Question

If a SQL Server 2008r2 database were receiving update statements to both a child table, and also to the parent table (related by a Foreign Key constraint) at the same time, would the update statements be able to create a deadlock situation?

Note based on comments: The fields being updated in this situation are not the key fields, they are just counter fields.

Thanks.

Answer 1

Yes you can. Here’s proof:

--setup
use tempdb;
create table Parent (
    ParentID int not null, 
    constraint PK_Parent primary key clustered (ParentId)
);
insert into Parent values (1), (2), (3);
create table Child (
    ChildId int identity, 
    constraint PK_Child primary key clustered (ChildId),
    ParentId int, 
    constraint FK_Child_Parent foreign key (ParentId) 
        references Parent (ParentId)
);
insert into Child (ParentId) values (2), (2), (3);

--in window 1
use tempdb;
begin tran;
update Child set ParentId = 1 where ParentId = 3;


--in window 2
use tempdb;
begin tran;
update Parent set ParentId = 4 where ParentId = 1;

--back in window 1
update Child set ParentId = 4 where ParentId = 2;

I tested this and was able to produce a deadlock.