SQL – many-to-many table primary key


This question comes up after reading a comment in this question:

Database Design

When you create a many-to-many table, should you create a composite primary key on the two foreign key columns, or create a auto-increment surrogate "ID" primary key, and just put indexes on your two FK columns (and maybe a unique constraint)? What are the implications on performance for inserting new records/re-indexing in each case?

Basically, this:

PartID (PK/FK)
DeviceID (PK/FK)

vs. this:

ID (PK/auto-increment)
PartID (FK)
DeviceID (FK)

The commenter says:

making the two IDs the PK means the
table is physically sorted on the disk
in that order. So if we insert
(Part1/Device1), (Part1/Device2),
(Part2/Device3), then (Part 1/Device3)
the database will have to break the
table apart and insert the last one
between entries 2 and 3. For many
records, this becomes very problematic
as it involves shuffling hundreds,
thousands, or millions of records
every time one is added. By contrast,
an autoincrementing PK allows the new
records to be tacked on to the end.

The reason I'm asking is because I've always been inclined to do the composite primary key with no surrogate auto-increment column, but I'm not sure if the surrogate key is actually more performant.

Best Solution

With a simple two-column many-to-many mapping, I see no real advantage to having a surrogate key. Having a primary key on (col1,col2) is guaranteed unique (assuming your col1 and col2 values in the referenced tables are unique) and a separate index on (col2,col1) will catch those cases where the opposite order would execute faster. The surrogate is a waste of space.

You won't need indexes on the individual columns since the table should only ever be used to join the two referenced tables together.

That comment you refer to in the question is not worth the electrons it uses, in my opinion. It sounds like the author thinks the table is stored in an array rather than an extremely high performance balanced multi-way tree structure.

For a start, it's never necessary to store or get at the table sorted, just the index. And the index won't be stored sequentially, it'll be stored in an efficient manner to be able to be retrieved quickly.

In addition, the vast majority of database tables are read far more often than written. That makes anything you do on the select side far more relevant than anything on the insert side.