PostgresSQL get last 20 rows and then get max value


I have a table that I need to list the last 20 rows from end_time and then base off that get the max value of the column run_time.


select run_time_sec,end_time FROM runinfo_history
where sched_table = 'TEST_TIMES'
  and job_mem_name = 'TEST_TIMES'
  and data_center = 'DEV'
order BY end_time DESC LIMIT 20

gets the last 20

run_time_sec     end_time            
 ---------------  --------------------
 4                8/19/2014 6:25:39 PM
 4                8/19/2014 6:25:07 PM
 4                8/19/2014 6:24:29 PM
 3                8/19/2014 6:23:40 PM
 3                8/19/2014 6:23:11 PM
 3                8/19/2014 6:23:01 PM
 4                8/19/2014 6:22:51 PM
 3                8/19/2014 6:22:41 PM
 4                8/19/2014 6:22:32 PM
 4                8/19/2014 6:22:21 PM
 4                8/19/2014 6:21:32 PM
 4                8/19/2014 6:21:22 PM
 4                8/19/2014 6:21:12 PM
 4                8/19/2014 6:20:52 PM
 4                8/19/2014 6:20:42 PM
 4                8/19/2014 6:20:32 PM
 3                8/19/2014 6:20:22 PM
 3                8/19/2014 6:20:02 PM
 3                8/19/2014 6:19:50 PM
 60               8/19/2014 6:11:13 PM

I do not need the column end_time. Its just hear as to show what it is but used to get the last 20 dates from that column.

Not sure who to combine to get max from the list.


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Phil PhillipsDirector of DevOps & Quality AssuranceCommented:
You can get the max run_time_sec from your query with something like:

select max(run_time_sec) from (
  select run_time_sec FROM runinfo_history
    where sched_table = 'TEST_TIMES'
      and job_mem_name = 'TEST_TIMES'
      and data_center = 'DEV'
      order BY end_time DESC LIMIT 20
) as t1;

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