Question

I am wondering if there is a best coding practice in regard to dealing with IDs for parent > child objects in the code, where the DB records use an auto incremented int as the ID (on the initial save). Of course when in the code you are unable to guess what this ID might be and therefore must leave it blank and presumably save all these items in a transaction grabbing the parent ID first and then setting it on all children before saving them

Guids on the other hand are much easier to deal with in the code, as of course you can happily generate the Id first and set it on everything and save without worry..

is there a nice easy way to deal with objects in code using auto-ints as their db keys?

thanks

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.

• 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!

Also: from a C# / .NET point of view – it depends on how you access your SQL Server database. If you use something like Linq-to-SQL or Entity Framework v4, your .NET objects will be automagically updated with the inserted IDs (from your INT IDENTITY column) – without you having to do anything at all. So to me, that’s one less reason why you should feel the need to use GUID’s….

GUID’s are horribly bad as SQL Server clustering keys – not just bad – really, truly, AWFUL