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I need to write a Batch class to delete records from Child to Parents objects:

Child__c --> Parent__c --> Parent_Of_Parent__c

Every object has approximately 5 million records to delete, so I want to make sure I don't hit Salesforce limits. The query in the start method has a semi-join:

Select id, Date__c, Parent__c from Child__c where Date __c < LAST_N_Days:365 and Id not in (SELECT Child__c from OtherObject)

I know that with QueryLocator I can retrieve 50 million records, but I don’t know if with semi-joins I’ll get Query Timeout Error (‘OtherObject’ has 1.5 million records). Also, I don’t know if I’ll hit any Governor Limit with the semi-join.

I found this answer about semi-joins: What are the limitations on using a semi-join in a batch apex query locator? but in the Developer Console it doesn't seem like semi-joins are being counted. Or am I wrong?

If it can help, this is my code (any suggestion is welcome):

Batch Class

global class BatchWorkaround implements Database.Batchable<sObject>{

    global Database.QueryLocator start(Database.BatchableContext BC){

        String query = 'Select id, Date__c, Parent__c from Child__c where Date __c < LAST_N_Days:365 and Id not in  (SELECT Child__c from OtherObject)';
       
        return Database.getQueryLocator(query);
    }

    global void execute(Database.BatchableContext BC, List<sObject> scope){

        DeleteOtherRecords.execute((List<Child__c>) scope);
        
        delete scope;
       
    }
        
    global void finish(Database.BatchableContext BC) {
        
    //do something

    }

DeleteOtherRecords Class

public class DeleteOtherRecords{
    
    private static List<Parent__c> cp = new List<Parent__c>();
    private static List<Parent_of_Parent __c> op = new List<Parent_of_Parent __c>();
    private static List<Parent_of_Parent __c> noOthers = new List<Parent_of_Parent __c>();
    
    public static void execute(List<Child__c> scope){
        
        List<Id> cList = new List<Id>();
        
            for(Child__c c: scope){
                
                cList.add(c.Parent__c);
            }
            
            cp = [SELECT ID, Parent_of_Parent__c FROM Parent__c WHERE ID IN : cList];
        
        List<Id> opList = new List<Id>();
        
            for(Parent__c a: cp){
                
                opList.add(a.Parent_of_Parent__c);
            }
        
            op = [SELECT ID FROM Parent_of_Parent__c WHERE ID IN : opList];     
            
        if(cp.size() > 0){
                                    
            delete cp;
        }

        if(op.size() > 0){
            
            noOthers = [SELECT ID FROM Parent_of_Parent__c WHERE ID IN: op AND ID NOT IN (SELECT Parent_of_Parent__c FROM Parent__c)];
            
            if(noOthers.size() > 0){                   
                
            delete noOthers;
                    
            }
        }
    }
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