We have a batch process which reads the base tables and performs some aggregation and then update the tables with an modified flag.
We have an update statement which updates around 3million rows.As a part of the business requirement we need to have table-level lock on the table which we are updating.
UPDATE TABLE1 t1 SET PARAMETER1=(SELECT p1 from TABLE2 t2 where t1.ROW_ID=ROWIDTOCHAR(t2.ROW_ID)
The observation today we made is that, update statement with table level lock is taking 35 mins while without table level lock is taking 20 mins.
I am not able to ascertain this observation. Please help!
Cheers,
Dwarak
Nobody but your database could tell you the reason of your observation. You'll have to do an AWR report.
However, it's not quite possible that the UPDATE would run longer because the table had been locked before.
Did you account for caching (both in the database and the filesystem) in your testing? Depending on what you did when, one statement might have run faster due to data already being in memory.
Related
I'm using user_tab_modifications table to monitor all my table's change in DB, but sometimes the records disappeared.
For example, I updated the data in table A, and ran the following SQL to flush the table user_tab_modifications so that I can see the latest information there.
exec DBMS_STATS.FLUSH_DATABASE_MONITORING_INFO;
Then
SELECT * FROM USER_TAB_MODIFICATIONS;
So I can see the record about table A in there.
But then I found the record about table A disappeared after about 1 minute even though I didn't do anything in Oracle.
(other records in user_tab_modifications do not change. No problems)
That's why and can I do some settings to change it (make sure the records there will not disappear)? Thank you.
From the documentation:
USER_TAB_MODIFICATIONS describes modifications to all tables owned by the current user that have been modified since the last time statistics were gathered on the tables.
You might want to check if some stat gathering process was running in the background on the concerned table between the time when the changes were done and when you saw the stat record disappear.
I have a pretty complex query where we make use of a temporary table (this is in Oracle running on AWS RDS service).
INSERT INTO TMPTABLE (inserts about 25.000 rows in no time)
SELECT FROM X JOIN TMPTABLE (joins with temp table also in no time)
DELETE FROM TMPTABLE (takes no time in a copy of the production database, up to 10 minutes in the production database)
If I change the delete to a truncate it is as fast as in development.
So this change I will of course deploy. But I would like to understand why this occurs. AWS team has been quite helpful but they are a bit biased on AWS and like to tell me that my 3000 USD a month database server is not fast enough (I don't think so). I am not that fluent in Oracle administration but I have understood that if the redo logs are constantly filled, this can cause issues. I have increased the size quite substantially, but then again, this doesn't really add up.
This is a fairly standard issue when deleting large amounts of data. The delete operation has to modify each and every row individually. Each row gets deleted, added to a transaction log, and is given an LSN.
truncate, on the other hand, skips all that and simply deallocates the data in the table.
You'll find this behavior is consistent across various RDMS solutions. Oracle, MSSQL, PostgreSQL, and MySQL will all have the same issue.
I suggest you use an Oracle Global Temporary table. They are fast, and don't need to be explicitly deleted after the session ends.
For example:
CREATE GLOBAL TEMPORARY TABLE TMP_T
(
ID NUMBER(32)
)
ON COMMIT DELETE ROWS;
See https://docs.oracle.com/cd/B28359_01/server.111/b28310/tables003.htm#ADMIN11633
I have two procedures that I want to run on the same table, one uses the birth date and the other updates the name and last name taken from a third table.
The one that uses the birthday to update the age field runs all over the table, and the one that updates the names and last name only updates the rows that appear on the third table based on a key.
So I launched both and got deadlocked! Is there a way to prioritize any of them? I read about the nowait and skip locked for the update but then, how would I return to the ones skipped?
Hope you can help me on this!!
One possibility is to lock all rows you will update at once. Doing all updates in a single update statment will accomplish this. Or
select whatever from T
where ...
for update;
Another solution is to create what I call a "Gatekeeper" table. Both procedures need to lock the Gatekeeper table in exclusive mode before updating the table in question. The second procedure will block until the first commits but won't deadlock. In 11g you can create a table with no space allocated.
A variation is to insert a row in the Gatekeeper. Then lock only that row with select for update. Then you can use the Gatekeeper in other situations.
I would guess that you got locked because the update for all the rows and the update for a small set of rows accessed rows in different orders.
The former used a full scan and reached Raw A first, then went on to other rows, eventually trying to lock Row B. However, the other query was driven from an index or a join and already had Row B locked, and was off to lock Row A when it found it was already locked.
So, the fix: firstly, having an age column that needs to be constantly modified is a really bad idea. Perhaps it was done to allow indexing of age, but with a correctly written query an index on date of birth will let you find the same records just as quickly. You've broken normalisation rules and ended up coding yourself a deadlocking application. Hopefully you are only updating the rows that need to be updated, not all of them regardless -- I mean, that would just be insane.
The best solution is to get rid of that design flaw.
The not so good solution is to deconflict your queries by running them at different times or by using DBMS_Lock so that only one of them can run at any time.
Recently, a very strange scenario has been reported from one of of our sites.
Based on our fields, we found that there should be some kind of delete that must have happenend for that scenario
In our application code, there is no delete for that table itself. So we checked in gv$sqlarea(since we use RAC) table whether there are any delete sql for this table. We found nothing.
Then we tried to do the same kind of delete through our PL/SQL Developer. We are able to track all delete through gv$sqlarea or gv$session. But when we use below query, lock, edit and commit in plsql developer, there is no trace
select t.*, t.rowid
from <table>
Something which we are able to find is sys.mon_mods$ has the count of deletes. But it is not stored for a long time, so that we can trace by timestamp
Can anyone help me out to track this down
Oracle Version: 11.1.0.7.0
Type : RAC (5 instances)
gv$sqlarea just shows the SQL statements that are in the shared pool. If the statement is only executed once, depending on how large the shared pool and how many distinct SQL statements are executed, a statement might not be in the shared pool very long. I certainly wouldn't expect that a one-time statement would still be in the shared pool of a reasonably active system after a couple hours.
Assuming that you didn't enable auditing and that you don't have triggers that record deletes, is the system in ARCHIVELOG mode? Do you have the archived logs from the point in time where the row was deleted? If so, you could potentially use LogMiner to look through the archived logs to find the statement in question.
I have a trigger that checks another couple of tables before allowing a row to be inserted. However between the time I check the other tables and insert the row the other tables may get updated.
How do I ensure the tables I'm checking remain in a consistent state until after the new row is inserted? I was thinking of taking locks out but everything I've read boils down to if you are not leaving locking to Oracle you're almost certainly doing it wrong.
Oracle is already doing this for you, when you perform a select it will look at all tables as of the time the transaction started ( the time of the first DML ). This wont stop the data from being changed under you though, your transaction just wont see it being changed. If you want to stop that data from being changed then you can use "SELECT FOR UPDATE" as Justin Cave suggests.
I would seriously question what you are doing though, triggers, except in the most trivial cases, almost always lead to unexpected side effects.