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

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:          Sébastien Celles
> Email address:      s.celles@gmail.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@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

+ As you are, so once was I.  As I am, so you will be. +
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