Thread: Help with writing a generate_series(tsmultirange, interval)

Hello all!

I’m excited for multi ranges, as they fit nicely into a scheduling app. What I’m trying to express is something along
thelines of « Every weekday from 2021-08-01 and 2021-10-01, from 9 AM to 8 PM, every 90 minutes ». You can think of
publictransit for the model. 

Initially, I was going to create a table with every departure recorded, and was going to refresh the table on every
writeto the parent table, but that means maintaining many rows for every change to the schedule. Then, I remembered
multiranges in PG14, and they fit nicely with what I had in mind. 

Now that I can store the info I want, I’d like to iterate over the ranges, so I can generate the exact departure times,
somethingsimilar to this: 

SELECT instant
FROM generate_series(
    '{["2021-08-02 08:00:00","2021-08-02 11:00:00"),["2021-08-03 08:00:00","2021-08-03 20:00:00"]’}
  , interval ’90 minutes’) as instant;

2021-08-02 08:00
2021-08-02 09:30
— 2021-08-02 11:00 excluded as the range excludes its upper bound
2021-08-03 08:00:00
2021-08-03 09:30:00
2021-08-03 11:00:00
2021-08-03 12:30:00
2021-08-03 14:00:00
2021-08-03 15:30:00
2021-08-03 17:00:00
2021-08-03 18:30:00
2021-08-03 20:00:00 — included, as the upper bound is inclusive

That function doesn’t exist, and I can’t seem to find a function to iterate over a multi range either. Does such a
functionexist? I’m specifically looking at https://www.postgresql.org/docs/14/functions-range.html. 

This is a toy application, a spike to see what’s possible.

I wanted to avoid maintaining a table with hundreds of rows per route, if a route runs frequently enough (every 10
minutes,8 AM to 8 PM, over a year). Of course, I can avoid refreshing the departures table if the schedule hasn’t
changed,but still, preparing this table will not take a constant amount of time; e.g. it will depend on the schedule’s
size.

Any tips appreciated!
François

On 7/31/21 11:59 AM, François Beausoleil wrote:
> Hello all!
> 
> I’m excited for multi ranges, as they fit nicely into a scheduling app. What I’m trying to express is something along
thelines of « Every weekday from 2021-08-01 and 2021-10-01, from 9 AM to 8 PM, every 90 minutes ». You can think of
publictransit for the model.
 
> 
> Initially, I was going to create a table with every departure recorded, and was going to refresh the table on every
writeto the parent table, but that means maintaining many rows for every change to the schedule. Then, I remembered
multiranges in PG14, and they fit nicely with what I had in mind.
 
> 
> Now that I can store the info I want, I’d like to iterate over the ranges, so I can generate the exact departure
times,something similar to this:
 
> 
> SELECT instant
> FROM generate_series(
>      '{["2021-08-02 08:00:00","2021-08-02 11:00:00"),["2021-08-03 08:00:00","2021-08-03 20:00:00"]’}
>    , interval ’90 minutes’) as instant;
> 
> 2021-08-02 08:00
> 2021-08-02 09:30
> — 2021-08-02 11:00 excluded as the range excludes its upper bound
> 2021-08-03 08:00:00
> 2021-08-03 09:30:00
> 2021-08-03 11:00:00
> 2021-08-03 12:30:00
> 2021-08-03 14:00:00
> 2021-08-03 15:30:00
> 2021-08-03 17:00:00
> 2021-08-03 18:30:00
> 2021-08-03 20:00:00 — included, as the upper bound is inclusive
> 
> That function doesn’t exist, and I can’t seem to find a function to iterate over a multi range either. Does such a
functionexist? I’m specifically looking at https://www.postgresql.org/docs/14/functions-range.html.
 
> 
> This is a toy application, a spike to see what’s possible.
> 
> I wanted to avoid maintaining a table with hundreds of rows per route, if a route runs frequently enough (every 10
minutes,8 AM to 8 PM, over a year). Of course, I can avoid refreshing the departures table if the schedule hasn’t
changed,but still, preparing this table will not take a constant amount of time; e.g. it will depend on the schedule’s
size.
> 
> Any tips appreciated!

How about:

SELECT
     *
FROM
     generate_series ('2021-08-02 08:00:00'::timestamp, '2021-08-02 
10:59:00'::timestamp, interval '90 minutes') AS instant
UNION
SELECT
     *
FROM
     generate_series ('2021-08-03 08:00:00'::timestamp, '2021-08-03 
20:00:00'::timestamp, interval '90 minutes') AS instant
ORDER BY instant;

  instant
---------------------
  2021-08-02 08:00:00
  2021-08-02 09:30:00
  2021-08-03 08:00:00
  2021-08-03 09:30:00
  2021-08-03 11:00:00
  2021-08-03 12:30:00
  2021-08-03 14:00:00
  2021-08-03 15:30:00
  2021-08-03 17:00:00
  2021-08-03 18:30:00
  2021-08-03 20:00:00

