I have inherited a few jobs and I am trying to understand why select statements would be in their steps. I would think that select statements would be pointless in an automated job that displays nothing for an end user.
It's hard to know why without seeing the code, but I can take a guess...
Perhaps it is a SELECT ... INTO ... statement where the results are stored in another table.
Another alternative might be just to test that the service is running correctly. If the select fails an exception might be thrown and this might be caught elsewhere to notify someone that there is a problem.
Related
I am stumped on an issue I am having. The true context is rather complicated, but I can boil it down to these functional points (everything else is not related to the problematic table):
I have a trigger function that contains several SELECTs and then an UPDATE
The update takes an unreasonable amount of time to execute ("unreasonable" = > 1.4s)
The same exact queries when run outside the trigger (for the same rows, parameters, etc.) do not have any issues (i.e. they execute in under 1-2ms)
I am pretty sure that indexes, etc., are working as necessary; i.e. there shouldn't be any issues.
There are no circular triggers
There is on trigger on the destination table, but even with that removed the behavior is the same.
I have done many tests to no avail, but these are pretty meaningful:
when the update is replaced with a SELECT, the response time is fast, as expected
when the update is replaced with a SELECT... FOR UPDATE, the response time is slow, the same as the update
^ this (as well as other things) has led me to possibly believe that the delay is spent waiting to achieve a lock
No other transactions are really happening on that table. I am truly bewildered.
Server context: This is being run in AWS/RDS on db.m5.xlarge.
What I am looking for is whether there is a way to get some information about locks that are happening mid-transaction or possibly even a history of acquired locks? Or anything else that can give me insight into what is causing the delay that seems so closely related to acquiring a lock on that table.
Unfortunately, just to make everything even more frustrating, I cannot replicate the issue when I attempt to use EXPLAIN in the function body. The only way to do this (that I know of) is to use the EXECUTE... syntax with a query string. That doesn't have a delay - its also useless for the trigger.
As far as I know, we can't use start transaction within functions, thus we can't use COMMIT and ROLLBACK in functions.
But how then we ROLLBACK by some if-condition?
How then we can perform a sequence of statements in a specific level of isolation? I mean a situation when an application wants to call a SQL (plpgsql) function and that function really needs to be run in a transaction with a certain isolation level. What to do in such a case?
In which cases then it is really practical to run ROLLBACK? Only when we manually write a script, check something and then ROLLBACK manually if we don't like the result. And in the same case, I see the practicality of savepoints. However, I feel like it is a serious constraint.
If you want to rollback the complete transaction, RAISE an exception.
If you only want to roll back part of your work, start a new block with a BEGIN at the point to which you want to roll back and add an EXCEPTION clause to the block.
Since the transaction is started outside the function, the isolation level already has to be set properly when you are in the function.
You can query
SELECT current_setting('transaction_isolation', TRUE);
and throw an error if the setting is not correct.
is too general or too simple to answer.
You roll back a transaction if you have reached a point in your processing where you want to undo everything you have done so far in the transaction.
Often, that happens implicitly rather than explicitly by throwing an error.
If I have a DB2 SQL procedure or function that's recursive, or it has a tricky loop, and the process has become infinite, how can I kill it if it's already running..? This is DB2-for-i v7.3, with development in DBeaver SQL.
Is it enough to simply cancel the query in the SQL IDE..? I've done so in the past with too-long queries, and I always get a notification for "Query has been cancelled". But with an infinitely looping or recursing procedure, is there any risk the process might still continue in the background until something crashes and the DBA staff come to pound on my door with pitchforks and flaming torches..?
EDIT: It was suggested my question is a duplicate of this one, but I read that one before my post, and it's not the same. That question is how to prevent an infinite loop from happening in the first place, but my question is about how to kill one that's already happening.
Cancel should work, but if you can find the right job in WRKACTJOB, then you can just end it with OPTION(*IMMED).
The server jobs should be in subsystem QUSRWRK, and are named QZDASOINIT. The one you are looking for will have your userid, and if it is looping, it will have a status of RUN. These jobs sit at TIMW normally when they are waiting for work. These are Prestart Jobs, and if you end one manually, it will come back if it needs to.
Build a cancel into the loop by some means to exit a long running loop.
--for example checking if any rows exist in a table.
loop...
set myexit = (select myexit from myexit fetch first 1 rows only);
if myexit is not null then return;
endloop...
Maybe some better solutions come of this.
We have a WebSphere cluster with four clones. Identical code runs on each of the clones. We have Quartz periodically kick off a job that runs the code.
The code tries to update a row in a table so that only one of the clones will be able to successfully update the table, and then that clone will run the rest of the job. Something like:
update <table> set status = 'RUNNING' where job_name = 'JOB1' and status = 'STOPPED'
We do not start a transaction when we execute the update statement.
What we see sometimes is that all four clones fail to update the table, and all get a lock timeout error (sql code -913).
We've also tried an alternative where we start a transaction, select to see if the row is marked as running, and if not, then performing an update and committing; and otherwise rolling back.
That had the same problem.
One solution we did not try yet is to modify the select to be a "select for update" although from my googleing, I have doubts as to whether that will help.
Any suggestions?
This ended up not being a problem (that's what I get for listening to someone without checking it out myself).
I tested this out in our development environment with two clones. One of the clones would see the -913 lock timeout error occasionally while the other clone would successfully update the table. Other than the ugly log message, everything worked as it should.
Usually, however, we would not get the -913 error, but rather a warning indicating that there was no row to update from one of the clones. Again, this behavior is fine.
So, as we originally thought, and Clockwork-Muse also suggests, using UPDATE statements in this manner to enforce a lock works just fine in DB2.
TSQL here. Specifically Server 2008(literally just upgraded)
Concerning stored procedures: Try/Catch
I was trying to make a list of cases when a Select Statement will throw an exception. The ones I can think of are syntax related(includes null variables) and divide by zero. I'm only guessing there are just a whole boat load of them for Insert/Alter and Create/Truncate.
If you happen to know of a good source link, that would be great.
This question came up when I was reading this exhaustive blog post about error handling for SQL server. It's titled for SQL Server 2000, but I think most of it still applies.
edit
Sorry, I meant to link this earlier. . .
http://msdn.microsoft.com/en-us/library/aa175920(v=sql.80).aspx
Outside for compile ("didnt' run") errors, you have at least these runtime errors
arithmetic errors
These change based on various SET statement
Example: get sql server to warn about truncation / rounding
overflow errors
example: one of the rows overflows smallint in some calculation
CAST errors
eg you try ISNUMERIC in a WHERE or CASE and try to cast 'bob^' or 1.23 to int
See Why use Select Top 100 Percent?
However, you'd always want to use TRY/CATCH though, surely...?
Adding to gbn's post, you can also get locking errors like lock wait timeouts and deadlocks.
If you are referencing #Temp tables, you can get "Invalid object name '#Temp'" errors, because these are unbound until the statement executes.
If you are in READ UNCOMMITTED or WITH (NOLOCK), you can get error: 601 - "Could not continue scan with NOLOCK due to data movement."
If your code runs .NET code, you would probably get exceptions from there.
If your code selects from a remote server, you could a whole different set of errors about connections.