Re: BUG #15698: to_char doesn't return expected value with negative INTERVAL

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: s(dot)celles(at)gmail(dot)com, pgsql-bugs(at)lists(dot)postgresql(dot)org
Subject: Re: BUG #15698: to_char doesn't return expected value with negative INTERVAL
Date: 2019-04-09 17:23:22
Message-ID: 20190409172322.qh4sz6ioxuva23ri@momjian.us
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On Sat, Mar 16, 2019 at 08:11:19PM +0000, PG Bug reporting form wrote:
> The following bug has been logged on the website:
>
> Bug reference: 15698
> Logged by: S├ębastien Celles
> Email address: s(dot)celles(at)gmail(dot)com
> PostgreSQL version: 10.5
> Operating system: Windows 10
> Description:
>
> Hello,
>
> This is my first bug report here (despite I'm using PostgreSQL since many
> years !)
>
> I don't know if it's a bug... but the following query
>
> SELECT to_char(-(3 * 60 * 60 * 1000 + 7*60 * 1000 + 12345) * INTERVAL '1
> millisecond', 'HH24:MI:SS.MS') as interv;
>
> doesn't return the result I was expecting.
>
> It returns:
>
> '-03:-07:-12.-345'
>
> I was expecting
>
> '-03:07:12.345'
>
> If it's not a bug (but a feature ;-) )... is there a way to return result as
> I was expecting.

I am sorry for my delay in replying.

Yes, I agree the current output looks odd. You would think that
to_char() could just roll the sign up to a single mention, but intervals
store values in three parts:

typedef struct
{
TimeOffset time; /* all time units other than days, months and
* years */
int32 day; /* days, after time for alignment */
int32 month; /* months and years, after time for alignment */
} Interval;

Those parts can have different signs. Here is an example:

SELECT to_char('-1 month 2 days -3 hours'::interval, 'MM DD HH') AS interv;
interv
------------
-01 02 -03

Therefore, we output _all_ units with separate signs. (I don't know how
I would pass a single negative value into to_char() for
timestamp/timestamptz.)

The values only cross the three unit boundaries when we call "justify"
functions:

List of functions
Schema | Name | Result data type | Argument data types | Type
------------+------------------+------------------+---------------------+------
pg_catalog | justify_days | interval | interval | func
pg_catalog | justify_hours | interval | interval | func
pg_catalog | justify_interval | interval | interval | func

Here is a psql query that optionally outputs the negative sign of your
calculation, and then passes the absolute value to to_char():

\set var -(3 * 60 * 60 * 1000 + 7*60 * 1000 + 12345)

SELECT CASE WHEN :var < 0 THEN '-' END ||
to_char(abs(:var) * INTERVAL '1 millisecond',
'HH24:MI:SS.MS') as interv;
interv
---------------
-03:07:12.345

I hope this helps.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ As you are, so once was I. As I am, so you will be. +
+ Ancient Roman grave inscription +

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