TID clarification

I'm implementing an O/R mapping tools for Java on top of JDBC. One of
the issues I'm dealing with are dirty checks.

An object is loaded from a row and modified in memory without acquiring
a lock. Prior to storing the object I would like to assure that the
memory copy is current and no changes occured to the database in the
meantime.

One way of doing that is by comparing fields from the original object
copy. Another solution is using a TIMESTAMP or RAWID.

In RDBMS that support timestamp, each update to a row updates the
timestamp, so loading the timestamp and comparing it during update is
sufficient to identity dirtyness.

In RDBMS that support RAWID (as opposed to ROWID), every update to the
row updates the RAWID, providing the same functionality as TIMESTAMP but
without the time value.

I ran a very simple test using ctid and noticed that each update to a
row (a three-row table) changes the ctid. I want to know if that is by
accident or by design. Assuming that I update a row, will ctid change
each time, or only when the row is repositioned in the database.

And, if ctid does not have to change when a row is updated, why does it
change in such a small table (less than the size of a page)?

arkin


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Assaf Arkin                                           www.exoffice.com
CTO, Exoffice Technologies, Inc.                        www.exolab.org