Clustered and nonclustered indexes performance

Question

I have a huge table (~ 10 million rows) with clustered PK on a random uniqueidentifier column. The most operations I do with this table is inserting a new row if there is not yet a row with the same pk. (To improve performance of it I use IGNORE_DUP_KEY = ON option)

My question is

Can I get rid of clustered index at all on this table? I mean when I insert a row into a table with clustered index it should rearrange data physicaly. May be it is better to drop clustered index and create nonclustered index on that colum to avoid data rearrangement?

I can't do an experiment on the live db because if performance falls down it will be a headache. On the test db I can only see 'Clustered Index Insert 100%' in the case with clustered index and 'table insert' + some seeking opertations in the nonclustered index in the case with non-clustered index.

Thanks in advance

Answer 1

GUIDs may seem to be a natural choice for your primary key - and if you really must, you could probably argue to use it for the PRIMARY KEY of the table. What I'd strongly recommend not to do is use the GUID column as the clustering key, which SQL Server does by default, unless you specifically tell it not to.

You really need to keep two issues apart:

1) the primary key is a logical construct - one of the candidate keys that uniquely and reliably identifies every row in your table. This can be anything, really - an INT, a GUID, a string - pick what makes most sense for your scenario.

2) the clustering key (the column or columns that define the "clustered index" on the table) - this is a physical storage-related thing, and here, a small, stable, ever-increasing data type is your best pick - INT or BIGINT as your default option.

By default, the primary key on a SQL Server table is also used as the clustering key - but that doesn't need to be that way! I've personally seen massive performance gains when breaking up the previous GUID-based Primary / Clustered Key into two separate key - the primary (logical) key on the GUID, and the clustering (ordering) key on a separate INT IDENTITY(1,1) column.

As Kimberly Tripp - the Queen of Indexing - and others have stated a great many times - a GUID as the clustering key isn't optimal, since due to its randomness, it will lead to massive page and index fragmentation and to generally bad performance.

Yes, I know - there's newsequentialid() in SQL Server 2005 and up - but even that is not truly and fully sequential and thus also suffers from the same problems as the GUID - just a bit less prominently so.

Then there's another issue to consider: the clustering key on a table will be added to each and every entry on each and every non-clustered index on your table as well - thus you really want to make sure it's as small as possible. Typically, an INT with 2+ billion rows should be sufficient for the vast majority of tables - and compared to a GUID as the clustering key, you can save yourself hundreds of megabytes of storage on disk and in server memory.

Quick calculation - using INT vs. GUID as Primary and Clustering Key:

• Base Table with 1'000'000 rows (3.8 MB vs. 15.26 MB)
• 6 nonclustered indexes (22.89 MB vs. 91.55 MB)

TOTAL: 25 MB vs. 106 MB - and that's just on a single table!

Some more food for thought - excellent stuff by Kimberly Tripp - read it, read it again, digest it! It's the SQL Server indexing gospel, really. As she shows in her "The Clustered Index Debate contiues", having a good clustering key (as opposed to none or a bad one) really does speed up pretty much all database operations! It's a good idea - but it has to be a good clustering key....

• GUIDs as PRIMARY KEY and/or clustered key
• The clustered index debate continues
• Ever-increasing clustering key - the Clustered Index Debate..........again!
• Disk space is cheap - that's not the point!

Marc