Default for date field: today vs CURRENT_DATE

Rich Shepard <rshepard@appl-ecosys.com> writes:

Reading the manual I saw that 'today' is a special value, but it did not
work when I used it as a column default; e.g.,
start_date date DEFAULT today,
Appending parentheses also failed. But, changing today to CURRENT_DATE
worked. I've not found an explanation and would appreciate learning why
'today' fails.

'today' is special as a date input string, so you can use it as a literal:

regression=# select 'today'::date;
date
------------
2019-01-02
(1 row)

But it's not a SQL keyword, nor a function name, so you can't write it
without quotes.

Also, it wouldn't be very useful for this purpose, because it's resolved
on sight in date_in(). Thus

regression=# create table wrong_thing (start_date date DEFAULT 'today');
CREATE TABLE
regression=# \d wrong_thing
Table "public.wrong_thing"
Column | Type | Collation | Nullable | Default
------------+------+-----------+----------+--------------------
start_date | date | | | '2019-01-02'::date

The default would effectively be the creation date of the table,
not the insertion date of any particular row.

So CURRENT_DATE or one of its sibling functions is what you want
here. On the other hand, something like

INSERT INTO my_table VALUES ('today', ...);

might be perfectly sensible code.

regards, tom lane

On Wed, 2 Jan 2019, Tom Lane wrote:

'today' is special as a date input string, so you can use it as a literal:

regression=# select 'today'::date;
date
------------
2019-01-02
(1 row)

But it's not a SQL keyword, nor a function name, so you can't write it
without quotes.

Tom,

Now I understand. I tried it with quotes, too, but this was in the table
definition, not as a selection criterion. I did not pick up this difference
when reading about the date-time data types.

Thanks very much for clarifying.

Best regards,

Rich