Code example for adding unique constraints in SQL Server using SQL

Using UNIQUEIDENTIFIER data types provides benefits beyond just ensuring uniqueness by preventing hot spots in data pages due to their random nature. However, it’s important to note that page splitting is unavoidable when using UNIQUEIDENTIFIER data types, especially if they are included in a clustered key.

Solution 1:

There is a certain amount of write overhead associated with any index, but it also provides benefits for read operations.

To ensure unique values, it is important for the search speed of [EventId] to outweigh the expenses of maintaining an index. Failing to implement


means that the database would not enforce uniqueness, resulting in a table scan when searching for an existing value.

You can minimize fragmentation by using a fill amount that is below 100.

Using newsequentialid as the default option can help decrease fragmentation.

Solution 2:

There is a performance impact associated with every index, primary key, or unique constraint. However, the benefit of improved retrieval performance outweighs the minimal impact on write performance.

The performance of indexing is influenced by the size of the key or value. The data type’s size is reflected when indexing a CHAR or VARCHAR field. For instance, a BIGINT is 64bits, while an INT is 32bits, and a UNIQUEIDENTIFIER is a 128bit value.

When using a UNIQUEIDENTIFIER, page splitting can cause performance issues. Pages hold up to 8K of records, and if a new record needs to be added in the middle of a full page, two new pages will need to be created to accommodate the additional data. This process can be especially problematic in a clustered index because it impacts the physical storage of records.

Employing UNIQUEIDENTIFIER data types in a clustered key amplifies the occurrence of page splitting, which is a natural aspect of its usage. Therefore, I advise against utilizing a clustered key for such data types.

Apart from ensuring uniqueness, the random nature of UNIQUEIDENTIFIER data types provides a benefit by preventing the occurrence of hot spots in data pages.

Your instance has an INT identity column and a UNIQUEIDENTIFIER column with a default value of NewID(). It piques my interest to learn the reason for having both, assuming this is not just an example.

Solution 3:

This hyperlink may be outdated, but it provides a response to your query. According to the query optimizer, unique constraints and unique indexes are essentially identical in terms of performance, and there is no advantage to using one over the other.

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