Is it safe to use DEFAULT currval()

[Richard Huxton]

I've got a table, and I'm trying to track changes to it, but can't do it via a
primary key including a revision-number (historical reasons).

CREATE TABLE foo (
id SERIAL,
blah text,
tracking int4 DEFAULT currval('foo_id_seq')
PRIMARY KEY (id)
);

I'm actually setting the DEFAULT on tracking using ALTER TABLE after the
event.

Now - it works, but is that a design feature or just luck?

It's convenient that tracking = id of the first entry, but not vital, so I can
always use a separate sequence if needs be.

--
Richard Huxton
Archonet Ltd

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[Tom Lane]

Richard Huxton <de*@archonet.com> writes:

CREATE TABLE foo (
id SERIAL,
blah text,
tracking int4 DEFAULT currval('foo_id_seq')
PRIMARY KEY (id)
); Now - it works, but is that a design feature or just luck?

You're essentially assuming that the column expressions for an INSERT
will be evaluated left-to-right. That's true at the moment, and I don't
see any foreseeable reason why we'd change it, but it surely is an
implementation dependency that could bite you someday.

regards, tom lane

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[Richard Huxton]

On Saturday 27 September 2003 18:40, Tom Lane wrote:

Richard Huxton <de*@archonet.com> writes:

CREATE TABLE foo (
id SERIAL,
blah text,
tracking int4 DEFAULT currval('foo_id_seq')
PRIMARY KEY (id)
);

Now - it works, but is that a design feature or just luck?

You're essentially assuming that the column expressions for an INSERT
will be evaluated left-to-right. That's true at the moment, and I don't
see any foreseeable reason why we'd change it, but it surely is an
implementation dependency that could bite you someday.

I feared as much. Ah well, use nextval(), a different sequence or write a
trigger I suppose.

--
Richard Huxton
Archonet Ltd

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