bank uses in its interactions with its customersand ( the requirements for the database system that manages the accounts the typical customer opens and closes accounts far less often than he or she accesses the account customers are willing to wait many minutes while accounts are created or deleted but are typically not willing to wait more than brief time for individual account transactions such as deposit or withdrawal these observations can be considered as informal specifications for the time constraints on the problem it is common practice for banks to provide two tiers of service human tellers or automated teller machines (atmssupport customer access to account balances and updates such as deposits and withdrawals special service representatives are typically provided (during restricted hoursto handle opening and closing accounts teller and atm transactions are expected to take little time opening or closing an account can take much longer (perhaps up to an hour from the customer' perspectivefrom database perspectivewe see that atm transactions do not modify the database significantly for simplicityassume that if money is added or removedthis transaction simply changes the value stored in an account record adding new account to the database is allowed to take several minutes deleting an account need have no time constraintbecause from the customer' point of view all that matters is that all the money be returned (equivalent to withdrawalfrom the bank' point of viewthe account record might be removed from the database system after business hoursor at the end of the monthly account cycle when considering the choice of data structure to use in the database system that manages customer accountswe see that data structure that has little concern for the cost of deletionbut is highly efficient for search and moderately efficient for insertionshould meet the resource constraints imposed by this problem records are accessible by unique account number (sometimes called an exact-match queryone data structure that meets these requirements is the hash table described in hash tables allow for extremely fast exact-match search record can be modified quickly when the modification does not affect its space requirements hash tables also support efficient insertion of new records while deletions can also be supported efficientlytoo many deletions lead to some degradation in performance for the remaining operations howeverthe hash table can be reorganized periodically to restore the system to peak efficiency such reorganization can occur offline so as not to affect atm transactions