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1. The overall database design is known as a schema and the collection of information stored in database at a particular instant is an instance . schemas are changed very rarely and if at all, by database administrator alone . A database can have one physical schema , one logical schema and several sub schemas 2. data independence ----- the ability to modify schema definition in one level without effecting the schema definition in next higher level of database is data independence . Physical data independence is ability to modify physical schema without causing application programs be rewritten, whereas logical data independence is ability to modify logical schema of database without causing application programs be rewritten obviously logical data independence is more difficult to achieve as application programs are heavily dependent on logical structure of data they access 3 a transaction is a unit of programs execution that accesses and possibly updates various data items , ACID properties of a transaction are as follows ---> Atomicity ------ either transaction executes in it s entirety or none of it executes at all > Consistency ----- the state of the database remains the same both before and after transaction occurs , despite system changes > Isolation ----- despite the simultaneous execution of transactions, each transaction proceeds independently of the other despite systems changes > Durability --- any modifications made to the database as a result of transaction persist even if there are system changes we now explain the ACID properties of a transaction through examples , we have a schedule involving accounts A and B .the initial values in A and B accounts are $ 1000 and $ 2000 , now this is consistent state as sum A+B is preserved throughout , now a transaction occurs which transfers $ 50 from A to B , as a result final values in A and B are $ 950 and $ 2050 , this is clearly a consistent state as sum A+B is preserved both before and after transaction occurs , we now suppose that a system failure occurs after write[A] committed but before write[B] commits so now the final values in A and B are 950 and 2000 clearly inconsistent state as sum A+B is no longer preserved both before and after transaction, even if systems failure did not occurs and transaction proceeded to completion, such an inconsistent state would not have been visible except during execution of transaction, this once agai.n reflects atomicity property I.e either the transaction executes in it s entirety or none executes at all. Isolation is preserved since despite concurrent execution of transactions, each transaction proceeds independently of others despite systems changes , durability is preserved through recovery management component of transactions 4 . query processor ---- database users can be application programers , sophisticated users , specialized users and naive users . Sophisticated users are software personnel who interact with database through writing their queries as sql statements that is forwarded to query processor who then submits it to query manager
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