RE: Group by wrong results?

Hmm.  

with session_count as

(

  select sample_time, count(1) sess_count, min(trunc(sample_time,’MI’) sample_minute

     from dba_hist_active_sess_history

    where sample_time > trunc(sysdate-(1/24))

    group by sample_time

)

select sample_minute, max(sess_count) max_sessions

   from session_count

  group by sample_minute

  order by sample_minute  

If I understand, you want at most a single time row per minute with that row being the sample with the most sessions (the high sampled session_count in each minute sampled).  

I don’t believe your code is WRONG, but I’m curious whether calculating the minute in the initial view gets you the correct answer. I just typed this in, so it’s possible I fubar’d the syntax.  

mwf  

From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Daniel Fink (Redacted sender "daniel.fink" for DMARC) Sent: Tuesday, August 07, 2018 12:26 PM
To: oracle-l_at_freelists.org
Subject: Group by wrong results?  

A script to extract session count from ash data is not returning the right results using a TO_CHAR(sample_time, 'YYYYMMDD hh24:mi') group by unless I use a factored subquery.  

Query 1

WITH session_count AS

( SELECT sample_time, count(1) sess_count

  FROM dba_hist_active_sess_history

  WHERE sample_time >= TRUNC(sysdate - (1/24))

  GROUP BY sample_time

)

SELECT TO_CHAR(sample_time, 'YYYYMMDD hh24:mi') sample_minute,

       MAX(sess_count) max_sessions

FROM session_count

GROUP BY TO_CHAR(sample_time, 'YYYYMMDD hh24:mi')

ORDER BY sample_minute

/

Wrong Results example

SAMPLE_MINUTE SESS_COUNT

• ----------

20180727 09:09 5

20180727 09:09 1

20180727 09:09 1

20180727 09:09 5

20180727 09:09 2

20180727 09:09 4

20180727 09:10 3

20180727 09:10 5

20180727 09:10 1

20180727 09:10 4

20180727 09:10 7

20180727 09:10 9  

If I add another factored subquery, I get the right aggregation

WITH session_count AS

( SELECT sample_time, count(1) sess_count

  FROM dba_hist_active_sess_history

  WHERE sample_time >= sysdate - (1/24)

  GROUP BY sample_time

),

session_minutes

AS

( SELECT TO_CHAR(sample_time, 'YYYYMMDD hh24:mi') sample_minute,

      sess_count

FROM session_count

)

SELECT sample_minute,

       MAX(sess_count)

FROM session_minutes

GROUP BY sample_minute

ORDER BY sample_minute  

Correct results

SAMPLE_MINUTE MAX(SESS_COUNT)

• ---------------

20180727 09:01 22

20180727 09:02 22

20180727 09:03 15

20180727 09:04 10

20180727 09:05 11

20180727 09:06 10

20180727 09:07 20

20180727 09:08 20

20180727 09:09 5

20180727 09:10 9

20180727 09:11 12

20180727 09:12 12

20180727 09:13 9

20180727 09:14 6

20180727 09:15 3  

Why is the first query (which looks correct to me) not properly aggregating?

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