I have a series of questions about Keys, Indexes and Constraints in SQL, SQL 2005 in particular. I have been working with SQL for about 4 years but I have never been able to get definitive answers on this topic and there is always contradictory info on blog posts, etc. Most of the time tables I create and use just have an Identity column that is a Primary Key and other tables point to it via a Foreign Key.
With join tables I have no Identity and create a composite Primary Key over the Foreign Key columns. The following is a set of statements of my current beliefs, which may be wrong, please correct me if so, and other questions.
So here goes:
As I understand it the difference between a Clustered and Non Clustered Index (regardless of whether it is Unique or not) is that the Clustered Index affects the physical ordering of data in a table (hence you can only have one in a table), whereas a Non Clustered Index builds a tree data structure. When creating Indexes why should I care about Clustered vs Non Clustered? When should I use one or the other? I was told that inserting and deleting are slow with Non-Clustered indexes as the tree needs to be “rebuilt.” I take it Clustered indexes do not affect performance this way?
I see that Primary Keys are actually just Clustered Indexes that are Unique (do they have to be clustered?). What is special about a Primary Key vs a Clustered Unique Index?
I have also seen Constraints, but I have never used them or really looked at them. I was told that the purpose of Constraints is that they are for enforcing data integrity, whereas Indexes are aimed at performance. I have also read that constraints are acually implemented as Indexes anyway so they are “the same.” This doesnt sound right to me. How are constraints different to Indexes?
Clustered indexes are, as you put it correctly, the definition as to how data in a table is stored physically, i.e. you have a B-tree sorted using the clustering key and you have the data at the leaf level.
Non-clustered indexes on the other hand are separate tree structures which at the leaf level only have the clustering key (or a RID if the table is a heap), meaning that when you use a non-clustered index, you’ll have to use the clustered index to get the other columns (unless your request is fully covered by the non-clustered index, which can happen if you request only the columns, which constitute the non-clustered index key columns).
When should you use one or the other ? Well, since you can have only one clustered index, define it on the columns which makes most sense, i.e. when you look up clients by ID most of the time, define a clustered index on the ID. Non-clustered indexes should be defined on columns which are used less often.
Regarding performance, inserts or updates that change the index key are always painfull, regardless of whether it is a clusted on non-clustered index, since page splits can happen, which forces data to be moved between pages (moving the pages of a clustered index hurts more, since you have more data in the leaf level). Thus the general rule is to avoid changing the index key and inserting new values so that they would be sequencial. Otherwise you’ll encounter fragmentation and will have to rebuild your index on a regular basis.
Finally, regarding constraints, by definition, they have nothing to do with indexes, yet SQL server has chosen to implement them using indexes. E.g. currently, a unique constraint is implemented as an index, however this can change in a future version (though I doubt that will happen). The type of index (clustered or not) is up to you, just remember that you can have only one clustered index.
If you have more questions of this type, I highly recommend reading this book, which covers these topics in depth.