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).
Related
It is a well known fact that in an Oracle database it is not possible to make a transaction out of multiple DDL statements.
However, is there any way to lock a specific set of database objects within the current connection so that after a DDL query is executed, all locks are held until they are explicitly released?
An obvious solution of this kind doesn't work, because executing the DDL statement automatically commits the transaction, and with it, the locks are released:
LOCK TABLE x ....;
LOCK TABLE y ....;
ALTER TABLE x ....; -- Does not work properly since table locks are released here
ALTER TABLE y ....;
ALTER TABLE x ....;
COMMIT;
The DBMS_LOCK option doesn't work either, because it is an advisory lock, and the concurrent thread must respect this lock and at least be aware of its existence.
Moreover, it is not controlled which statements can be executed by concurrent threads/sessions. It is possible to execute a query only in the current session, and it must be ensured that no intermediate queries on tables X and Y are executed from other sessions until the current session has ended.
Are there any ideas how this can be implemented?
PS: Please don't mention the high-level task or XY problem. There is no high-level task. The question is posed exactly as it is.
A bit of a joke (breaks all dependent PL/SQL), but... ;)
ALTER TABLE x RENAME TO x__my_precious;
ALTER TABLE y RENAME TO y__my_precious;
ALTER TABLE x__my_precious ...;
ALTER TABLE y__my_precious ...;
ALTER TABLE x__my_precious RENAME TO x;
ALTER TABLE y__my_precious RENAME TO y;
I'm pretty sure what you're trying to do isn't possible with Oracle's native transaction control. DDL will always end a transaction, so no lock on that object is going to survive it. Even if you immediately attempted to lock it after the DDL, another waiting session could slip in and obtain the lock before you do.
You can, however, serialize access to the table by utilizing another dummy table or row in a dummy table, assuming you control the code of any process wishing to access the table. If this is the case, then before accessing the table, attempt to lock the dummy table or a row in it first, and only if it succeeds continue with accessing the main table. Then the process that does DDL can take out that same lock (preventing other processes from proceeding), then do the DDL in a subroutine (named PL/SQL block) with PRAGMA AUTONOMOUS_TRANSACTION. That way the DDL ends the autonomous transaction rather than the main one, which still holds the lock on the dummy table.
You have to use a dummy table because if you tried to use the same table you want to modify you'll deadlock yourself. Of course, this only works if you can make all other processes do the lock-the-dummy-table safety check before they can proceed.
Lastly, albeit what I said above should work, it is likely that you're trying to do something you shouldn't do. DDL against hot objects isn't a good idea. Whatever you're trying to do, there is probably a better way than modifying objects on the fly like this. Even if you are able to keep other locked out, you are likely to cause object reference errors, package invalidations, SQL cursor invalidations, etc.. it can be a real mess.
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.
in my pl\sql process, there is an execution of "alter table exchange parition.."
on some table.
the problem is that during that operation - other users can try to access the target table.
one process that executed a select query on that table, got this error:
ORA-08103 object no longer exists.
i think that it is not the same like 'object or view doesn't exist'.
i think that 'object no longer exists' error, come when the process start ok,
and then the exchange (or other operation) come from the side and the process
can't bo done.
the chance is very low that it will happen, because the exchange is very very fast.
but for this case, there is any idea how to solve it? how to prevent this situation?
maybe a way to execute the exchange only if no-one touch the table?
thanks.
This will happen when session #2 alters the table in the table after session #1 has opened its cursor but before it is finished fetching the rows from that cursor.
I don't think there is a foolproof way to prevent the exchange from happening unless you are willing to change the code that other users are using to access the target table.
LOCK TABLE mytable IN EXCLUSIVE MODE will not wait for SELECT statements to complete, nor will it prevent new SELECT statements from starting, so acquiring an exclusive lock before attempting the ALTER TABLE will not work.
If you want to prevent the ALTER TABLE from happening at the same time as SELECTs, you need both to depend on acquiring the same lock. A relatively robust way to do that would be to use the DBMS_LOCK package to allocate a lock for that table. Call that lock "mytablelock".
Then, using DBMS_LOCK, your SELECT sessions would need to acquire "shared" locks on "mytablelock" before progressing. Your alter table sessions would need to acquire an "exclusive" lock on "mytablelock" before progressing.
This scheme would allow multiple SELECT sessions to run without interfering with each other, but it would prevent the ALTER TABLE from running while any SELECT was running.
A (much) less robust, but simpler, way to do it would be to change the SELECT statements issued from the other query into SELECT FOR UPDATES. But that's a recipe for lots of unnecessary waits and deadlock errors.
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.
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