I have a table set up in my sql server that keeps track of inventory items (in another database) that have changed. This table is fed by several different triggers. Every 15 minutes a scheduled task runs a batch file that executes a number of different queries that send updates on the items flagged in this table to update several ecommerce websites. The last query in the batch file resets the flags.
As you can imagine there is potential to lose changes if an item is flagged while this batch file is running. I have worked around this by replaying the last 25 hours of updates every 24 hours, just in case this scenario happened. It works, but IMO is kind of clumsy.
What I would like to do is delay any writes to this table until my script finishes, and resets the flags on all the rows that were flagged when the script started. Then allow all of these delayed writes to happen.
I've looked into doing this with table hints (TABLOCK) but this seems to be limited to one query--unless I'm misunderstanding what I have read, which is certainly possible. I have several that run in succession. TIA.
Alex
Could you modify your script into a stored procedure that extracts all the data into a temporary table using a select statement that applies a lock to the production table. You could then drop your lock on the main table and do all your processing in the temporary table (or permanent table built for the purpose) away from the live system. It will be a lot slower and put more load on your SQL box but speed shouldn't be an issue if you have a point in time snapshot of it.
If that option is not applicable then maybe you could play with wrapping the whole thing in a transaction and putting a table lock on your production table with the first select statement.
Good luck mate
Related
We are working on a data warehouse using IBM DB2 and we wanted to load data by partition exchange. That means we prepare a temporary table with the data we want to load into the target table and then use that entire table as a data partition in the target table. If there was previous data we just discard the old partition.
Basically you just do "ALTER TABLE target_table ATTACH PARTITION pname [starting and ending clauses] FROM temp_table".
It works wonderfully, but only for one operation at a time. If we do multiple loads in parallel or try to attach multiple partitions to the same table it's raining deadlock errors from the database.
From what I understand, the problem isn't necessarily with parallel access to the target table itself (locking it changes nothing), but accesses to system catalog tables in the background.
I have combed through the DB2 documentation but the only reference to the topic of concurrent DDL statements I found at all was to avoid doing them. The answer to this question, can't be to simply not attempt it?
Does anyone know a way to deal with this problem?
I tried to have a global, single synchronization table to lock if you want to attach any partitions, but it didn't help either. Either I'm missing something (implicit commits somewhere?) or some of the data catalog updates even happen asynchronously, which makes the whole problem much worse. If that is the case, is there are any chance at all to query if the attach is safe to perform at any given moment?
I have an issue with my data flow task locking, this task compares a couple of tables, from the same server and the result is inserted into one of the tables being compared. The table being inserted into is being compared by a NOT EXISTS clause.
When performing fast load the task freezes with out errors when doing a regular insert the task gives a dead lock error.
I have 2 other tasks that perform the same action to the same table and they work fine but the amount of information being inserted is alot smaller. I am not running these tasks in parallel.
I am considering using no locks hint to get around this because this is the only task that writes to a cerain table partition, however I am only coming to this conclusion because I can not figure out anything else, aside from using a temp table, or a hashed anti join.
Probably you have so called deadlock situation. You have in your DataFlow Task (DFT) two separate connection instances to the same table. The first conn instance runs SELECT and places Shared lock on the table, the second runs INSERT and places a page or table lock.
A few words on possible cause. SSIS DFT reads table rows and processes it in batches. When number of rows is small, read is completed within a single batch, and Shared lock is eliminated when Insert takes place. When number of rows is substantial, SSIS splits rows into several batches, and processes it consequentially. This allows to perform steps following DFT Data Source before the Data Source completes reading.
The design - reading and writing the same table in the same Data Flow is not good because of possible locking issue. Ways to work it out:
Move all DFT logic inside single INSERT statement and get rid of DFT. Might not be possible.
Split DFT, move data into intermediate table, and then - move to the target table with following DFT or SQL Command. Additional table needed.
Set a Read Committed Snapshot Isolation (RCSI) on the DB and use Read Committed on SELECT. Applicable to MS SQL DB only.
The most universal way is the second with an additional table. The third is for MS SQL only.
I need to alter the size of a column on a large table (millions of rows). It will be set to a nvarchar(n) rather than nvarchar(max), so from what I understand, it will not be a long change. But since I will be doing this on production I wanted to understand the ramifications in case it does take long.
Should I just hit F5 from SSMS like I execute normal queries? What happens if my machine crashes? Or goes to sleep? What's the general best practice for doing long running updates? Should it be scheduled as a job on the server maybe?
Thanks
Please DO NOT just hit F5. I did this once and lost all the data in the table. Depending on the change, the update statement that is created for you actually stores the data in memory, drops the table, creates the new one that has the change you want, and populates the data from memory. However in my case one of the changes I made was adding a unique constraint so the population failed, and as the statement was over the data in memory was dropped. This left me with the new empty table.
I would create the table you are changing, with the change(s) you want, as a new table. Then select * into the new table, then re-name the tables in a single statement. If there is potential for data to be entered into the table while this is running and that is an issue, you may want to lock the table.
Depending on the size of the table and duration of the statement, you may want to save the locking and re-naming for later, and after the initial population of the new table do a differential population of new data and re-name the tables.
Sorry for the long post.
