SQL: Store and query `cron` field - sql

Question

I'm building a cron-as-a-service, to allow users to input their cron expression, and do some stuff periodically.
Here's a simple version of my table:
create table users (
id serial not null primary key,
-- `text` seems the most straightforward data type, any other suggestions?
cron text not null,
constraint valid_cron_expression CHECK (cron ~* '{some_regex}'),
-- snip --
-- maybe some other fields, like what to do on each cron call
)
My backend runs a SQL query every minute. How can I query all the rows whose cron field match the current timestamp (rounded to the closest minute)?
Edit: users input the cron field as a cron expression, e.g. 5 4 * * *.
Edit 2: corrected the fact that cron time resolution is minute, not second.

Answer 1

First of all you don't need to query every second because the cron has only a one minute resolution.
Next, comparing a cron scheduler expression to a timestamp is not a trivial task.
I'm not aware of any PostgreSQL module that would be able to parse the cron expressions.
There are two options, either you write your own function to do the comparison, or else you use a external library in the programming language you are using to do the comparison outside of the Database.
Here you will find an example implementation of such a function for Oracle that could easily be ported to PostgreSQL: SQL Query to convert cron expression to date/time format
It is incomplete because it doesn't handle complex expressions like */5 or 5,10,15 for individual fields of the cron expression but this is where I would start.

Answer 2

You might need to adjust some functions to mach your SQL dialect, but this seems to work, your only entry here is the value for ETL_CRON and the output is a boolean RUN_ETL.
The example returns TRUE at every 12th minute from 3 through 59 past every hour from 2 through 22 on Monday, Tuesday, Wednesday, Thursday, and Friday.:
select
'3/12 2-22 * * 1,2,3,4,5' as etl_cron
, split_part(etl_cron,' ',1) as etl_minute
, split_part(etl_cron,' ',2) as etl_hour
, split_part(etl_cron,' ',3) as etl_daymonth
, split_part(etl_cron,' ',4) as etl_month
, split_part(etl_cron,' ',5) as etl_dayweek
, convert_timezone('Europe/Amsterdam',current_timestamp) as etl_datetime
, case
when etl_minute = '*' then true
when contains(etl_minute,'-') then minute(etl_datetime) between split_part(etl_minute,'-',1) and split_part(etl_minute,'-',2)
when contains(etl_minute,'/') then mod(minute(etl_datetime),split_part(etl_minute,'/',2)) - split_part(etl_minute,'/',1) = 0
else array_contains(minute(etl_datetime)::varchar::variant, split(etl_minute,','))
end as run_minute
, case
when etl_hour = '*' then true
when contains(etl_hour,'-') then hour(etl_datetime) between split_part(etl_hour,'-',1) and split_part(etl_hour,'-',2)
when contains(etl_hour,'/') then mod(hour(etl_datetime),split_part(etl_hour,'/',2)) - split_part(etl_hour,'/',1) = 0
else array_contains(hour(etl_datetime)::varchar::variant, split(etl_hour,','))
end as run_hour
, case
when etl_daymonth = '*' then true
when contains(etl_daymonth,'-') then day(etl_datetime) between split_part(etl_daymonth,'-',1) and split_part(etl_daymonth,'-',2)
when contains(etl_daymonth,'/') then mod(day(etl_datetime),split_part(etl_daymonth,'/',2)) - split_part(etl_daymonth,'/',1) = 0
else array_contains(day(etl_datetime)::varchar::variant, split(etl_daymonth,','))
end as run_daymonth
, case
when etl_month = '*' then true
when contains(etl_month,'-') then month(etl_datetime) between split_part(etl_month,'-',1) and split_part(etl_month,'-',2)
when contains(etl_month,'/') then mod(month(etl_datetime),split_part(etl_month,'/',2)) - split_part(etl_month,'/',1) = 0
else array_contains(month(etl_datetime)::varchar::variant, split(etl_month,','))
end as run_month
, case
when etl_dayweek = '*' then true
when contains(etl_dayweek,'-') then dayofweek(etl_datetime) between split_part(etl_dayweek,'-',1) and split_part(etl_dayweek,'-',2)
when contains(etl_dayweek,'/') then mod(dayofweek(etl_datetime),split_part(etl_dayweek,'/',2)) - split_part(etl_dayweek,'/',1) = 0
else array_contains(dayofweek(etl_datetime)::varchar::variant, split(etl_dayweek,','))
end as run_dayweek
, run_minute and run_hour and run_daymonth and run_month and run_dayweek as run_etl;

Answer 3

This addresses the original version of the question.
Assuming that cron is a timestamp of some sort, you would use:
where cron >= date_trunc('second', now()) and
cron < date_trun('second', now()) + interval '1 second'