how to create a column constant in Postgresql - postgresql

Question

create table test(
t_id SERIAL primary key,
t_date CONSTANT date default CURRENT_DATE
);
ERROR: syntax error at or near "date"
LINE 3: t_date CONSTANT date default CURRENT_DATE
^
********** Error **********
ERROR: syntax error at or near "date"
SQL state: 42601

Answer 1

For a default value you can use a function,
CREATE TABLE test(
t_id SERIAL primary key,
t_date date DEFAULT now()
);
about constant, I never used, even other SQL (!), only in a PL/SQL context ...
If you need a "no update" constraint, you can use a trigger. Example:
CREATE FUNCTION correct_update() RETURNS trigger AS $$
BEGIN
NEW.t_date=OLD.t_date;
RETURN NEW;
END
$$ LANGUAGE plpgsql;
CREATE TRIGGER no_date_update
BEFORE BEFORE UPDATE ON test
FOR EACH ROW
WHEN (OLD.t_date IS DISTINCT FROM NEW.t_date)
EXECUTE PROCEDURE correct_update();
For a complete control, you need also trigg the INSERT event, (and does not need a default value anymore because insert trigger will do):
create table test(
t_id SERIAL primary key,
t_date date -- a default will be redundant
);
CREATE FUNCTION correct_date() RETURNS trigger AS $$
BEGIN
IF TG_OP = 'INSERT' THEN
NEW.t_date=now(); -- default value
ELSIF TG_OP = 'UPDATE' THEN -- optional AND OLD.t_date != NEW.t_date
NEW.t_date=OLD.t_date; -- "constant" behaviour
END IF;
RETURN NEW;
END
$$ LANGUAGE plpgsql;
CREATE TRIGGER constant_date
BEFORE INSERT OR UPDATE ON test
FOR EACH ROW
EXECUTE PROCEDURE correct_date();
The OLD.t_date != NEW.t_date comparison is optional, because not affects performance... But is a good practice to use it. Another way is to check in the trigger, by WHEN, but only update triggers can use OLD... So, the best create-triggers for the same correct_date() function (with no old/new comparison) are:
CREATE TRIGGER constant_date_ins
BEFORE INSERT ON test
FOR EACH ROW
EXECUTE PROCEDURE correct_date();
CREATE TRIGGER constant_date_upd
BEFORE UPDATE ON test
FOR EACH ROW
WHEN (OLD.t_date IS DISTINCT FROM NEW.t_date)
EXECUTE PROCEDURE correct_date();
Contextualizing in a scenario
As commented above in the question, there are a lack of contextualization , ex. explaining "why you think this should work and what it should do".
Scenario-1: the db-master need to block careless programmers
We can imagine a framework like CakePHP with a "created" field and a database-master that wants that this field have a "constant behaviour", preventing that careless programmers affects this "expected constraint".
That kind of scenario was used in the anwser.
Scenario-2: the project decision is to alert by error
This is the suggestion #IgorRomanchenko ...
... now here as a Wiki, you can EDIT and add new solution/example ...

Answer 2

You want a check constraint
create table test(
t_id SERIAL primary key,
t_date date default CURRENT_DATE check(t_date = current_date)
);
insert into test(t_date) values (default);
INSERT 0 1
insert into test(t_date) values ('2014-01-01');
ERROR: new row for relation "test" violates check constraint "test_t_date_check"
DETAIL: Failing row contains (2, 2014-01-01).
Or may be a foreign key constraint which allows multiple possible values and can be updated without altering the table's schema
create table date_constraint (
date_constraint date primary key
);
insert into date_constraint (date_constraint) values (current_date);
create table test(
t_id SERIAL primary key,
t_date date
default CURRENT_DATE
references date_constraint(date_constraint)
);
insert into test(t_date) values (default);
INSERT 0 1
insert into test(t_date) values ('2014-01-01');
ERROR: insert or update on table "test" violates foreign key constraint "test_t_date_fkey"
DETAIL: Key (t_date)=(2014-01-01) is not present in table "date_constraint".
http://www.postgresql.org/docs/current/static/ddl-constraints.html