手动为主键创建索引是一种好习惯吗？

Question

I am recently working with Entity Framework Core and I have some issue about the relation between the primary key and the indexes.

To be more concrete, I found out that in a table containing composite primary keys an index is created for the second property of the key.

You can see an example here

Can you explain me if I should manually create another index for the first one? Or is a clustered index created for that first property?

Answer 1

Generally an index on a set of columns can be used even if a query is only searching some of the columns, with the restriction that the query must ask for columns from the index left to right, no gaps

Thus if a set of columns A,B,C,D are indexed, this index can still be used to answer queries that are filtering on A , A and B , A and B and C .

Thus you don't need to index NoteID separately, because the index that aids the primary key (NoteID, CategoryID) can be used by queries calling for just NoteID. It cannot, however, be used to answer queries calling for just CategoryID, hence the separate index being created

As an aside, you might find, in some cases, that you can supply values in a where clause that have no purpose other than to encourage use of an index that covers them. Suppose, for example, that a table has an index on Name, Gender, Age , and you want all 20 year old people named Steven. If you can reasonably assert that Steven is always male, you can WHERE Name = 'Steven' AND Gender = 'M' AND Age = 20 - even though the Gender of M is redundant, specifying it will let the DB engine use that index. Omitting it means the DB will have a much harder job of figuring out whether to use the index or not

You can also re-arrange index ordering to help your application perform, and give the DB fewer indexes to maintain.. If all your queries will only ever ask for A, A+C or A+B+C, it would be better to specify the index for the columns in the order A,C,B then a single index can cover all queries, rather than having to maintain an index of A+B+C and another of A+C

Answer 2

You don't need to create an index on NoteID because it is the first column of the primary key. Generally, you want all foreign keys to be the first column in at least one index. In your case, NoteID is the first column in the primary key which acts as a clustered unique index.