Sharing Rules allow fine-grained control of access to records in Force.com based on categories such as role membership or fields matching given criteria – for example, a sharing rule might state that managers based at headquarters have access to all account records, whereas those of the Timbuktu office only have access to account records in the Timbuktu village. Ordinarily, the platform reevaluates sharing rules automatically as records are created, updated and deleted, so, in our example, when an employee is promoted to manager, they gain access to all the data they need to do their job.
Occasionally, though, administrators need to make large numbers of changes to data such as roles, territories, groups and users, perhaps in response to business events such as a reorganization or merger. Since large scale changes result in a correspondingly large number of sharing rule reevaluations, it's useful to be able to control when those calculations happen, for example, outside the working day. In the past, the only option has been for administrators to stay up late and perform such large scale changes at the desired time, but now there's a better way.
The Summer '11 release adds the ability to defer sharing calculations, suspending them temporarily and performing them in bulk at a later time, perhaps during an organization's scheduled maintenance period. If you're an admin for a large org, this is a must have feature! It isn't enabled by default, though – you'll need to contact salesforce.com to enable it, and users need all of the “Manage Users”, “View Setup and Configuration” and “Manage Sharing Calculation Deferral” permissions to be able to wield its power. As with all new features, see the release notes for full details, and sign up for the Summer '11 pre-release to try it out now!