Sharing Rules Recalculation

 1.Manual Sharing Rule Recalculation (Parallel Calculation)

2.Auto Sharing Rule Recalculation (One by One Calculation)

Manual Sharing Rule Recalculation (Parallel Calculation) :

whenever a sharing rule is recalculated manually, it is always done in parallel meaning all sharing rules calculation 

would start together and would run simultaneously.

If the number of records that are impacted by the recalculation are less than 25k or 25000 then in this case 

you would not receive an email upon completion of the recalculation.

We can track Manual Sharing Recalculation activity in the 'Background Jobs'.

we will find the start time along with the specific sharing criteria , percentage completed and the end time.

Note :

Manual sharing recalculation or parallel recalculation is faster than the Auto Sharing recalculation.

Auto Sharing Rule Recalculation (One by One Calculation) :

The Auto Recalculation of sharing rules is when some activity triggers the modification to the sharing rules and 

so a recalculation is done for the same by Salesforce without explicitly clicking on Recalculate button.

This happens when for example a record type is deleted or there is a change in the Role Hierarchy or any activity 

that sees a modification in the sharing rules. As soon as the activity is performed, Salesforce itselves starts the 

recalculation of the sharing rules for the whole org, this is done in a one by one mode meaning only one sharing rule at a time.

As only one sharing rule is calculated at a time it takes quite a long time depending on your org data.

To get an estimate of how much time would it take to recalculate the Sharing Rules on your org, 

Salesforce suggests that you run the same on a full copy sandbox. Ideally the time taken on the Full Copy sandbox 

is a replica of how much time it would take on your production instance.

Defer Sharing Calculations:

Deferring sharing calculation is ideal if you make a large number of changes to roles, territories, groups, users, 

portal account ownership, or public groups participating in sharing rules, and want to suspend the 

automatic sharing calculation to a later time.