Understanding Transactions And Locking In PostgreSQL

CREATE OR REPLACE FUNCTION buysomething(varchar(255),TEXT) RETURNS integer AS $$
    DECLARE
        account_balance NUMERIC(22,9);
    BEGIN
        LOCK TABLE accounting IN EXCLUSIVE MODE;
        SELECT (SUM(COALESCE(credit,0)) - SUM(COALESCE(debit,0)) INTO account_balance
            FROM accounting
            WHERE account_id = $1;
 
        IF account_balance >= 5 THEN
            INSERT INTO purchases (SomeTextCol) VALUES ($2);
            INSERT INTO accounting (account_id, debit) VALUES ($1, 5);
        ELSE
            RAISE EXCEPTION ‘The account % does not have enough free balance’, $1;
        END IF;
    END;
$$ Language plpgsql;