Dead Lock occur when foreign key exists on table - sql-server

Question

There are two table exist in DB, Audit and AuditField, following is the Create table code:
-- Primary key: ID
CREATE TABLE [dbo].[Audit](
[ID] [int] IDENTITY(1,1) NOT NULL PRIMARY KEY,
[TypeName] [varchar](50) NOT NULL
)
GO
-- Primary key: ID
CREATE TABLE [dbo].[AuditField](
[ID] [int] IDENTITY(1,1) NOT NULL PRIMARY KEY,
[AuditID] [int] NOT NULL,
[Field1] [varchar](50) NOT NULL
)
GO
-- Set foreign key on AuditField table
ALTER TABLE [dbo].[AuditField]
ADD CONSTRAINT [FK_AuditFiled_Audit] FOREIGN KEY([AuditID])
REFERENCES [dbo].[Audit] ([ID])
GO
Then I prepared some test data:
DECLARE #audit TABLE
(
ID int not null,
TypeName varchar(50)
)
DECLARE #auditField TABLE
(
AuditID int not null,
Field1 varchar(50)
)
-- ADD TEST DATA
DECLARE #i int = 1
DECLARE #rowCount int = 500
WHILE #i<=#rowCount
BEGIN
INSERT INTO #audit
VALUES(#i, 'SomeTypeName')
INSERT INTO #auditField
(AuditID,Field1)
VALUES(#i,'SomeThing')
SET #i += 1
END
Finally, i run following transaction to insert test data to these two table:
begin transaction
INSERT INTO dbo.Audit
SELECT TypeName
FROM #audit
ORDER BY ID
declare #lastIdentity int = ##identity
declare #offSet int = #lastIdentity - #rowCount
INSERT INTO dbo.AuditField
SELECT AuditID+#offSet AS AuditID, Field1
FROM #auditField
ORDER BY AuditID
commit transaction
When this transaction run concurrent, dead lock occur, one process are failed, the other got an error：
Msg 547, Level 16, State 0, Line 40 The INSERT statement conflicted
with the FOREIGN KEY constraint "FK_AuditFiled_Audit". The conflict
occurred in database "MyDB", table "dbo.Audit", column 'ID'.
There is no trigger on Audit and AuditField table.
Sorry for the format of the code, I really need an answer why this dead lock occur, thanks.
One thing should be clear, the data of AuditField table comes from #auditField, As #Bogdan answer I rewrite like this:
begin transaction
INSERT INTO dbo.Audit
OUTPUT inserted.ID INTO #temp
SELECT TypeName
FROM #audit
INSERT INTO #idMapping
SELECT ROW_NUMBER() OVER(ORDER BY ID) AS RowNumber, ID
FROM #temp
INSERT INTO dbo.AuditField
SELECT m.ID AS AuditID, Field1
FROM #auditField af
INNER JOIN #idMapping m ON af.AuditID = m.RowNumber
commit transaction

Answer 1

This is and Read - Write deadlock:
As you can see, every transaction has successfully acquired an [e]X[clusive] lock and it requests a S[hared] lock. The question is why a transaction try to read rows X locked by another transaction.
And the answer is bellow:
1) Following piece of source code
declare #lastIdentity int = ##identity
declare #offSet int = #lastIdentity - #rowCount
assumes that IDENTITY values generated by every
INSERT INTO dbo.Audit
SELECT TypeName
FROM ...
statement are continues. This is completely wrong as you can see in the following picture:
This means that at some point in time, a transaction could successfully get X locks on inserted rows and then
1) Because inserted rows into Audit aren't continuous and
2) Because of
declare #lastIdentity int = ##identity
declare #offSet int = #lastIdentity - #rowCount
INSERT INTO dbo.AuditField
SELECT AuditID+#offSet AS AuditID, Field1 ...
this last INSERT tries to insert into dbo.AuditField, AuditID values that belong to another transaction and this requires FK validation and, also, means that SQL Server needs to read rows from dbo.Audit. For this S[hared] locks are needed.
To be clear: the root cause of this deadlock is not the FK constraint. The real problem is that source code.
Solution: I would rewrite thus:
begin transaction
INSERT INTO dbo.Audit
OUTPUT inserted.ID, inserted.TypeName INTO #audit (ID, TypeName)
SELECT TypeName
FROM #audit
-- ORDER BY ID -- Isn't necessary
... do something (ex. DELETE) with rows from #audit
INSERT INTO dbo.AuditField (AuditID, ...)
SELECT x.ID, ...
FROM #audit x
-- ORDER BY AuditID
/* or
INSERT INTO dbo.AuditField (AuditID, Field1, ....)
SELECT y.ID, y.ColumnName, ...
FROM (
SELECT x.ID, ...
FROM #audit x
UNPIVOT( ColumnValue FOR ColumnName IN ([TypeName], ...) )
) y
WHERE y.....
*/
commit transaction -- Isn't necessary

Answer 2

You are trying to insert an invalid foreign key value into dbo.AuditField:
SELECT AuditID+#offSet AS AuditID, Field1
Why the #offset? You won't necessarily have an AuditId with that value in the dbo.Audit table.