At Realtime, efficiently comparing records in DynamoDB with record in RDS (PostgreSQL)

I don't think there is any efficient way of doing this. The most efficient thing you can do is to ensure your streaming logic from DynamoDB to RDS is robust and handles failures elegantly.

You can then periodically takes dumps of DynamoDB and RDS and compare them in an offline manner.

The other approach, would be to also stream your DynamoDB changes to an SQS queue, and process them changes with a delay of 3 minutes, and then read those events back from RDS to make sure it was committed.

You can leverage DynamoDB streams to get the MODIFY events.

DynamoDB Streams captures a time-ordered sequence of item-level modifications in any DynamoDB table and stores this information in a log for up to 24 hours. Applications can access this log and view the data items as they appeared before and after they were modified, in near-real time.

You can then use a Lambda trigger or a custom stream consuming logic to compare the records from stream with your RDS rows.