Re: Passing a dynamic interval to generate_series()

On Sun, Jun 30, 2024 at 3:51 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Igal Sapir <igal(at)lucee(dot)org> writes:
> > But this throws an error (SQL Error [42601]: ERROR: syntax error at or
> near
> > "'1 '"):
>
> > SELECT generate_series(
> > date_trunc('month', current_date),
> > date_trunc('month', current_date + interval '7 month'),
> > interval ('1 ' || 'month')::interval
> > )
>
> You're overthinking it.
>
> SELECT generate_series(
> date_trunc('month', current_date),
> date_trunc('month', current_date + interval '7 month'),
> ('1 ' || 'month')::interval
> );
> generate_series
> ------------------------
> 2024-06-01 00:00:00-04
> 2024-07-01 00:00:00-04
> 2024-08-01 00:00:00-04
> 2024-09-01 00:00:00-04
> 2024-10-01 00:00:00-04
> 2024-11-01 00:00:00-04
> 2024-12-01 00:00:00-05
> 2025-01-01 00:00:00-05
> (8 rows)
>

Thank you, Tom. I thought that I tried that too, but apparently I did not
because it works the way you wrote it.

>
> It might help to read this:
>
>
> https://www.postgresql.org/docs/current/sql-syntax-lexical.html#SQL-SYNTAX-CONSTANTS-GENERIC
>
> and to experiment with what you get from the constituent elements
> of what you tried, rather than trying to guess what they are from
> generate_series's behavior. For example,
>
> select (interval '1 ');
> interval
> ----------
> 00:00:01
> (1 row)
>
> select (interval '1 ' || 'month');
> ?column?
> ---------------
> 00:00:01month
> (1 row)
>

I actually did test the expression that I posted, but it might be casting
it twice. While your examples that you wrote show 1 month correctly:

SELECT (interval '1 ' || 'month');

?column? |
-------------+
00:00:01month|

SELECT ('1 ' || 'month')::interval;

interval|
--------+
1 mon|

When the expression includes the "::interval" suffix as in the example that
I posted it returns 1 second, possibly because it is casting to interval
twice (at least on PostgreSQL 16.2 (Debian 16.2-1.pgdg120+2)):

SELECT (interval '1 ' || 'month')::interval;

interval|
--------+
00:00:01|

Anyway, you solved my issue, so thank you very much as always,

Igal

>
> regards, tom lane
>