Do I ever need to change it?
The isolation_level parameter in $CUBRID/conf/cubrid.conf file is set by default to 3 which is equal to REPEATABLE READ CLASS with READ UNCOMMITTED INSTANCES.
This means that another transaction T2 cannot update the schema of table A while transaction T1 is viewing table A. Transaction T1 may experience R' read (dirty read) for the record that was updated but not committed by another transaction T2 which is indicated by READ UNCOMMITTED INSTANCES.
To obtain the current value of isolation_level you can execute GET TRANSACTION ISOLATION LEVEL query.
Regarding whether you ever need to change it, in most cases you will never need to, especially if you are not sure what this is for. Just leave the default values as it is. If you really understand what you're doing, consult the manual for details.
You can read more about the isolation levels in the manual http://www.cubrid.org/manual/831/en/SET%20TRANSACTION%20ISOLATION%20LEVEL.
More explanation is also covered at Database Concurrency section in the manual and in CUBRID Transaction and Lock tutorial.
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