Hello Srikant ,

When dealing with a large number of records and performing custom rollup calculations in Salesforce, it's essential to optimize the solution to ensure efficient processing. Here's a suggested optimized solution using a Batch Apex class to perform the custom rollup on more than 5 Lakh records:

Batch Apex Class:
Create a Batch Apex class to perform the rollup calculations. The batch class allows you to process records in smaller chunks, which is more efficient for large data volumes.

public class CustomRollupBatch implements Database.Batchable<SObject> {

    public Database.QueryLocator start(Database.BatchableContext context) {
        // Query the child records with necessary fields for rollup calculation
        return Database.getQueryLocator('SELECT Field1_c, Field2c, Parent_Accountc FROM Child_Object_c');
    }

    public void execute(Database.BatchableContext context, List<SObject> scope) {
        // Map to store rollup results for each parent account
        Map<Id, Decimal> parentRollupMap = new Map<Id, Decimal>();

        for (SObject record : scope) {
            // Perform rollup calculation for each record
            // Replace 'Field1_c' and 'Field2_c' with your actual fields for rollup
            Decimal rollupValue = (Decimal)record.get('Field1_c') + (Decimal)record.get('Field2_c');
            
            Id parentId = (Id)record.get('Parent_Account__c');
            if (parentRollupMap.containsKey(parentId)) {
                parentRollupMap.put(parentId, parentRollupMap.get(parentId) + rollupValue);
            } else {
                parentRollupMap.put(parentId, rollupValue);
            }
        }

        // Update parent account records with the rollup result
        List<Account> parentUpdates = new List<Account>();
        for (Id parentId : parentRollupMap.keySet()) {
            Account parentAccount = new Account(Id = parentId);
            parentAccount.Custom_Rollup_Field__c = parentRollupMap.get(parentId);
            parentUpdates.add(parentAccount);
        }
        update parentUpdates;
    }

    public void finish(Database.BatchableContext context) {
        // Optionally, you can perform any final cleanup or notification logic here.
    }
}

Scheduling the Batch Job:
To schedule the batch job, navigate to "Apex Classes" in Salesforce Setup and click "Schedule Apex." Choose the "CustomRollupBatch" class, set the preferred frequency (e.g., daily or weekly), and specify the start time for the job.

By using this Batch Apex approach, the rollup calculations will be performed efficiently in smaller chunks, which can handle large data volumes without hitting governor limits. The code should be customized to fit your specific child object fields and parent account field for rollup. Additionally, ensure that the field types and data volume align with the appropriate data types to avoid any data-related issues.

If this helps , please mark this as Best Answer.
Thank you.