List Throtlling:

List throttling is one of the new option in sharepoint 2010 that enable to set limits on how severely users can put the beat down on the servers. In simple way, what it does is allows to set a limit for how many rows of data can be retrieved for a list or library at any one time. The best example of this would be if we had 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, and user get a message that it cant be retrieved i.e. all data requested  cannot be retrieved because it exceeds the throttle limit for this list.
The action that can impact a large number of rows whose execution would fall under list throttling limit i.e. we had a list with 6000 items and a throttle limit of 5000 and create a view that only display 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 we 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 site deletion. sp we could again hit the throttling limit.
How does list throttling works
it all starts in central admin. list throttling is an attribute that you will generally manage at the web application level. So we go into central admin, click application management, then click on manage web applications. Click on single web application to select, then in the ribbon click on general settings drop down and select the Resource Throttling menu item. It displays dialogue box with several options.
List View Threshold: this is the maximum number of items that can be retrived in on request. the default value is 5000. The smallest we make is 2000.
Object Model Override: If you choose to allow object model override, users to whom you grant sufficient permission can override the List View Threshold programmatically for particular queries. (yes)
List View Threshold for auditors and administrators: Specify the maximum number of items that an object model database query can involve at one time for users to whom you grant sufficient permissions through Security Policy. (20000)
List View Lookup Threshold : Specify the maximum number of Lookup, Person/Group, or workflow status fields that a database query can involve at one time. (8)
Daily Time Window for Large Queries : Specify a daily time window when large queries can be executed. Specify a time outside of working hours for this window because large queries may cause excessive server load.
List Unique Permissions Threshold : Specify the maximum number of unique permissions that a list can have at one time. this option allows you to create a block of time during the day, typically when usage is low, that you will allow queries to run and not enforce the list throttling limits. The one thing to remember here is that if you execute a query during that time period, it will run until complete.  So if it’s still running when the daily window period closes, the query will continue to execute until all results have been returned.(50000)
Backward-Compatible Event Handlers : Turn on or off backward-compatible event handlers for this Web application. If this is turned off, users cannot bind document libraries to backward-compatible event handlers. (off)
HTTP Request Monitoring and Throttling :Turn on or off the HTTP request throttling job. This job monitors front-end Web server performance, and in the event of HTTP request overload, rejects (throttles) low priority requests. You can turn request throttling off to allow services such as Search to run uninterrupted on the farm, however, in an unthrottled farm experiencing overload, front-end Web servers become less responsive, and may stop working. (on)
Change Log : Specify how long entries are kept in the change log. (after 60 days)

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