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I am having some confusion in understanding the meaning of the term data item .

when we write

READ(A)  

A=A-10

WRITE(A)

Now I am unable to get one point here that when we have a file consisting of various blocks and those blocks consisting of various record and when we say that we are accessing a particular data item for reading then is this data item is a record or some particular field inside that record .

 

And major confusion is that how can we simply read a field from a record during disk access , since disk access is expensive hence we must be able to fetch the entire block from the disk , but we write only this statement like :

READ(A) , is this block or only a single attribute ?

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I AM NOT SURE  BUT I AM GIVING A GO

just imagine about the granularity of DB..  DB-> REGIONS ->FILES->RECORDS

record is a collection of fields .fields are ATTRIBUTES so whatever you do that is in domain of attributes.

well notice even metadata is stored in disk ..  

but there is buffer cache due to which the frequency of access of secondry storage is reduced

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