Oracle isolation level – xaazg编程网站

l Isolation level and concurrency are mutually contradictory : The higher the degree of isolation, the worse the database concurrency : The lower the degree of isolation, the better database concurrency .

l Non-repeatable read (nonrepeatable read): The same query multiple times in the same transaction, as do other firms submitted modify or delete , Return different result sets each time, non-repeatable read occurs at this time .(A transaction rereads data it has previously read and finds that another committed transaction has modified or deleted the data. Data recovery jaipur )

l Phantom (phantom read): The same query multiple times in the same transaction, because other firms do insert submitted , Each return a different result sets, phantom read occurs at this time .(A transaction reexecutes a query returning a set of rows that satisfies a search condition and finds that another committed transaction has inserted additional rows that satisfy the condition. Database platforms )

l Serializable Isolation level provides a read-only firm to provide read consistency ( Transaction-level read consistency ), DML operations while allowing .

l If the serializable transaction started when the uncommitted transactions in the serializable Before the end of the transaction to modify the serializable transaction will be carried out to modify the line and submit , The serializable transaction can not read these changes , Therefore, the occurrence of errors can not serialize access .( A different interpretation : As long as serializable transaction start to finish in between other commitments on serializable Affairs to modify things were changed and committed changes occurred not serialize access error .)

l If a serializable transaction contains data manipulation language (DML) that attempts to update any resource that may have been updated in a transaction uncommitted at the start of the serializable transaction, ( And changes in the later submitted without rollback ),then the DML statement fails. Data recovery group The error returned was ORA-08177: Cannot serialize access for this transaction.

l ORACLE Recent record in the data block rows of data modify operation of the N-transaction information , Purpose is to determine whether the transaction in the beginning of uncommitted transactions to modify this transaction to modify the line.

Database xls See in English :Oracle permits a serializable transaction to modify a data row only if it can determine that prior changes to the row were made by transactions that had committed when the serializable transaction began. C database library To make this determination efficiently, Oracle uses control information stored in the data block that indicates which rows in the block contain committed and uncommitted changes. 510 k database In a sense, the block contains a recent history of transactions that affected each row in the block. How to become a database administrator The amount of history that is retained is controlled by the INITRANS parameter of CREATE TABLE and ALTER TABLE. Database xml Under some circumstances, Oracle may have insufficient history information to determine whether a row has been updated by a “too recent” transaction. Database terminology This can occur when many transactions concurrently modify the same data block, or do so in a very short period. Database theory You can avoid this situation by setting higher values of INITRANS for tables that will experience many transactions updating the same blocks. Database 1 to 1 relationship Doing so will enable Oracle to allocate sufficient storage in each block to record the history of recent transactions that accessed the block.

l The INITRANS Parameter:Oracle stores control information in each data block to manage access by concurrent transactions. Database testing Therefore, if you set the transaction isolation level to serializable, you must use the ALTER TABLE command to set INITRANS to at least 3. Database graph This parameter will cause Oracle to allocate sufficient storage in each block to record the history of recent transactions that accessed the block. Database naming conventions Higher values should be used for tables that will undergo many transactions updating the same blocks.

l Comply with the transaction-level read consistency, only to see before the start of this transaction to submit the changes by other transactions .

l read only Be serializable subset . Database entity They are to avoid non-repeatable reads and phantom. Database developer The difference is read only Is read-only : In serializable can be DML Operating .

l Transactions before transaction 1 2 Start, and keep uncommitted state . Data recovery plan Transaction 2 wants to modify the transaction is being 1 Modified rows. Data recovery kansas city Affairs 2 Wait if the transaction 1 Rollback, the transaction 2( Or whether it is read committed serializable Mode ) The changes it wants to do if the transaction 1 Submitted, then when the transaction 2 Is read committed mode , The changes it wants to do : When the transaction is 2 serializable Mode of failure and error “Cannot serialize access”, 2 invisible transactions because the transaction 1 Submit changes, and the transaction 2 Want to modify the transaction on the basis of a change do. N k database See specific English :Both read committed and serializable transactions use row-level locking, and both will wait if they try to change a row updated by an uncommitted concurrent transaction. Data recovery 2016 The second transaction that tries to update a given row waits for the other transaction to commit or roll back and release its lock. 510 k database fda If that other transaction rolls back, the waiting transaction (regardless of its isolation mode) can proceed to change the previously locked row, as if the other transaction had not existed. Database programmer However, if the other (blocking) transaction commits and releases its locks, a read committed transaction proceeds with its intended update. Data recovery osx A serializable transaction, however, fails with the error “Cannot serialize access”, because the other transaction has committed a change that was made since the serializable transaction began.

l On SET TRANSACTION READ WRITE:read write And read committed is the same as . Database integrity In the reading area, they are avoiding dirty read , But they can not achieve repeatable read, although not documented read write In writing in line with the read committed , But clearly it will be added in the writing of the exclusive lock to avoid lost updates in the process of locking , If you encounter locked resources can not be locked, should wait and not give up . Database backup This is consistent with read committed .

l ORACLE Statement-level read consistency to ensure that a statement of the processed data set is a single point in time in the data set , The start time is the time this statement .

l For DML statements , It can not see the changes made by their own, that is DML Statement itself is beginning to see the implementation of the pre-existing data .