Update Trigger on Parent record on Roll-up summary field when child records are inserted or deleted?

Question

I want to build an update trigger on a rollup summary field.

Say I have a parent record A, A have 1000 children records linked with a Master-Detail relationship.

1.First Operation: delete 1000 records, if the code is like

List<Child__c> children = [select id from Child__c where parentid = A.Id];
delete children;

How many times would the update trigger on parent record A run?

Is it 5? What if the second 200 batch is being deleted when the first update trigger is running? I'm confused with the execution order and the events start&end order.

2.Second Operation: insert 1000 child records from data loader or any external app.

How many times would the update trigger on parent record A run? Is it 5? Is it similar to the delete operation?

Thanks

Update

The Roll up summary field is a "SUM" of it's children records field num__c which has a number type.

The question assumes the num__c field of the deleted&inserted children records have value on this field.

The question also specified 1000 child records belongs to a same parent record

Answer 1

The update triggers on parent will be fire .200 child records are processed at a time when you delete 1000 records .This means there will be 5 chunks .Lets say 200 child belongs to 10 parent records ,trigger.new will have 10 parent records .

The trigger on Parent would run for update case .In your case you are deleting or inserting and that should trigger update triggers on parent records .

To confirm this behavior just did a quick experiment

1.Create a parent object and a child object .Associate them via master detail .

2.Create a number field on the child record .

3.Create a roll up summary field on the parent record summing the number field that's created in previous field

4.Create a trigger on the parent record like below

trigger triggerSpeaker on Parent__c (before update) {
  if(trigger.isbefore && trigger.isupdate){
    for(Parent__c sp:trigger.old ){
      sp.addError('You cannot update me');
    }
  }
 }

5.Delete or insert the data into child objects via data loader or developer console.You should see the add error indicating triggers fire