Re: [DOCS] to_char(): 'FM' also suppresses *trailing* zeroes - Mailing list pgsql-docs

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.