Question :
I have some triggers to log changes on a table to Log table.
On insert and delete I add row to the Log table
and for update I add two rows.
The log table contain identity column, and I want the 2 update rows to be sequential ( by the the id = identity)
for example:
assuming the following table:
Create table t1 ([name] nvarchar(40) primary key, [value] nvarchar(max))
the log table is:
Create table t1_log
([log_id] identity(1,1),[log_ts] DateTime default GETDATE(),
[log_action] varchar(20), log_session_id int default @@SPID,
[name] nvarchar(40), value nvarchar(max))
And I have 3 triggers to update the log:
Create trigger t1_ins on t1 After Insert as
begin
Insert into t1_log([log_action],[name],[value]) select 'insert', [name], [value] from inserted
end
Go
create trigger t1_del on t1 After delete as
begin
Insert into t1_log([log_action],[name],[value]) select 'delete', [name], [value] from deleted
end
Go
create trigger t1_upd on t1 After update as
begin
Insert into t1_log([log_action],[name],[value])
select [log_action], [name], [value] from (
(select ROW_NUMBER() OVER (ORDER BY (SELECT 0)) As ROW_ID, 'update from' as [log_action], [name], [value] from deleted)
UNION
(select ROW_NUMBER() OVER (ORDER BY (SELECT 0)) As ROW_ID, 'update to' as [log_action], [name], [value] from inserted)
) as temp_tbl
Order By [temp_tbl].ROW_ID, [temp_tbl].[log_action]
end
Go
In this solution, when I do update from several sessions, there is a chance to several updates in the same time and it break the update sequence.
I can see 2 ‘update from’ rows and then two ‘update to’ rows and I want to prevent it.
The only solution I can think of it to lock the t1_log table in the update trigger using :
Select * from t1_log with (TABLOCKX)
But what if the t1_log have many rows? I guess select *
will be slow, and each update will return the selected *.
So I’m using the following:
create trigger t1_upd on t1 After update as
begin
declare @tt
Begin transaction
select @tt=1 from t1_log with (TABLOCKX)
Insert into t1_log([log_action],[name],[value])
select [log_action], [name], [value] from (
(select ROW_NUMBER() OVER (ORDER BY (SELECT 0)) As ROW_ID, 'update from' as [log_action], [name], [value] from deleted)
UNION
(select ROW_NUMBER() OVER (ORDER BY (SELECT 0)) As ROW_ID, 'update to' as [log_action], [name], [value] from inserted)
) as temp_tbl
Order By [temp_tbl].ROW_ID, [temp_tbl].[log_action]
Commit trasaction
end
this works better, but I still wonder if there is a fastest way to lock a table?
Answer :
To further expand on my comment, here is some sample code for you to look at. I don’t like the idea of introducing intentional locking on what appears to be a core table in your system. It will effectively slow everyone down to a single-threaded access.
The ideal solution would remove the need to have the update from and update to logging actions in a specific sequence. You can do this by adding a guid or some other identifier to the log table and use that to group the update from and update to actions together.
This example assumes that [Name] is a constant value and won’t be changing.
/** Build up our table and triggers
Note that I have consolidated the trigger logic into a single trigger
and the additional column on T1_Log
**/
CREATE TABLE dbo.T1
(
[Name] NVARCHAR(40) PRIMARY KEY NOT NULL
, [Value] NVARCHAR(MAX) NOT NULL
)
CREATE TABLE dbo.T1_Log
(
Log_ID INT IDENTITY(1,1) NOT NULL PRIMARY KEY
, Log_ActionGUID UNIQUEIDENTIFIER NOT NULL
, Log_TS DATETIME DEFAULT GETDATE() NOT NULL
, Log_Action VARCHAR(20) NOT NULL
, Log_Session_ID INT DEFAULT @@SPID NOT NULL
, [Name] NVARCHAR(40) NOT NULL
, [Value] NVARCHAR(MAX) NOT NULL
)
GO
CREATE OR ALTER TRIGGER trg_T1_Log ON dbo.T1
AFTER INSERT, UPDATE, DELETE
AS
BEGIN
DECLARE @Log_ActionGUID UNIQUEIDENTIFIER = NEWID()
;WITH CTE_Actions AS
(
SELECT Log_Action = CASE WHEN D.[name] IS NULL THEN 'insert'
WHEN I.[name] IS NULL THEN 'delete'
ELSE 'update from'
END
, Log_Sort = 1
, [Name] = COALESCE(D.[name], I.[name])
, [Value] = COALESCE(D.[Value], I.[Value])
FROM inserted AS I
FULL OUTER JOIN deleted AS D ON D.[Name] = I.[Name]
UNION ALL
SELECT 'update to' AS Log_Action
, Log_Sort = 2
, I.[Name]
, I.[Value]
FROM inserted AS I
WHERE EXISTS (SELECT TOP (1) 1 FROM deleted AS D WHERE D.[name] = I.[name])
)
INSERT INTO dbo.T1_Log
(Log_ActionGUID, Log_Action, [Name], [Value])
SELECT @Log_ActionGUID
, Log_Action
, [Name]
, [Value]
FROM CTE_Actions AS A
ORDER BY Log_Sort
END
GO
/** Test Statements **/
INSERT INTO dbo.T1
([Name], [Value])
VALUES
('John Smith', 'Smith Value 1')
UPDATE dbo.T1
SET [Value] = 'New Value'
WHERE [Name] = 'John Smith'
DELETE FROM dbo.T1 WHERE [Name] = 'John Smith'
/** Show Log Data **/
SELECT * FROM dbo.T1_Log
ORDER BY Log_TS, Log_ActionGUID
/** Cleanup **/
DROP TABLE IF EXISTS dbo.T1_Log
DROP TABLE IF EXISTS dbo.T1