What is a "dirty read" ?

Posted: 9/18/2003 6:03:00 PM

java-programmer, What is a "dirty read" ? Hi group,

I am new to JDBC but have idea about Oracle transaction management. I
am confused about JDBC documentation saying "dirty read"s allow
changes made by a trasaction visible to another before it is
committed. What does this mean ? Does it mean that the concurrent
transaction can be from different connections ? If so how is it
possible ? I believe read dirty reads are reads from dirty blocks
which is accessible only by this session.

TIA

Pramod R

Posted: 9/18/2003 7:08:00 PM

java-programmer >> What is a "dirty read" ? In article <email***@***.com>, email***@***.com (Pramod Ramachandran) wrote:
>I am new to JDBC but have idea about Oracle transaction management. I
>am confused about JDBC documentation saying "dirty read"s allow
>changes made by a trasaction visible to another before it is
>committed. What does this mean ? Does it mean that the concurrent
>transaction can be from different connections ? If so how is it
>possible ? I believe read dirty reads are reads from dirty blocks
>which is accessible only by this session.

Yes, by setting the isolation level to READ UNCOMMITTED you accept that you
may read uncommitted data from any other transaction. JDBC does not support
multiple open transactions on the same connection.

Exactly how the database server implements this and that isolation level is
outside the scope of JDBC and I am sure there are just as many implementations
as there are products.

-
Robert Sundstr?, Mimer SQL Development
Upright Database Technology AB, http://www.mimer.com
Validate your SQL statements at http://developer.mimer.com/parser

Posted: 9/18/2003 11:28:00 PM

java-programmer >> What is a "dirty read" ?

Pramod Ramachandran wrote:

> Hi group,
>
> I am new to JDBC but have idea about Oracle transaction management. I
> am confused about JDBC documentation saying "dirty read"s allow
> changes made by a trasaction visible to another before it is
> committed. What does this mean ? Does it mean that the concurrent
> transaction can be from different connections ? If so how is it
> possible ? I believe read dirty reads are reads from dirty blocks
> which is accessible only by this session.

Don't worry. Dirty reads are impossible in Oracle. Dirty reads are when
a query done by one user can see data that another user is inserting, before
that second user has committed the data. This is associated with one of the
lower transaction isolation levels, but oraccle doesn't provide those levels.
Oracle only has read_committed, and an odd problematic serializable mode.
Joe Weinstein at BEA

>
>
> TIA
>
> Pramod R