Re: is there an immutable function to switch from date to character?

[Celia McInnis <celia.mcinnis@xxxxxxxxx>]

Sorry I pasted in the wrong code, I had wanted a column with the character version of the date (ie., YYYY-Mon-DD). Steve Baldwin's hack pointed me in the right direction. Here is the example:

create temporary table junk as select now()::date as evtdate;
SELECT 1

alter table junk add column chardate text GENERATED ALWAYS AS (cmm_date_to_char(evtdate)) STORED;

select * from junk;
  evtdate   |  chardate  
------------+-------------
 2024-04-24 | 2024-Apr-24
(1 row)

where cmm_date_to_char is defined as:

create or replace function cmm_date_to_char(i_date in date) returns text immutable language sql as $$ select to
_char(i_date, 'YYYY-Mon-DD') $$;

Thanks!

On Wed, Apr 24, 2024 at 5:54 PM Tom Lane <tgl@xxxxxxxxxxxxx> wrote:

Celia McInnis <celia.mcinnis@xxxxxxxxx> writes:
> create temporary table junk as select now()::date as evtdate;
> alter table junk add column chardate text GENERATED ALWAYS AS
> (to_char(evtdate,'YYYY-Mon-DD')) STORED;

> ERROR:  generation _expression_ is not immutable

Probably not; I think all the available conversion functions
respond to some combination of datestyle, lc_time, and timezone
settings.  (Type date doesn't depend on timezone, but that keeps you
from using anything that shares functionality with timestamptz ...
and your to_char call promotes the date to timestamptz.)

I find your example not terribly compelling.  Why expend storage
space on such a column?

If you're bound and determined to do it, writing a wrapper
function that's labeled immutable should work:

=# create function mytochar(date) returns text
strict immutable parallel safe
as $$ begin return to_char($1::timestamp, 'YYYY-Mon-DD'); end $$
language plpgsql;
CREATE FUNCTION
=# alter table junk add column chardate text GENERATED ALWAYS AS
(mytochar(evtdate)) STORED;
ALTER TABLE

It's on you to be sure that the function actually is immutable,
or at least immutable enough for your use-case.  I believe my
example is pretty safe: neither datestyle nor timezone should
affect the timestamp-without-timezone variant of to_char(),
and this particular format string doesn't depend on lc_time.

                        regards, tom lane