static Map<Id, Account> cache = new Map<Id, Account>();

Account[] getAccountsById(Set<Id> idSet) {
		Boolean complete = true;
		Account[] toReturn = new Account[]{};

		for (Id id : idSet) {
			if (!cache.containsKey(id)) {
				complete = false;
				break;
			}
			toReturn.add(cache.get(id));
		}
		if (complete == true) return toReturn;

    Account[] accList = [SELECT Name FROM Account];
    for (Account acc : accList) {
       cache.put(acc.Id, acc);
    }
    return accList;
}