List throttling is one of the new options in SharePoint 2010 that enable to set limits on how severely users can put the beat down on your servers. In a nutshell, what it does is allow you to set a limit for how many rows of data can be retrieved for a list or library at any one time. The most basic example of this would be if you had a list with thousands of items, and someone created a view that would return all of the items in the list in a single page. List throttling ensures that such a request would not be allowed to execute. The hit on the server is alleviated, and the user gets a nice little message that says sorry, we can’t retrieve all of the data you requested because it exceeds the throttle limit for this list.

These kinds of operations that can trigger hitting this limit though aren’t limited to viewing data – that’s just the easiest example to demonstrate. There are other actions that can impact a large number of rows whose execution would fall into the list throttle limits. For example, suppose you had a list with 6000 items and a throttle limit of 5000. You create a view that only displays 50 items at a time, but it does a sort on a non-indexed column. Behind the scenes, this means that we need to sort all 6000 items and then fetch the first 50. If you are going to delete a web with large flat lists you potentially have the same problem. We need to select all of the items for all of the lists as part of the site deletion, so we could again hit the throttling limit. These are just a few examples but you can start to imagine some of the others.

For code and samples check MSDN.