'm NOT saying you shouldn't have a primary key, I'm saying the performance impact of a primary key is in the index itself, not the fact that it's a primary key)

On the other hand, changing your clustered index to include FileID would likely have a bigger impact as lookups using those columns would not need to search the index then look up the data - the data pages would be right there with the index values.