Re: Missing documentation for uses of extract(field from interval)

I have added documentation for EXTRACT(DAY) usage with intervals. We
had something for MONTH, but for some reason nothing for DAY. Applied
patch attached.

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Daniel Grace wrote:
> (Note: I'm not subscribed to pgsql-docs, so please CC me on any
> replies. Thanks!)
>
> I pulled up the documentation today to see if I could get just the
> number of days in an interval using EXTRACT. The answer provided was
> vague: While you can use EXTRACT(field from interval) (shown in table
> 9-27), section 9.9.1 only lists:
>
> day
>
> The day (of the month) field (1 - 31)
>
> SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40');
> Result: 16
>
>
>
> It turns out that EXTRACT(day FROM interval) does work as expected --
> so perhaps the documentation should be updated to something like:
>
> day
>
> The day (of the month) field (1 - 31), or the number of days in the interval
>
> SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40');
> Result: 16
>
> SELECT EXTRACT(DAY FROM INTERVAL '42 days 21:00:00');
> Result: 42
>
> Likewise for month, year, century and maybe some others. (hour,
> minute, and *second{s?} probably don't need updated text).
>
> There also should probably be some warning in here about EXTRACT on
> intervals however. Namely cases like this:
>
> SELECT EXTRACT(YEAR FROM INTERVAL '400 days') --> Result: 0
> (because the interval is '400 days' not '1 year 35 days')
> SELECT EXTRACT(DAY FROM INTERVAL '1 year') --> Result: 0 (because
> the interval is '1 year' not '365 days')
> SELECT EXTRACT(DAY FROM INTERVAL '1 month') --> Result: 0
>
> Though, this works:
> SELECT EXTRACT(CENTURY FROM INTERVAL '99 years') --> Result: 0
> SELECT EXTRACT(CENTURY FROM INTERVAL '100 years') --> Result: 1
> --
> Daniel Grace
> AGE, LLC
> System Administrator and Software Developer
>
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--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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