How does Oracle database handle that situation, if multiple users try to modify the rows in the same block? I know that they cannot modify the same row. But how does it work for the different rows in the same block?
I mean, how does Oracle handle row level lock, while it modifies the whole block when you modify one row?
You should ask this question on dba.stackexchange.com in order to get a better response.
A row-level lock happens when DML (Insert, Update, Delete) or SELECT FOR UPDATE is done. This row-level lock is controlled by the transaction that contains the DML that created the lock. When the transaction commits or rolls back, the lock is released. If another transaction tries to update the same row, it has to wait until the initial transaction commits or rolls back.
The block header contains an ITL (Interested Transaction List) with slots allocated for each transaction that wants to modify the block. The INITRANS table setting is a number that allows for multiple transactions on the table blocks. The default is low and for high concurrency tables, this is set to anything from 10 to 50. If INITRANS is set to 1, then there will be a wait for the next transaction that tries to modify the same block.
So, the whole block is not modified; the row lock is handled within the ITL and the number allocated to the table.
Related
What is the difference between row lock and table lock in Oracle database.
will for loop with update statement trigger table lock ??
Any DML statement on a table is going to acquire a table lock. But it is terribly unlikely that this table lock is going to affect another session in a way that limits concurrency. When your session updates rows, there will be a row exclusive table lock which will stop another session from doing DDL on the table (say, adding or removing a column) while there are active, uncommitted transactions involving the table. But presumably, you're not generally trying to modify the structure of the table at the same time that you're updating rows in the table (or understand that when you deploy these DDL changes that you'll block other sessions for a short period of time and you're picking your deployment times accordingly).
The specific rows that you are updating will be locked in order to prevent another session from modifying those rows until your transaction either commits or rolls back. Those row level locks are generally the locks that cause performance and scalability issues. Ideally, your code would be structured to hold the locks for as little time as possible (updating data in sets is much faster than doing row-by-row updates) and to minimize the probability that two sessions will try to update the same row simultaneously.
It says on the documentation that oracle 12c, aquires a table lock when a row lock is aquired.That is not so in sql server, it is very baffling.
A row lock, also called a TX lock, is a lock on a single row of a table. A transaction acquires a row lock for each row modified by one of the following statements: INSERT, UPDATE, DELETE, MERGE, and SELECT ... FOR UPDATE. The row lock exists until the transaction commits or rolls back.
***When a transaction obtains a row lock for a row, the transaction also acquires a table lock for the table in which the row resides***. The table lock prevents conflicting DDL operations that would override data changes in a current transaction.
Can somebody elucidate this?
The table lock that occurs is a shared lock. There can be any number of shared locks allowed on the same table at the same time: they do not interfere with one another.
What they do do is prevent anything from acquiring an exclusive lock on that table: say, to change the structure of the table.
I have a pl/sql script that clears (via delete from statement) and populates several depended tables like this:
delete from table-A
insert into table-A values(...)
delete from table-B
insert into table-B values(...)
These operations require ~ 10 seconds to complete and I'd like to stop all sql queries that try to read data from table-A or table-B while tables are updating. These queries should stop and continue execution when table-A and table-B are completely updated.
What is the proper way to do this?
As others have pointed out, Oracle's basic concurrency model is that writers do not block readers and readers do not block writers. You can't stop a simple select from running. Your queries will see the data as of the SCN that they started executing (assuming that you're using the default read committed transaction isolation level) so they will have a consistent view of the data before your updates started.
You could potentially acquire a custom named lock using dbms_lock.request. You would need to acquire this lock before running your updates and every session that queries the tables would also need to acquire the lock before it starts to query the tables. That will, obviously, decrease the scalability of your application but it will accomplish what you appear to be asking for. Presumably, the sessions doing queries can acquire the lock in shared mode while the session doing the updates would need to acquire it in exclusive mode.
