I thought this might be a good idea for speeding up account balance requests, but I'm not sure how to ensure the table is initialized properly through restarts. This reminds me, Wordpress has a wp-cron.php script to do ongoing maintenance to the system, perhaps something like that would be helpful for PBooks, like ensuring any MEMORY tables are properly initialized.
CREATE TABLE pb_account_ledger_balances ENGINE=MEMORY
SELECT account_id,sum(entry_amount) as total_amount
FROM pb_general_ledger
GROUP BY account_id;