How to cancel a long-running Database operation?

Question

Currently working with Oracle, but will also need a solution for MS SQL.

I have a GUI that allows users to generate SQL that will be executed on the database. This can take a very long time, depending on the search they generate. I want the GUI/App to responsive during this search and I want the user to be able to cancel the search.

I'm using a Background Worker Thread.

My problem is that, when the user cancels the search, I can't interrupt the call to the database. It waits until it is finished and then, it can poll the 'CancelationPending' property. Not only does this waste resources on the database, but it creates problems for my code.

If the user hits 'Search' on a very long query, then clicks 'Cancel' and then 'Search' again - the first search is still chugging away on the database. The background worker is still busy when they hit search again. The only solution I've got to this problem is to make a new background worker.

It seems like a really ugly way to do things. The database keeps working I'm creating new instances of background workers....when I really want to STOP the database call and re-use the same worker.

How can I do that?

Answer 1

I think the best solution seems to kill sessions via monitoring table.

With Oracle you can make it as says Burnsys

In Firebird 2.5 it will looks the same

I hope something similar exist in Ms SQL

Answer 2

If you're using ADO.NET and SQL data provider, take a look at SqlCommand.Cancel method. That does what you're looking for. However, it tries to cancel and the cancellation may take time. Basically, it's up to SQL Server to decide when to grant your cancellation request. When the query is cancelled, you should get a SqlException that indicates that the operation was cancelled by user. Apparently, you don't want to treat this exception as exception and handle it specially such as if SqlException is due to user cancelling the operation, just swallow it.

Answer 3

I have tried both Cancel and Close with ADO 2.8 and SQLOLEDB or SQL Server native client. With Cancel, the recordset stops fetching data, but in the backround the reading from the server continues and consumes memory from the application. In a 32 bit application it can happen that you get an "out of memory" message some minutes later. When I close the recordset (or the connection, with or without Cancel before), ADO 2.8 waits until all records are fetched.

I don't know if ADO.NET does it better, but I think it's a good idea to monitor memory and network access after Cancel/Close to be sure that ADO really stops reading data.

Answer 4

You could have the background worker fire off the actual database call on a different thread, and then periodically check to see if either the database call has finished, or cancel has been pressed, at which point you could kill off the database thread. This wouldn't actually help the database load any (as your query has been sent and is still processing) but it does release your local resources related to it.