I am trying to find the duplicate emails under same account.
I have limited soql abilities so I didn't use "group" in my query at the time.
i wrote a batch apex class to process all the contacts to find the duplicate pairs but I got error "Batch Apex Error: Batchable Instance Is Too Big"
I tried the following:
1. Change the batch size to be 150 or 180.  
2. Searched this error in this forum, people encountered this when they were importing csv files or when they mistakenly added "Database.Stateful" keyword.

I don't quite understand the heap limit here, for each batch, what is the limit?  How can I know which part of my code comsumes most of the heap so I can try to fix my code?
Thank you!

 

global class ContactsCleaning implements Database.Batchable<sObject>,Database.Stateful{
    //query: select id,Accountid,email from Contact where email !=null and email!='\'\'' and accountid!=null and accountid!='\'\''
    public Map<AccountEmail,List<Contact>> duplicatePairs=new Map<AccountEmail,List<Contact>>();
    public integer totalemail=0;
    public integer totalcontact=0;
    global final String query;
    global ContactsCleaning(String q){
        query=q;
    }    
    global Database.QueryLocator start(Database.BatchableContext BC){
        return Database.getQueryLocator(query);
    }
    global void execute(Database.BatchableContext BC, List<Contact> scope)
    {
        for(Contact c : scope)
        {                        
                //check if email&account pair exists
                AccountEmail ae=new AccountEmail(c.AccountId,c.Email);
                if(duplicatePairs.containsKey(ae))
                {
                    duplicatePairs.get(ae).add(c);                  
                }
                else
                {                    
                    List<Contact> cl=new List<Contact>();
                    cl.add(c);
                    duplicatePairs.put(ae, cl);                    
                }            
        }                
    }    
    global void finish(Database.BatchableContext BC)
    {
        Document d=new Document();
        d.Name='Duplicate Emails';
        d.folderid = UserInfo.getUserId();
        String content='';
        for(AccountEmail ae : duplicatePairs.keySet())
        {
            if(duplicatePairs.get(ae).size()<=1)
            {
                duplicatePairs.remove(ae);
            }
            else
            {
                List<Contact> ls=duplicatePairs.get(ae);
                content=content+'Account: '+ae.AccountID+'   '+'Email:   '+ae.Email+'   ';
                Integer i=1;
                for(Contact c : ls)
                {
                    content=content+i+'   '+'ContactId:  '+c.Id+'   ';
                    i++;
                }
                content+='\n';
            }
        }
        d.Body=Blob.valueOf(content);
        d.ContentType='text/plain';
        d.Type='txt';
        insert d;
        Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
        String[] toAddresses = new String[] {'jack.wang@accelerationretirement.com'};
        mail.setToAddresses(toAddresses);
        mail.setSubject('Duplicate Email Stats');
        mail.setPlainTextBody('Total Contacts:   '+totalcontact+'\n'+'Total Emails:   '+totalemail+'\n'+'Deplicate Pairs:   '+duplicatePairs.size());
        mail.setHtmlBody('<div>Total Contacts:   '+totalcontact+'</div>'+'Total Emails:   '+totalemail+'</div><div>Deplicate Pairs:   '+duplicatePairs.size()+'</div>');
        Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });                                
    }    
}

//This class is the key to dedup email of Contact
public class AccountEmail {
    public String AccountID;
    public String Email;
    public AccountEmail()
    {
    }
    public AccountEmail(String acctid,String e)
    {
        AccountID=acctid;
        Email=e;
    }
    public Boolean equals(Object obj) {
        if (obj instanceof AccountEmail) {
            AccountEmail p = (AccountEmail)obj;
            return ((AccountID==p.AccountID) && (Email==p.Email));
        }
        return false;
    }

    public Integer hashCode() {
        return (31 * AccountID.hashCode()) ^ (Email.hashCode());
    }

}