Re: Support for jsonpath .datetime() method

On 23.07.2019 16:44, Peter Eisentraut wrote:

> I think the best way forward here is to focus first on patch 0002 and
> get the additional format templates in, independent of any surrounding
> JSON functionality.
>
> In particular, remove parse_datetime() and all the related API changes,
> then it becomes much simpler.
>
> The codes FF1..FF6 that you added appear to be correct, but reading the
> spec I find there is more missing, specifically
>
> - RRRR and RR

It seems that our YY works like RR should:

SELECT to_date('69', 'YY');
   to_date
------------
  2069-01-01
(1 row)

SELECT to_date('70', 'YY');
   to_date
------------
  1970-01-01
(1 row)

But by the standard first two digits of current year should be used in YY.


Oracle follows the standard but its implementation has the different
rounding algorithm:

SELECT TO_CHAR(TO_DATE('99', 'YY'), 'YYYY') from dual;
2099

SELECT TO_CHAR(TO_DATE('49', 'RR'), 'YYYY') from dual;
2049

SELECT TO_CHAR(TO_DATE('50', 'RR'), 'YYYY') from dual;
1950


So it's unclear what we should do:
  - implement YY and RR strictly following the standard only in .datetime()
  - fix YY implementation in to_date()/to_timestamp() and implement RR
  - use our non-standard templates in .datetime()

> - SSSSS (currently only SSSS is supported, but that's not standard)

SSSSS template can be easily added as alias to SSSS.

> Also in some cases we allow timestamps with seven digits of fractional
> precision, so perhaps FF7 should be supported as well.  I'm not quite
> sure about the details here.  You tests only cover 6 and 9 digits.  It
> would be good to cover 7 and perhaps 8 as well, since those are the
> boundary cases.

FF7-FF9 weer present in earlier versions of the jsonpath patches, but they
had been removed (see [1]) because they were not completely supported due
to the limited precision of timestamp.

> Some concrete pieces of review:
>
> +       <row>
> +        <entry><literal>FF1</literal></entry>
> +        <entry>decisecond (0-9)</entry>
> +       </row>
>
> Let's not use such weird terms as "deciseconds".  We could say
> "fractional seconds, 1 digit" etc. or something like that.
And what about "tenths of seconds", "hundredths of seconds"?
> +/* Return flags for DCH_from_char() */
> +#define DCH_DATED  0x01
> +#define DCH_TIMED  0x02
> +#define DCH_ZONED  0x04
>
> I think you mean do_to_timestamp() here.  These terms "dated" etc. are
> from the SQL standard text, but they should be explained somewhere for
> the readers of the code.

[1] 
https://www.postgresql.org/message-id/885de241-5a51-29c8-a6b3-f1dda22aba13%40postgrespro.ru

-- 
Nikita Glukhov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company