Question

I’m new in SQL Server development and I’m currently reading a book, the thing is that in this book I read that if I update an indexed column I will affect all the records in that column because an index is like the index of a book.

In my work, the SSN (social security Number) is the primary key table, and I’ve been asked to enable de SSN modification on our application.

So I am wondering…. what is the cost of modifying a clustered index column?

Thanks in advance!

Answer 1

The cost is quite high – another reason why a good clustering key should be static, e.g. never change.

The clustered index column(s) are actually present inside each and every non-clustered index, too – so if you have a table with 10 non-clustered indices, updating the value of a column used in the clustering key on that table will need to go out and update all 10 non-clustered indices, too.

Read Kimberly Tripp’s blog post Ever-increasing clustering key – the Clustered Index Debate……….again! to learn what constitutes a good clustering key on a table.

To sum up briefly – a good clustering key should be:

• narrow – 4 bytes is perfect, 8 bytes is tolerable, anything beyond that is getting bad on performance….

• unique – the clustering key is the ultimate lookup for your data – if that column (or set of columns) isn’t unique, SQL Server will add a 4-byte uniquefier to your data – not recommended….

• static – the value of the clustering column(s) should never change – for performance reasons

• ever-increasing – to avoid index fragmentation

INT IDENTITY or BIGINT IDENTITY are thus the best options for you – if you can, use one of those. Avoid large compound indices, avoid large variable-length string columns, avoid GUID (heavy on fragmentation and thus bad for performance)