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From: Celia McInnis <[email protected]>
To: Tom Lane <[email protected]>
Cc: pgsql-generallists.postgresql.org <[email protected]>
Subject: Re: is there an immutable function to switch from date to character?
Date: Wed, 24 Apr 2024 18:05:12 -0400
Message-ID: <CAGD6t7LsWi4pYPye-eY9V-=WG8FxB+AzJ-ziwNaDK9A27a_WVg@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <CAGD6t7+GykxBs=BVeg2un_rQOrjXz==pXt-UWFVTNCgeYCSB5A@mail.gmail.com>
	<[email protected]>

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 <[email protected]> wrote:

> Celia McInnis <[email protected]> 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
>

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