How to query multiple objects in apex batch

Question

I want to retrieve Leads and Accounts in Apex start function and update the owner field How can that be achieved?

    global Database.QueryLocator start(Database.BatchableContext BC) {
        //query leads and accounts
    }

    global void execute(Database.BatchableContext BC, List<sObject> object) {
           //update leads and accounts
    }   

    global void finish(Database.BatchableContext BC) {
    }

Edit

I have a custom object that has a field that contains a user Id. If this field is updated from user A Id to user B Id for example, a trigger calls the batch method. Leads and Accounts where the owner is A need to be retrieved and then the owner field should be updated by the new value which is user Id B.

Answer 1

You should never execute batches from a trigger context. It is highly likely that doing so will lead to many avoidable headaches. Optimal design pattern here would be to add a change tracking object you can batch over. Whenever your trigger detects this change, insert a record into the tracking table. Make sure it contains lookups Old User and New User, as well as a checkbox Is Processed. Your batch would simply pick up any records where the checkbox is false, find and update relevant records, then set the checkbox to true.