for developers to manage concurrent access to data. Internally,

data consistency is maintained by using a multiversion

model (Multiversion Concurrency Control, <acronym>MVCC</acronym>).

This means thatwhile querying a databaseeachtransaction sees

This means that eachSQL statement sees

a snapshot of data (a <firstterm>database version</firstterm>)

as it was some

time ago, regardless of the current state of the underlying data.

Thisprotects the transactionfrom viewing inconsistent datathat

could be causedby(other)concurrenttransaction updates on the same

Thisprevents statementsfrom viewing inconsistent dataproduced

by concurrenttransactions performing updates on the same

data rows, providing <firstterm>transaction isolation</firstterm>

for each database session. <acronym>MVCC</acronym>, by eschewing

the locking methodologies of traditional database systems,