Please anyone explain locking mode in Oracle i.e. Share, Exclusive and Update lock. I found many theories on this and according to that
Share lock : Nobody can change data,Read only purpose
Exclusive lock : Only one user/connection are allow to change the data.
Update lock : Rows are locked till user made commit/rollback.
Then, I tried shared to check how it works
SQL> lock table emp in share mode;
Table(s) Locked.
SQL> update emp set sal=sal+10;
14 rows updated.
Then, I found that, user can change data after share lock. Then, what makes it different from exclusive lock and update lock.
Another question, how Update lock and exclusive lock are different with each other, even they seems almost equivalent.
Posting explanation for future visitors, and it also gives the answer.
Shared lock
Before I begin let me first say that there are 5 types of table locks - row shared, row exclusive, shared, shared row exclusive and exclusive. And shared lock is one among these. Also, please note that there are row locks, which are different than table locks. Follow the link I have provided in end to read about all this.
A shared lock is acquired on the table specified in following statement – LOCK TABLE table IN SHARE MODE;
This lock prevents other transactions from getting “row exclusive” (this lock is used by INSERT, UPDATE and DELETE statement), “shared row exclusive” and “exclusive” table locks, otherwise everything is permitted.
So, this means that a shared lock will block other transactions from executing INSERT, UPDATE and DELETE statements on that table but will allow other transactions to update the rows using “SELECT … FOR UPDATE” statement because for this statement a “row shared” lock is required, and it is permitted when a “shared” lock is required.
Below table is a good summary of locks and what's permitted.
Since many users will follow this question so I decided to go one more step further and put my learning notes, I hope folks will be benefited from it:
Source of this information and also excellent reading about Oracle locks.
It's very well explained in the documentation: http://docs.oracle.com/cd/E11882_01/server.112/e41084/ap_locks001.htm#SQLRF55502
In your example you locked the table in shared mode. This does not prevent other sessions locking the same object in shared mode, but it does prevent them from locking it in exclusive mode so you could not drop the table (which requires an exclusive lock) while it is being updated (which has a shared lock).
I get ORA-00054 while loading large data files(~ 10 gb)
The error occurs when this a new file is loaded after a previous file.
Any ideas how I can solve this?
One possible scenario.
Is this a direct path load ? If so, please check the v$locked_object view and see if is being locked by someone during your load.
select dbao.object_name
from v$locked_object vlo,
dba_objects dbao
where vlo.object_id = dbao.object_id
and dbao.object_name = 'Table that you are trying to load...'
From the Oracle Documentation at http://download.oracle.com/docs/cd/B10500_01/server.920/a96524/c21dlins.htm
Locking Considerations with
Direct-Path INSERT
During direct-path INSERT, Oracle
obtains exclusive locks on the table
(or on all partitions of a partitioned
table). As a result, users cannot
perform any concurrent insert, update,
or delete operations on the table, and
concurrent index creation and build
operations are not permitted.
Concurrent queries, however, are
supported, but the query will return
only the information before the insert
operation.
Maybe this is linked to tablespace datafile sizes, table size, because ORA-00054 usually appears when an ALTER statement is run.
I do not pretend to be right here.
Check those views.
DBA_BLOCKERS – Shows non-waiting sessions holding locks being waited-on
DBA_DDL_LOCKS – Shows all DDL locks held or being requested
DBA_DML_LOCKS - Shows all DML locks held or being requested
DBA_LOCK_INTERNAL – Displays 1 row for every lock or latch held or being requested with the username of who is holding the lock
DBA_LOCKS - Shows all locks or latches held or being requested
DBA_WAITERS - Shows all sessions waiting on, but not holding waited for locks
http://www.dba-oracle.com/t_ora_00054_locks.htm
Your table seems to be locked: ORA-00054
It can be because of the way that Oracle driver handles the BLOB types (the driver locks the record, opens an stream to write the binary data, and needs "some help" to release the record).
I would try the next secuence:
Load the first file
COMMIT;
Load the second file