Add column with If Not Exists

Hi

I would like to suggest the addition of the "If not exists" to the Add
Column feature of Postgres.  There are quite common situations where
ensuring a column exists is important so that an update to remote
devices will not fail but it is not so important that deprecated fields
be removed.  This is often the case with backward compatibility.  New
columns will not affect old systems but allows all remote devices
running older software to be updated using the same process as new
devices.  Once the hardware reaches end of life it will be replaced and
the new hardware will use the new columns.  So having the ability to
Alter the table with a series of Add Column commands ensures that the
new records included in the update are added to the table.  Having the
"if not exists" would remove all of this potential complexity and allows
a quick and easy method to ensure the column exists in the table using
simple SQL commands that will not cause errors if the column is already
present.

I understand that some people will feel this is not the recommended way
to proceed.  I understand there are lots of ways this can be handled
differently as I have been working with database of various kinds for
over 30 years.  All I am suggesting is that this feature would be nice
to have in some scenarios and thus would like it considered.  I do not
wish to start a discussion on whether using it is a sound approach or
not as that is a different discussion.

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Regards,
Simon White
dCipher Computing
705-500-0191