Check if the value was changed in the original object in a trigger

Question

I'm writing a rollup trigger to update a field at account level based on some criteria. Let's say there's a boolean field with true/ false values.

Should I check if the value has changed at original object before updating this field in my trigger ( for optimizing trigger performance )? ( Also meaning that I will include this field in a query ).

If the value is the same in both cases, will salesforce commit a change ( changing value to the same value ) or will salesforce be smart enough to skip the record if no change occurred?

Answer 1

A simple way to check for that would be to put a formula field on the child object that tells you the value of the parent field. Then you don't even need to query; just check the value in the formula field and decide if you want to process.

for (MyObject__c obj : Trigger.new)
    if (obj.Formula_to_ParentField__c != correctValue)
        //process this record

To answer your other question: If you include a record in an update call, it will do the update and run triggers and workflows and change the LastModifiedDate, even if there are no field value changes.