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The default operation is that you can only read data that was committed before your read began.
I got this a while back from bytes.com.... looks like their source was a book called Oracle 9i Concepts:
Oracle provides these transaction isolation levels.
"Read committed"
This is the default transaction isolation level. Each query executed
by a transaction sees only data that was committed before the query
(not the transaction) began. An Oracle query never reads dirty
(uncommitted) data.
Because Oracle does not prevent other transactions from modifying the
data read by a query, that data can be changed by other transactions
between two executions of the query. Thus, a transaction that executes
a given query twice can experience both nonrepeatable read and
phantoms.
"Serializable"
Serializable transactions see only those changes that were committed
at the time the transaction began, plus those changes made by the
transaction itself through INSERT, UPDATE, and DELETE statements.
Serializable transactions do not experience nonrepeatable reads or
phantoms.
"Read-only"
Read-only transactions see only those changes that were committed at
the time the transaction began and do not allow INSERT, UPDATE, and
DELETE statements.