Обсуждение: Clarify: default precision on timestamps is 6

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/16/datatype-datetime.html
Description:

https://www.postgresql.org/docs/current/datatype-datetime.html says:

> time, timestamp, and interval accept an optional precision value p which
specifies the number of fractional digits retained in the seconds field. By
default, there is no explicit bound on precision. The allowed range of p is
from 0 to 6.

I think it would be more accurate to say "By default, the bound on precision
is 6 (that is, microsecond precision). The allowed range of p is from 0 to
6. Values with greater precision than the type allows will be rounded to its
maximum precision, and values with less precision than the type allows will
be stored as given."

The following experiment confirms that 'timestamp with time zone' behaves
exactly like 'timestamp(6) with time zone'; both of them round any precision
beyond microseconds, and neither returns timestamps with greater precision
than the value that was inserted.

```sql
# CREATE TABLE tstest (a timestamp with time zone, b timestamp(6) with time
zone, c timestamp(2) with time zone);
CREATE TABLE
# \d tstest
                         Table "public.tstest"
 Column |            Type             | Collation | Nullable | Default
--------+-----------------------------+-----------+----------+---------
 a      | timestamp with time zone    |           |          |
 b      | timestamp(6) with time zone |           |          |
 c      | timestamp(2) with time zone |           |          |

-- microsecond precision
# INSERT INTO tstest (a, b, c) VALUES ('2024-01-01 00:00:00.123456Z',
'2024-01-01 00:00:00.123456Z', '2024-01-01 00:00:00.123456Z');
INSERT 0 1

-- nanosecond precision
# INSERT INTO tstest (a, b, c) VALUES ('2024-01-01 00:00:00.123456789Z',
'2024-01-01 00:00:00.123456789Z', '2024-01-01 00:00:00.123456789Z');
INSERT 0 1

-- millisecond precision
# INSERT INTO tstest (a, b, c) VALUES ('2024-01-01 00:00:00.123Z',
'2024-01-01 00:00:00.123Z', '2024-01-01 00:00:00.123Z');
INSERT 0 1

-- second precision
# INSERT INTO tstest (a, b, c) VALUES ('2024-01-01 00:00:00Z', '2024-01-01
00:00:00Z', '2024-01-01 00:00:00Z');
INSERT 0 1

--- results
# SELECT * FROM tstest;
               a               |               b               |
c
-------------------------------+-------------------------------+---------------------------
 2024-01-01 00:00:00.123456+00 | 2024-01-01 00:00:00.123456+00 | 2024-01-01
00:00:00.12+00
 2024-01-01 00:00:00.123457+00 | 2024-01-01 00:00:00.123457+00 | 2024-01-01
00:00:00.12+00
 2024-01-01 00:00:00.123+00    | 2024-01-01 00:00:00.123+00    | 2024-01-01
00:00:00.12+00
 2024-01-01 00:00:00+00        | 2024-01-01 00:00:00+00        | 2024-01-01
00:00:00+00
(4 rows)
```

both of them round any precision
beyond microseconds, and neither returns timestamps with greater precision
than the value that was inserted.

Kirk Parker <khp@equatoria.us> writes:
> On Fri, Oct 13, 2023 at 7:32 AM David G. Johnston <
> david.g.johnston@gmail.com> wrote:
>> That is precisely what a no default with maximum of six means.  If we say
>> the default is six that would imply storage of less precise values pads
>> significant zeros until there are six.

> Not sure that last statement is correct.  In 13 (only system I have access
> to at the moment) it doesn't look like casting to a precision greater than
> the value originally had causes any padding:

The timestamp types don't have any explicit notion of precision
(unlike, say, numeric).  The stored value is an integer number of
microseconds, nothing else.  I've not checked the output function
recently but it makes sense to me that it'd just drop trailing
zeroes from the display, independently of any claimed precision
for the column.  Meanwhile, when casting a timestamp value to
a declared precision, we handle that by just rounding off the
microseconds count.  This doesn't buy any space savings or anything
like that, it's just for pro-forma compliance with the spec.

I don't see anything particularly wrong with the existing docs.
The limitation to 1-microsecond precision is spelled out in the
table just above the para you quote.

            regards, tom lane