In SQL Server, do composite primary keys increase the chance of a deadlock?
sql, sql-server
Solution
If you are using composite PKs and are inserting large amounts of data in parallel you might get hit by resource hash collision. See "The Curious Case of the Dubious Deadlock and the Not So Logical Lock" for a real world example.
And for the explanation to resource hash collision I'll cite Remus Rusanu in "%%lockres%% collision probability magic marker: 16,777,215" (recommended read):
The lock manager in SQL Server doesn’t know what it locks, it just locks ‘resources’ (basically strings). It is the job of higher level components like the the access methods of the storage engine to present the ‘resource’ to the lock manager and ask for the desired lock. When locking rows in a heap or a b-tree the storage engine will synthesize a ‘resource’ from the record identifier. Since these resources have a limited length, the storage engine has to reduce the effective length of a key to the maximum length is allowed to present to the lock manager, and that means that the record’s key will be reduced to 6 bytes. This is achieved by hashing the key into a 6 byte hash value.
[...]
On 6 bytes there are 281,474,976,710,656 distinct possible values. Its a pretty big number? Not that big actually. [...] So the SQL %%lockres%% hash will produce two records with same hash, with a 50% probability, out of the table, any table, of only 16,777,215 record.
Problem
I was wondering if, either because of increased row locking, and/or because of increased time spent holding locks, having a composite primary key defined on a table would increase the chance of having a deadlock when a it is being updated my multiple threads at once? Thank you for any help.