Re: to_char(): 'FM' also suppresses *trailing* zeroes

On 28 August 2017 at 22:37, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Erwin Brandstetter <brsaweda(at)gmail(dot)com> writes:
> > In Table 9-27. "Template Pattern Modifiers for Numeric Formatting" it
> says:
> > FM | prefix fill mode (suppress leading zeroes and padding blanks) |
> FM9999
>
> > In fact, 'FM' also suppresses *trailing* zeroes after the comma. To fix,
> > this might be changed to:
> > suppress insignificant zeroes and padding blanks
>
> Not necessarily. A bit of experimentation says that it also matters
> whether you use "0" or "9" as the format character:
>
> So it appears to me that the bit you point out is flat out backwards;
> what FM actually suppresses is trailing zeroes not leading zeroes.
>
> I'm tempted to propose that in table 9-26, we need to write
>
> 9 digit position (can be dropped if insignificant)
>
> 0 digit position (cannot be dropped, even if insignificant)
>
> and then in 9-27 say
>
> FM fill mode: suppress trailing zeroes and padding spaces
>
> Also, in between those two tables, I see
>
> * 9 results in a value with the same number of digits as there are
> 9s. If a digit is not available it outputs a space.
>
> This seems outright wrong per the above examples, and in any case is not
> very useful since it doesn't explain the difference from "0". Perhaps
> rewrite as
>
> * 0 specifies a digit position that will always be printed,
> even if it contains a leading/trailing zero. 9 also specifies
> a digit position, but if it is a leading zero then it will be
> replaced by a space, while if it is a trailing zero and fill mode
> is specified then it will be deleted.

Leading zeros are removed completely for 9 with FM. And without FM they are
actually replaced with spaces before the sign. Maybe:

* 0 specifies a digit position that will always be printed, even as
insignificant zero.
9 also specifies a digit position, but leading zeros are replaced with
spaces before the optional sign. And if fill mode is specified (FM
modifier) insignificant leading and trailing zeros are removed completely.

I created a more comprehensive test matrix for to_char(numeric, text) to
aid in a quick review of the *FM* modifier:

CREATE TEMP TABLE t(t_id int, template text);
INSERT INTO t(t_id, template)
VALUES
( 1, '00.00')
, ( 2, '09.90')
, ( 3, '90.09')
, ( 4, '99.99')
, ( 5, '090.090')
, ( 6, '909.909')
, (11, 'FM00.00')
, (12, 'FM09.90')
, (13, 'FM90.09')
, (14, 'FM99.99')
, (15, 'FM090.090')
, (16, 'FM909.909');

CREATE TEMP TABLE v (v_id int, val numeric);
INSERT INTO v
VALUES
(1, -0.1 )
, (2, 0.12 )
, (3, -0.123)
, (4, -1.1 )
, (5, 12.12 )
, (6, -123.123);

SELECT val, template, to_char(val, template)
FROM v, t
ORDER BY v_id, t_id;

To test online:
http://dbfiddle.uk/?rdbms=postgres_10&fiddle=cfb98de7ea2f0e0fbe8205765fcb4a
d0
<http://dbfiddle.uk/?rdbms=postgres_10&fiddle=cfb98de7ea2f0e0fbe8205765fcb4ad0%29*>

I think the root of the confusion is that the FM does for date/time
formatting what the manual says (table 9-25):

FM | prefix fill mode (suppress leading zeroes and padding blanks) | FMMonth
>

test=# SELECT to_char(timestamp '0910-09-03 01:00:03.04', template)
test-# FROM (
test(# VALUES
test(# ('YYYY-MM-DD HH24:MI:SS.US')
test(# , ('FMYYYY-MM-DD HH24:MI:SS.US')
test(# , ('FMYYYY-MM-DD FMHH24:MI:SS.US')
test(# , ('FMYYYY-FMMM-FMDD FMHH24:FMMI:FMSS.FMUS')
test(# ) t(template);
to_char
----------------------------
0910-09-03 01:00:03.040000
910-09-03 01:00:03.040000
910-09-03 1:00:03.040000
910-9-3 1:0:3.040000

Note how trailing zeros are not suppressed for microseconds. (It would seem
to make more sense to suppress those, though.)

The same explanation is given for numeric formatting (table 9-27):

FM prefix | fill mode (suppress leading zeroes and padding blanks) | FM9999
>

But it does not apply there - like you pointed out. The exact behavior
seems to be:

For the pattern character 0 leading and trailing zeroes are always printed,
no matter what.
For the pattern character 9 ...
without FM modifier
leading zeros are replaced with padding blanks (before the sign if
it's there).
trailing zeros after the decimal point are printed.
with FM modifier
leading and trailing zeros are removed (unless overruled by a 0).
There is additional blank padding for the sign if not printed - also
removed with FM.

Regards
Erwin