alter table constraint on unlock oracle - oracle

Question

I am trying to write a script to be executed by a client who has no real knowledge of pl/sql,
I do a bunch of transactions in PL/sql to clean up their landscape and then have to add in some constraints to keep this from happening again, I commit automatically after the cleaning, and would like to finish my PL block with adding these constraints
only issue is I have no guarantee that the tables will not be locked when trying to add constraints to them, is there a wait until unlock type of command in oracle?
thank you, new to oracle and cannot seem to find this, I have combed through a bit of API but am at the point of time sensitivity and is proving very difficult to find when i think it seems like it would be a pretty regular issue with DB management

Answer 1

in your pl/sql block before execute any DDL you should call via execute immedaite :
LOCK TABLE <table_name> IN EXCLUSIVE MODE WAIT <n>;
where is number of seconds to wait.
But bear in mind even if you get lock after your first ddl it will be released because DDL commits automatically. So there is no way to guarantee that list of DDL commands will be executed one-by-one as in one transaction.

Answer 2

The command would be:
LOCK TABLE tab IN EXCLUSIVE MODE;
This will wait indefinitely if there is another session with a similar lock (which there should not be).
You can lock a table explicitly (with "WAIT") prior to a DDL, but that won't necessarily solve all of your problems if you are worried about another session holding an exclusive lock. Sounds more like you may have experienced an ORA-00054: resource busy and acquire with NOWAIT specified or timeout expired due to a DML transaction in another session. This will eventually finish, so issuing the above lock command prior to ALTER TABLE will help you here. Just be aware that if there is a hung session elsewhere the LOCK TABLE command can wait indefinitely unless you have set a timeout.
So lets do an example:
In session A I start a transaction on tab
SQL> INSERT INTO tab VALUES(...);
In session B I attempt to add a constraint to the table.
SQL> alter table tab add constraint uk_name unique(name);
alter table tab add constraint uk_name unique(name)
*
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified or timeout expire
So I'd have to try again. But instead if you precede your DDL with a LOCK ... EXCLUSIVE (WAIT)
SQL> lock table tab in exclusive mode;
-- indefinite period while the session A transaction blocks session B
Then session A commits or rolls back
SQL> commit;
Session B immediately continues when we see the lock command return Table(s) locked
Table(s) Locked.
SQL> alter table tab add constraint uk_name unique(name);
Table altered. (**comment Lock is released by the implicit commit issued the DDL statement)
SQL>
Since the DDL statement releases the lock, each DDL will need to be preceded with a fresh LOCK statement. If you want to use this, I recommend adding a timeout (as suggested in comments by Justin). Lets wait up to a minute before giving up.
SQL> lock table tab in exclusive mode WAIT 60;
Although if this is an unattended script on a busy database, I'd probably go with something more than 60 seconds. Just log everything to a spool file, and check the log later for errors.
Anything more than this sort of maintenance probably requires that you quiesce your database first, else just deal with the contention on a case by case basis.