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.
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.
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.
Is it possible to somehow emulate shared row locks (as they exist in other DBMS, like PostgreSQL) in Oracle DB?
That is row locks, that allow other session to obtain the same (shared) locks on the same row, but prevent them from obtaining exclusive locks. Also, an existing exclusive lock on a row prevent others obtaining any kind of row lock.
For reference: Oracle supports only exclusive row locks.
Note: The question is about Oracle DB 11.2g
As you know, Oracle needs to get a row level lock when DML happens on a table.
I wonder if we can check the number of lock request to a table for changing Oracle blocks.
The number of locks on A table.
Is there any dictionary view to find it in Oracle?
Thanks.
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).