A row is treated as a variable length row when an ALTER TABLE has added a
new column to the table. My understanding is that future INSERT'ed and
UPDATE'ed rows have an indicator set in the row header marking these rows as
variable length. Additionally there is an indication in the DBD. It
follows that a REORG will standardize the row lengths and remove the
variable length row indicator from the row header, but is the row now really
treated as a fixed length row, or is a MODIFY also needed before these rows
are treated as fixed length rows? And if so, what if you don't use DB2
Image Copy to back up your data? The documentation states that MODIFY must
find at least one COPY row in SYSCOPY before it will maintain the DBD and
log range table. We are on Version 8 of DB2 for z/OS.
QUESTION POSED ON: 01 FEB 2005
QUESTION ANSWERED BY:
Craig Mullins
There is nothing in the row header, other than the row length, to indicate
that a row is variable length. If you have alter added a column, there is an
indicator in the Record OBD that a table might have a variable length row
(OBDREVLF). That way DB2 doesn't end with an error if it encounters a row
that doesn't contain the newly added column. It is the old rows that are
treated as exceptions because their row length is shorter than the maximum.
A subsequent REORG instantiates the new column in all rows, but it does not
turn off the OBDREVLF bit. A MODIFY that is run after the reorg and deletes
all recovery resources prior to the reorg does turn it off.
This is the way it works prior to DB2 V8. Haven't tested in V8, but I doubt
this has changed.
(Thanks to Michael Murley from BMC Software R+D for his assistance in
preparing this answer.)
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