> François
> 
> 
> 


-- 
Adrian Klaver
adrian.klaver@aklaver.com

On 7/31/21 5:16 PM, François Beausoleil wrote:
> Hello Adrian,
> 

> Yes, in fact, I wrote the following:
> 
>
--------------------------------------------------------------------------------------------------------------------------
> 
> CREATE FUNCTION generate_series(tstzrange, interval) RETURNS SETOF 
> timestamp with time zone AS $$
>    SELECT n
>    FROM generate_series(lower($1), upper($1), $2) AS t0(n)
>    WHERE $1 @> n
> $$ LANGUAGE sql immutable;
> 
> CREATE FUNCTION generate_series(tsrange, interval) RETURNS SETOF 
> timestamp without time zone AS $$
>    SELECT n
>    FROM generate_series(lower($1), upper($1), $2) AS t0(n)
>    WHERE $1 @> n
> $$ LANGUAGE sql immutable;
> 
> That was the easy part. My end goal is to iterate over a tsmultirange: I 
> would like to get each individual range from a given multi range. 
> Ideally, I’d like to do that without parsing the textual version of the 
> multi range.
> 
> While mowing the lawn, I thought that since the syntax of multi ranges 
> is similar to arrays, maybe I could use unnest(), but sadly, that was 
> not to be the case:
> 
> # select 
> unnest('{[2021-08-02,2021-08-04],[2021-08-07,2021-08-09)}'::tsmultirange);
> ERROR:  function unnest(tsmultirange) does not exist
> LINE 1: select unnest('{[2021-08-02,2021-08-04],[2021-08-07,2021-08-...
> 
> Apparently, PG can accept multi range values, but can’t do much with 
> them at the time, except to check for inclusion/exclusion.

I see your mowing the lawn and raise walking the dog. This rang some 
bells and then I remembered when in doubt consult depesz:

https://www.depesz.com/2021/07/15/how-to-get-list-of-elements-from-multiranges/

https://www.postgresql.org/message-id/20210715121508.GA30348@depesz.com

> 
> Thanks for your time!
> François
> 
>>> François
>>
>>
>> --
>> Adrian Klaver
>> adrian.klaver@aklaver.com <mailto:adrian.klaver@aklaver.com>
> 


-- 
Adrian Klaver
adrian.klaver@aklaver.com

=?utf-8?Q?Fran=C3=A7ois_Beausoleil?= <francois@teksol.info> writes:
> While mowing the lawn, I thought that since the syntax of multi ranges is similar to arrays, maybe I could use
unnest(),but sadly, that was not to be the case: 
> # select unnest('{[2021-08-02,2021-08-04],[2021-08-07,2021-08-09)}'::tsmultirange);
> ERROR:  function unnest(tsmultirange) does not exist

That's fixed for beta3:

regression=# select unnest('{[2021-08-02,2021-08-04],[2021-08-07,2021-08-09)}'::tsmultirange);
                    unnest
-----------------------------------------------
 ["2021-08-02 00:00:00","2021-08-04 00:00:00"]
 ["2021-08-07 00:00:00","2021-08-09 00:00:00")
(2 rows)


            regards, tom lane

> On 1 Aug 2021, at 3:30, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> =?utf-8?Q?Fran=C3=A7ois_Beausoleil?= <francois@teksol.info> writes:
>> While mowing the lawn, I thought that since the syntax of multi ranges is similar to arrays, maybe I could use
unnest(),but sadly, that was not to be the case: 
>> # select unnest('{[2021-08-02,2021-08-04],[2021-08-07,2021-08-09)}'::tsmultirange);
>> ERROR:  function unnest(tsmultirange) does not exist
>
> That's fixed for beta3:
>
> regression=# select unnest('{[2021-08-02,2021-08-04],[2021-08-07,2021-08-09)}'::tsmultirange);
>                    unnest
> -----------------------------------------------
> ["2021-08-02 00:00:00","2021-08-04 00:00:00"]
> ["2021-08-07 00:00:00","2021-08-09 00:00:00")
> (2 rows)
>
>
>             regards, tom lane

If what you need is behaving similar to arrays, perhaps arrays of ranges suit your problem?

development=> select unnest(array['[2021-08-02,2021-08-04]'::tsrange,'[2021-08-07,2021-08-09)'::tsrange]);
                    unnest
-----------------------------------------------
 ["2021-08-02 00:00:00","2021-08-04 00:00:00"]
 ["2021-08-07 00:00:00","2021-08-09 00:00:00")
(2 rows)

The drawback of that approach is probably with the operators and functions you have to your avail. It seems to me
thoughthat several of those not available for arrays could be emulated using array functions such as array_position(…)
forthe contains operator, unnest with tsrange functions for others, etc. 


Another approach could be to store the “rules” of the schedule and generate the relevant portion of the multirange as a
setof tsrange rows on-the-fly. That may well perform better than storing the entire range in a table of tsrange
records.

I’ve done something like that for a hierarchical query on versioned items where I had to base how to slice through the
hierarchyon a reference timestamp. That performed adequately on a production data warehouse, as long as you
sufficientlyconstrained the inputs. You can join such a function (laterally) to some other data set too. 

Regards,

Alban Hertroys
--
There is always an exception to always.