Thread: Understanding years part of Interval

> On 06/02/2023 12:20 CET Marcos Pegoraro <marcos@f10.com.br> wrote:
>
> I was just playing with some random timestamps for a week, for a month,
> for a year ...
>
> select distinct current_date+((random()::numeric)||'month')::interval from generate_series(1,100) order by 1;
> It´s with distinct clause because if you change that 'month' for a 'year'
> it´ll return only 12 rows, instead of 100. So, why years part of interval
> works differently than any other ?
>
> select '1.01 week'::interval; --> 0 years 0 mons 7 days 1 hours 40 mins 48.00 secs
> select '1.01 month'::interval; --> 0 years 1 mons 0 days 7 hours 12 mins 0.00 secs
> select '1.01 year'::interval; --> 1 years 0 mons 0 days 0 hours 0 mins 0.00 secs

Explained in https://www.postgresql.org/docs/15/datatype-datetime.html#DATATYPE-INTERVAL-INPUT:

    Field values can have fractional parts: for example, '1.5 weeks' or
    '01:02:03.45'. However, because interval internally stores only
    three integer units (months, days, microseconds), fractional units
    must be spilled to smaller units. Fractional parts of units greater
    than months are rounded to be an integer number of months, e.g.
    '1.5 years' becomes '1 year 6 mons'. Fractional parts of weeks and
    days are computed to be an integer number of days and microseconds,
    assuming 30 days per month and 24 hours per day, e.g., '1.75 months'
    becomes 1 mon 22 days 12:00:00. Only seconds will ever be shown as
    fractional on output.

    Internally interval values are stored as months, days, and
    microseconds. This is done because the number of days in a month
    varies, and a day can have 23 or 25 hours if a daylight savings time
    adjustment is involved.

--
Erik

> On 06/02/2023 18:33 CET Marcos Pegoraro <marcos@f10.com.br> wrote:
>
> Em seg., 6 de fev. de 2023 às 10:59, Erik Wienhold <ewie@ewie.name> escreveu:
> > > On 06/02/2023 12:20 CET Marcos Pegoraro <marcos@f10.com.br> wrote:
> >  >
> >  > I was just playing with some random timestamps for a week, for a month,
> >  > for a year ...
> >  >
> >  > select distinct current_date+((random()::numeric)||'month')::interval from generate_series(1,100) order by 1;
> >  > It´s with distinct clause because if you change that 'month' for a 'year'
> >  > it´ll return only 12 rows, instead of 100. So, why years part of interval
> >  > works differently than any other ?
> >  >
> >  > select '1.01 week'::interval; --> 0 years 0 mons 7 days 1 hours 40 mins 48.00 secs
> >  > select '1.01 month'::interval; --> 0 years 1 mons 0 days 7 hours 12 mins 0.00 secs
> >  > select '1.01 year'::interval; --> 1 years 0 mons 0 days 0 hours 0 mins 0.00 secs
> >
> >  Explained in https://www.postgresql.org/docs/15/datatype-datetime.html#DATATYPE-INTERVAL-INPUT:
> >
> >  Field values can have fractional parts: for example, '1.5 weeks' or
> >  '01:02:03.45'. However, because interval internally stores only
> >  three integer units (months, days, microseconds), fractional units
> >  must be spilled to smaller units. Fractional parts of units greater
> >  than months are rounded to be an integer number of months, e.g.
> >  '1.5 years' becomes '1 year 6 mons'. Fractional parts of weeks and
> >  days are computed to be an integer number of days and microseconds,
> >  assuming 30 days per month and 24 hours per day, e.g., '1.75 months'
> >  becomes 1 mon 22 days 12:00:00. Only seconds will ever be shown as
> >  fractional on output.
> >
> >  Internally interval values are stored as months, days, and
> >  microseconds. This is done because the number of days in a month
> >  varies, and a day can have 23 or 25 hours if a daylight savings time
> >  adjustment is involved.
> >
> I´ve sent this message initially to general and Erik told me it's documented,
> so it's better to hackers help me if this has an explaining why it's done that way.
>
> select '1 year'::interval = '1.05 year'::interval -->true ?
> I cannot agree that this select returns true.

The years are converted to months and the fractional month is rounded half up:

    1.05 year = 12.6 month
    => 1 year 0.6 month
    => 1 year 1 month        (after rounding)

Compare that to 12.5 months to see when the rounding occurs:

    12.5 month / 12 month
    => 1.0416... years

Plug 1.0416 and 1.0417 into the interval to observe the rounding:

    =# select '1.0416 year'::interval, '1.0417 year'::interval;
     interval |   interval
    ----------+--------------
     1 year   | 1 year 1 mon

--
Erik