Why Single Key Column Indexes Aren’t Good Choices

Choose To Lose


Most tables you encounter in SQL Server will have a clustered row store index on them, and probably some nonclustered row store indexes too.

If you live in the data warehouse world, you’ll probably see more and more column store indexes (hopefully), and if you need to report straight from your OLTP tables, you might even see some nonclustered column store indexes.

If your non-staging tables are heaps, you’ve probably got other things you should be doing than reading this post, like figuring out a good clustered index for them.

But anyway! Let’s focus on the most common types of tables, so that most everyone can happily follow along.

Drown Out


When you’ve got a table with a row store clustered index, all of your nonclustered indexes will “inherit” the keys of the clustered index. Where they end up depends on if the nonclustered index is defined as unique or not.

  • Non unique nonclustered row store indexes will store them in the key
  • Unique nonclustered row store indexes will store them as includes

There are times when a single key column index can be useful, like for a unique constraint.

But for the most part, outside of the occasional super-critical query that needs to be tuned, single key column indexes either get used in super-confusing ways, or don’t get used at all and just sit around hurting your buffer pool and transaction log, and increasing the likelihood of lock escalation.

Expansive


I can hear a lot of you saying that you use them to help foreign keys, and while a single key column index may get used for those processes, you most likely have many other queries that join tables with foreign key relationships together.

Those queries aren’t gonna sit around with just join columns. You’re gonna select, filter, group, and order those columns too, and wider indexes are gonna be way more helpful for that, and wider indexes are just as useful for helping foreign keys do their job.

If you have a single key column index, and a wider index that leads with the same key column, you really need to ask yourself why you have that single key column index around anymore.

In extreme cases, I see people create a single key column index on every column in a table. That’s beyond absurd, and a recipe for disaster in all of the ways listed above.

If you truly need an index on every single column, then you need a column store index.

Thanks for reading!



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