Edit:
Also, if the connection times out due to duration, then run the insert statement locally on the DB server. You could also create a job and run that, however it is essentially the same thing.
I need to update a table in sql server 2008 along the lines of the Merge statement - delete, insert, updates. Table is 700k rows and I need users to still have read access to it assuming an isolation level of read committed.
I tried things like ALTER TABLE table SET (LOCK_ESCALATION=DISABLE) to no avail. I tested by doing a select top 50000 * from another window, obvious read uncommitted worked :). Is there anyway around this without changing the user's isolation level and retaining an 'all or nothing' transaction behaviour?
My current solution of a cursor that commits in batches of n may allow users to work but loses the transactional behaviour. Perhaps I could just make the bulk update fast enough to always be less than 30 seconds (for timeout). The problem is the user's target db's are on very slow machines with only 512mb ram. Not sure the processor but assume it is really slow and I don't have access to them at this time!
I created a test that causes an update statement to need to run against all 700k rows:
I tried an update with a left join on my dev box (quite fast) and it was 17 seconds
The merge statement was 10 seconds
The FORWARD ONLY cursor was slower than both
These figures are acceptable on my machine but I would feel more comfortable if I could get the query time down to less than 5 seconds before allowing locks.
Any ideas on preventing any locking on the table/rows or making it faster still?
It sounds like this table may be queried a lot but not updated much. If it really is a true read-only table for everyone else, but you want to update it extremely quickly, you could create a script that uses this method (could even wrap it in a transaction, I believe, although it's probably unnecessary):
Make a copy of the table named TABLENAME_COPY
Update the copy table
Rename the original table to TABLENAME_ORIG
Rename the copy table to TABLENAME
You would only experience downtime in between steps 3 and 4, and a scripted table rename would probably be quicker than an update of so many rows.
This does all assume that no else can update the table while your process is running, but they will still be able to read it fully at any point except between 3 & 4
On our live/production database I'm trying to add a trigger to a table, but have been unsuccessful. I have tried a few times, but it has taken more than 30 minutes for the create trigger statement to complete and I've cancelled it.
The table is one that gets read/written to often by a couple different processes. I have disabled the scheduled jobs that update the table and attempted at times when there is less activity on the table, but I'm not able to stop everything that accesses the table.
I do not believe there is a problem with the create trigger statement itself. The create trigger statement was successful and quick in a test environment, and the trigger works correctly when rows are inserted/updated to the table. Although when I created the trigger on the test database there was no load on the table and it had considerably less rows, which is different than on the live/production database (100 vs. 13,000,000+).
Here is the create trigger statement that I'm trying to run
CREATE TRIGGER [OnItem_Updated]
ON [Item]
AFTER UPDATE
AS
BEGIN
SET NOCOUNT ON;
IF update(State)
BEGIN
/* do some stuff including for each row updated call a stored
procedure that increments a value in table based on the
UserId of the updated row */
END
END
Can there be issues with creating a trigger on a table while rows are being updated or if it has many rows?
In SQLServer triggers are created enabled by default. Is it possible to create the trigger disabled by default?
Any other ideas?
The problem may not be in the table itself, but in the system tables that have to be updated in order to create the trigger. If you're doing any other kind of DDL as part of your normal processes they could be holding it up.
Use sp_who to find out where the block is coming from then investigate from there.
I believe the CREATE Trigger will attempt to put a lock on the entire table.
If you have a lots of activity on that table it might have to wait a long time and you could be creating a deadlock.
For any schema changes you should really get everyone of the database.
That said it is tempting to put in "small" changes with active connections. You should take a look at the locks / connections to see where the lock contention is.
That's odd. An AFTER UPDATE trigger shouldn't need to check existing rows in the table. I suppose it's possible that you aren't able to obtain a lock on the table to add the trigger.
You might try creating a trigger that basically does nothing. If you can't create that, then it's a locking issue. If you can, then you could disable that trigger, add your intended code to the body, and enable it. (I do not believe you can disable a trigger during creation.)
Part of the problem may also be the trigger itself. Could your trigger accidentally be updating all rows of the table? There is a big differnce between 100 rows in a test database and 13,000,000. It is a very bad idea to develop code against such a small set when you have such a large dataset as you can have no way to predict performance. SQL that works fine for 100 records can completely lock up a system with millions for hours. You really want to know that in dev, not when you promote to prod.
Calling a stored proc in a trigger is usually a very bad choice. It also means that you have to loop through records which is an even worse choice in a trigger. Triggers must alawys account for multiple record inserts/updates or deletes. If someone inserts 100,000 rows (not unlikely if you have 13,000,000 records), then looping through a record based stored proc could take hours, lock the entire table and cause all users to want to hunt down the developer and kill (or at least maim) him because they cannot get their work done.
I would not even consider putting this trigger on prod until you test against a record set simliar in size to prod.
My friend Dennis wrote this article that illustrates why testing a small volumn of information when you have a large volumn of information can create difficulties on prd that you didn't notice on dev:
http://blogs.lessthandot.com/index.php/DataMgmt/?blog=3&title=your-testbed-has-to-have-the-same-volume&disp=single&more=1&c=1&tb=1&pb=1#c1210
Run DISABLE TRIGGER triggername ON tablename before altering the trigger, then reenable it with ENABLE TRIGGER triggername ON tablename