Code used in "How many batches will be executed? 20000 records & 250 batch size | Salesforce Batch Apex Interview" (https://youtu.be/LgHxpR_6Vrs)

batch-apex-mechanism-detail.cls

-> 'Create a Batch Class to opt out contact records from email, call and fax whose Unsubscribe Date has passed'

20,000 records -> total
200 records

How many batches?

-> 20,000 / 200 -> 100 batches in total

// * Execute batch
Database.executeBatch(new UpdateContactsBatch());
Database.executeBatch(new UpdateContactsBatch(), 75);

Secret Mechanism behind Batch Apex
-----------------------------------

20,000 records -> total
250 records

Total number of batches = 20,000 / 250 = 80 batches

=> 100 batches

// * Hypothetical implementation
class Database {

    public static Id executeBatch(Database.Batchable batchableClassInstance) {
        ApexJob job = new ApexJob(); -> '7070p00000qU6iW'
        Database.QueryLocator queryLocatorInstance = batchableClassInstance.start(job);
        setqueryOptions(400);
        QueryResult result = query(queryLocatorInstance.getQuery());

        List<sObject> records = result.records;
        // * How many records will be returned? -> 400 records

        batchableClassInstance.execute(job, first 250 records);
        batchableClassInstance.execute(job, next 150 records);

        // * First 400 records
        while(!result.done) {
            result = queryMore(result.queryLocator);
            records = result.records;
            // * next 400 records
            batchableClassInstance.execute(job, first 250 records);
            batchableClassInstance.execute(job, next 150 records);
        }

        batchableClassInstance.finish(job);
    }

    public static Id executeBatch(Database.Batchable batchableClassInstance, Integer scope) {
        ApexJob job = new ApexJob(); -> '7070p00000qU6iW'
        Database.QueryLocator queryLocatorInstance = batchableClassInstance.start(job);
        setqueryOptions(2000);
        QueryResult result = query(queryLocatorInstance.getQuery());

        List<sObject> records = result.records;
        // * How many records will be returned? -> 2000 records

        // batchableClassInstance.execute(job, first 500 records);
        // batchableClassInstance.execute(job, next 500 records);
        // batchableClassInstance.execute(job, next 500 records);
        // batchableClassInstance.execute(job, next 500 records);

        Integer totalRecordsProcessed = 0; // * 750 records
        List<sObject> recordsToProcess = new List<sObject>();
        while(records.size() > totalRecordsProcessed) {
            recordsToProcess.add(records.get(totalRecordsProcessed));
            totalRecordsProcessed++;
            if(Math.mod(totalRecordsProcessed,scope)==0) {
                batchableClassInstance.execute(job, recordsToProcess); // * 500 records as scope is 500
                recordsToProcess.clear();
            }
        }

        if(!recordsToProcess.isEmpty()) {
            batchableClassInstance.execute(job, recordsToProcess); // * 250 records however scope is 500
            recordsToProcess.clear();
        }

        // * First 400 records
        while(!result.done) {
            result = queryMore(result.queryLocator);
            records = result.records;
            // * next 400 records
            // batchableClassInstance.execute(job, first 200 records);
            // batchableClassInstance.execute(job, next 200 records);
            totalRecordsProcessed = 0;
            while(records.size() > totalRecordsProcessed) {
                recordsToProcess.add(records.get(totalRecordsProcessed));
                totalRecordsProcessed++;
                if(Math.mod(totalRecordsProcessed,scope)==0) {
                    batchableClassInstance.execute(job, recordsToProcess); // * 500 records as scope is 500
                    recordsToProcess.clear();
                }
            }

            if(!recordsToProcess.isEmpty()) {
                batchableClassInstance.execute(job, recordsToProcess); // * 250 records however scope is 500
                recordsToProcess.clear();
            }
        }

        batchableClassInstance.finish(job);
    }
}

class ApexJob implements Database.BatchableContext {

    public Id getJobId() {

    }

    public Id getChildJobId() {

    }
}


1 <= batchSize <= 100 -> retrieveChunkSize = 100
101 <= batchSize <= 400 -> retrieveChunkSize = 400 ->
401 <= batchSize <= 2000 -> retrieveChunkSize = 2000


-> 500 records

Code for UpdateContactsBatch class can be found here: https://gist.github.com/rahulmalhotra/89df8331054be604546bb22e1d6e5ee4