SQL Query to convert cron expression to date/time format

Question

I have some cron jobs scheduled on a Linux host. Each job writes execution log in an Oracle database table. The table has a result column that can be valued with 'OK' or 'KO'. The table also has a timestamp column that is valued with the last execution time for the job. In this way I'm sure about the job result. Now I need to be sure that the job has actually ran. I have another table with job ID and cron expression, for example:

JOB_ID        SCHEDULE
102           00 09 * * *

How can I write a sql select to check the last execution time against the SCHEDULE field (cron expression)?

I will appreciate any suggestion, I can change the my approach but I would like to use the cron syntax inside the database table

Regards Giova

Answer 1

You can use the regexp_substr function to extract individual elements from the schedule tuple. For example regexp_substr(schedule, '[0-9*]+', 1,1) will extract the minute element, while regexp_substr(sched, '[0-9*]+', 1, 2) will extract the hour element. The 4th parameter selects the desired element. You can then use either the EXTRACT or TO_CHAR function to get at various portions of your timestamp for comparison.

with cron(ID, Sched) as (
  select 102, '00 9 * * * *' from dual
), exec(id, ts) as (
  select 102, to_timestamp('2017-11-05 9:00:00', 'yyyy-mm-dd hh24:mi:ss') from dual union all
  select 102, to_timestamp('2017-11-05 9:05:00', 'yyyy-mm-dd hh24:mi:ss') from dual
), c2 as (
select id
     , sched 
     , regexp_substr(sched,'[0-9*]+',1,1) min
     , regexp_substr(sched,'[0-9*]+',1,2) hour
     , regexp_substr(sched,'[0-9*]+',1,3) day
     , regexp_substr(sched,'[0-9*]+',1,4) mon
     , regexp_substr(sched,'[0-9*]+',1,5) wday
     , regexp_substr(sched,'[0-9*]+',1,6) year
  from cron
)
select c2.*
     , exec.ts
     , case when (year = '*' or to_number(to_char(ts,'yyyy')) = to_number(year))
             and (mon  = '*' or to_number(to_char(ts,'mm')  ) = to_number(mon ))
             and (day  = '*' or to_number(to_char(ts,'dd')  ) = to_number(day ))
             and (hour = '*' or to_number(to_char(ts,'hh24')) = to_number(hour))
             and (min  = '*' or to_number(to_char(ts,'mi')  ) = to_number(min ))
             and (wday = '*' or to_number(to_char(ts,'d')   ) = to_number(wday))
            then 'OK'
            else 'KO'
       end Match
  from exec 
  join c2 
    on c2.id = exec.id;

move the logical expression from the case statement in part or whole as needed to get the results you need.