I am having table ABC and using in procedures with Delete action at starting and select action at end. (Delete with no where clause)
Now if a process A invokes the procedure and it is at select on table ABC, then at same time another process B invokes the procedure which reached to Delete on ABC without any where clause.
So my question is, would process A can find the data as Delete with no where clause is happening at same time.
Literally, is synchronization would be there among tables.
I'd suggest you read about Oracle multi-versioning and ACID transactions
http://docs.oracle.com/cd/E18283_01/server.112/e16508/consist.htm
https://en.wikipedia.org/?title=ACID
Things that happen in a session within a transaction are not available to another session. This continues until a commit is issued. You have your own version until a commit or rollback is issued.
Oracle starts a transaction by default unlike some other database servers. Other database servers also have their own defaults and different implementations of ACID.
Related
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.
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 am keeping two passwords (actually the same password as far as website human user knows,but hashed and salted using two different algorithms and salts) for my website. One is place in WebUsers table and that is where password is verified when users login at website. The other is password of an oracle user, each website user corresponds to an oracle user.
When making change password procedure, I have to change password in WebUsers table as well as in oracle, in a transaction. The problem is, the DDL statement [alter user ...] commit before performing any action. If [alter user ...] fail and in the exception handling code I rollback the transaction, the changes in the WebUsers table are not rollbacked. Is there some work around?
There is just no way to have a DML and DDL in one atomic transaction under Oracle.
In Oracle a a DDL will always explicitly result in a commit before running the DDL.
Typically in these sort of scenarios, you should execute the step that's most likely to fail first, i.e. in your case the DDL 'Alter User'.
And then run your DML , only if the DDL succeeds , but then again, there is this issue of the DML failing, and then not being able to rollback the DDL, at which point you'll simply have to run another DDL to revert the changes made by the first DDL.
The idea is that the second step is far more likely to fail than the first step.
This pattern is also used, where you've to deal with say something like JMS + JDBC , and you can't use a JTA transaction to wrap the JMS + JDBC activities in a single transcation.
I need to do a task but I have no idea how to do it.
Here is the problem:
I have about 1000 tables on a Oracle Database and many processes.
Each process does one or more SELECT on one or many tables.
Because it's almost impossible to look in the source code to find which process does which SELECT on which tables, I would like to have some kind of trigger on SELECT on every table.
The idea is that I will launch the processes one by one to be able to see which tables will query.
I know that there is no trigger on SELECT, but is there anything else?
I need to do this in a one shot, just to recover the necessary info, it will not run every day.
You could activate auditing. You can audit all SELECT with:
AUDIT SELECT TABLE;
You can specify BY SESSION so that only one record will be written to the audit trail per table accessed per session.
Your AUDIT_TRAIL parameter must be set to either DB or OS. If it is set to DB, the audit trail will be written to the SYS.AUD$ table.
Assuming that you can map a "process" in your terminology to a particular Oracle session, you could trace the Oracle session. That would show you all the SQL statements executed by that session.
You could also potentially do a SQL*Net trace from whatever the client machine is (note that the "client machine" in a three-tier environment is the application server). A SQL*Net trace tends not to be nearly as easy to work with, however.
I am working on a system to track a project's history. There are 3 main tables: projects, tasks, and clients then 3 history tables for each. I have the following trigger on projects table.
CREATE OR REPLACE TRIGGER mySchema.trg_projectHistory
BEFORE UPDATE OR DELETE
ON mySchema.projects REFERENCING NEW AS New OLD AS Old
FOR EACH ROW
declare tmpVersion number;
BEGIN
select myPackage.GETPROJECTVERSION( :OLD.project_ID ) into tmpVersion from dual;
INSERT INTO mySchema.projectHistiry
( project_ID, ..., version )
VALUES
( :OLD.project_ID,
...
tmpVersion
);
EXCEPTION
WHEN OTHERS THEN
-- Consider logging the error and then re-raise
RAISE;
END ;
/
I got three triggers for each of my tables (projects, tasks, clients).
Here is the challenge: Not everything changes at the same time. For example, somebody could just update a certain tasks' cost. In this case, only one trigger fires and I got one insert. I'd like to insert one record into 3 history tables at once even if nothing changed in the projects and clients tables.
Also, what if somebody changes a project's end_date, the cost, and say the picks another client. Now, I have three triggers firing at the same time. Only in this case, I will have one record inserted into my three history tables. (which I want)
If i modify the triggers to do insert into 3 tables for the first example, then I will have 9 inserts when the second example happens.
Not quite sure how to tackle this. any help?
To me it sounds as if you want a transaction-level snapshot of the three tables created whenever you make a change to any of those tables.
Have a row level trigger on each of the three tables that calls a single packaged procedure with the project id and optionally client / task id.
The packaged procedure inserts into all three history tables the relevant project, client and tasks where there isn't already a history record for that key and transaction (ie you don't want duplicates). You got a couple of choices when it comes to the latter. You can use a unique constraint and either a BULK select and insert with FORALL/SAVE EXCEPTIONS, DML error logging (EXCEPTIONS INTO) or a INSERT...SELECT...WHERE NOT EXISTS...
You do need to keep track of your transactions. I'm guessing this is what you were doing with myPackage.GETPROJECTVERSION. The trick here is to only increment versions when you have a new transaction. If, when you get a new version number, you hold it in a pacakge level variable, you can easily tell whether your session has already got a version number or not.
If your session is going to run multiple transaction, you'll need to 'clear' out the session-level version number if it was part of a previous transaction. If you get DBMS_TRANSACTION.LOCAL_TRANSACTION_ID and store that at the package/session level as well, you can determine if you are in a new transaction, or part of the same transaction.
From your description, it looks like you would be capturing the effective and end date for each of the history rows once any of the original rows change.
Eg. Project_hist table would have eff_date and exp_date which has the start and end date for a given project. Project table would just have an effective date. (as it is the active project).
I don't see why you want to insert rows for all three history tables when only one of the table values is updated. You can pretty much get the details as you need (as of a given date) using your current logic. (inserting old row in the history table for the table that has been updated only.).
Alternative answer.
Have a look at Total Recall / Flashback Archive
You can set the retention to 10 years, and use a simple AS OF TIMESTAMP to get the data as of any particular timestamp.
Not sure on performance though. It may be easier to have a daily or weekly retention and then a separate scheduled job that picks out the older versions using the VERSIONS BETWEEN syntax and stores them in your history table.