postgresql/doc
Tom Lane 9b9c5f279e Clarify behavior of adding and altering a column in same ALTER command.
The behavior of something like

ALTER TABLE transactions
  ADD COLUMN status varchar(30) DEFAULT 'old',
  ALTER COLUMN status SET default 'current';

is to fill existing table rows with 'old', not 'current'.  That's
intentional and desirable for a couple of reasons:

* It makes the behavior the same whether you merge the sub-commands
into one ALTER command or give them separately;

* If we applied the new default while filling the table, there would
be no way to get the existing behavior in one SQL command.

The same reasoning applies in cases that add a column and then
manipulate its GENERATED/IDENTITY status in a second sub-command,
since the generation expression is really just a kind of default.
However, that wasn't very obvious (at least not to me; earlier in
the referenced discussion thread I'd thought it was a bug to be
fixed).  And it certainly wasn't documented.

Hence, add documentation, code comments, and a test case to clarify
that this behavior is all intentional.

In passing, adjust ATExecAddColumn's defaults-related relkind check
so that it matches up exactly with ATRewriteTables, instead of being
effectively (though not literally) the negated inverse condition.
The reasoning can be explained a lot more concisely that way, too
(not to mention that the comment now matches the code, which it
did not before).

Discussion: https://postgr.es/m/10365.1558909428@sss.pgh.pa.us
2020-01-21 16:17:21 -05:00
..
src Clarify behavior of adding and altering a column in same ALTER command. 2020-01-21 16:17:21 -05:00
KNOWN_BUGS
MISSING_FEATURES
Makefile Remove maintainer-check target, fold into normal build 2013-10-10 20:11:56 -04:00
TODO Change documentation references to PG website to use https: not http: 2017-05-20 21:50:47 -04:00