I am faced with the dilemma of changing my primary keys from int identities to Guid. I’ll put my problem straight up. It’s a typical Retail management app, with POS and back office functionality. Has about 100 tables. The database synchronizes with other databases and receives/ sends new data.
Most tables don’t have frequent inserts, updates or select statements executing on them. However, some do have frequent inserts and selects on them, eg. products and orders tables.
Some tables have upto 4 foreign keys in them. If i changed my primary keys from ‘int’ to ‘Guid’, would there be a performance issue when inserting or querying data from tables that have many foreign keys. I know people have said that indexes will be fragmented and 16 bytes is an issue.
Space wouldn’t be an issue in my case and apparently index fragmentation can also be taken care of using ‘NEWSEQUENTIALID()’ function. Can someone tell me, from there experience, if Guid will be problematic in tables with many foreign keys.
I’ll be much appreciative of your thoughts on it…
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Answer
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!
So if you really must change your primary keys to GUIDs – try to make sure the primary key isn’t the clustering key, and you still have an INT IDENTITY field on the table that is used as the clustering key. Otherwise, your performance is sure to tank and take a severe hit .