Read committed is an isolation level that
guarantees that any data read is committed at the moment it is
read. It simply restricts the reader from seeing
any intermediate, uncommitted, 'dirty' read.
This level emulates serial transaction execution
for all committed transactions; as if transactions
had been executed one after another, serially,
rather than concurrently.
A transaction re-reads data it has previously read
and finds that data has been modified by another
transaction (that committed since the initial read).
A transaction re-executes a query returning a set of
rows that satisfy a search condition and finds that
the set of rows satisfying the condition has changed
due to another recently-committed transaction.
Multiversion concurrency control (MCC or MVCC), is a
concurrency control method commonly used by database
management systems to provide concurrent access to
the database.
A deadlock is a state in which each member of a group
is waiting for another member, including itself, to
take action, such as sending a message or more
commonly releasing a lock.