Mittwoch, 24. Juni 2020

Oceans, Islands, and Rivers in ASH

The Idea of Oceans, Islands and Rivers in performance data from Oracle sessions comes from Cary Millsap. I recommend reading his Presentation - especially page 33+. 
To summarize it very short: in an ocean of idle states (where the application doesn't interact with the session at all) there are islands of activity, but within these islands, there still can be rivers of idle events - when there is communication between the server process and the application. 

Even the algorithm was defined for trace data, I'm sure it can be implemented based on ASH data. Of course, the resolution isn't as fine as trace data, but if only big islands are of interest, and these islands doesn't have to many rivers, sampled ASH sould be acceptable.

This can be done quite easy with a statement like this.
I tried to keep it simple by having a CONST CTE at the beginning. there the RIVER_WIDTH can be set. It defines how many idle (non visible) samples define the widest river. Everything greater is an ocean which separates islands. 
In the 2nd CTE FILTER_FIRST, additional filters can be applied to ASH. Everything which narrows the focus can help, like USER_ID, SAMPLE_ID or timestamps. The idea matches proper scoping in SQL*Trace.

WITH CONST as (
SELECT /*+ qb_name(QB_CONST) */
    100 as GANTT_LENGTH -- unit: characters
  , 1   as RIVER_WIDTH  -- unit: sample_periode of ASH. --everything else IDLE is an OCEAN
FROM DUAL  
) ,FILTER_FIRST as (
SELECT /*+ qb_name(QB_FILTER_FIRST) */  ash.* 
FROM gv$active_session_history ash
WHERE 1=1 
--  AND USER_ID=123
--  AND ....
--  AND SAMPLE_ID < 487871
), ISLANDS as (
SELECT /*+ qb_Name(QB_ISLANDS) */
      min(BEGIN_SAMPLE_ID) OVER () total_min_sample_ID
   ,  max(END_SAMPLE_ID) OVER () total_max_sample_ID      
   ,   BEGIN_SAMPLE_ID
   ,  END_SAMPLE_ID
   ,  END_SAMPLE_ID - BEGIN_SAMPLE_ID +1  as ISLAND_LENGTH
   ,  ACTIVE_COUNT
   ,  inst_id
   ,  session_id
   ,  session_serial#
FROM FILTER_FIRST ff
       MATCH_RECOGNIZE(
         PARTITION BY inst_id, session_id, session_serial#
         ORDER BY SAMPLE_ID 
         MEASURES 
           first(SAMPLE_ID) as BEGIN_SAMPLE_ID,
           LAST(sample_id)  as END_SAMPLE_ID,
           COUNT(sample_id) as ACTIVE_COUNT
         ONE ROW PER MATCH
         PATTERN( frst cont*)
         DEFINE cont as SAMPLE_ID - prev(SAMPLE_ID) <= (SELECT RIVER_WIDTH FROM CONST) 
       )  
)
SELECT /*+ qb_name(QB_MAIN)*/ isl.begin_sample_id
     , isl.end_sample_id
     , isl.island_length
     , isl.active_count
     , isl.inst_id
     , isl.session_id
     , isl.session_serial#
FROM ISLANDS isl
Order by isl.begin_sample_id
        , isl.inst_id
        , isl.session_id
/
The result on my sandbox is like this:

BEGIN SAMPLE_ID END SAMPLE_ID ISLAND LENGTH ACTIVE COUNT INST ID SESSION ID SESSION SERIAL#
487760 487776 17 17 1 21 6605
487777 487780 4 4 1 274 7693
487781 487782 2 2 1 264 65268
487785 487785 1 1 1 278 22792
487791 487792 2 2 1 22 17104
487794 487794 1 1 1 22 37292
487795 487795 1 1 1 284 36814
487796 487797 2 2 1 278 22792
487798 487798 1 1 1 22 37292
487800 487800 1 1 1 31 44098
487800 487803 4 4 1 278 22792
487802 487875 74 71 1 22 37292
487807 487807 1 1 1 282 7351

BEGIN_SAMPLE_ID, END_SAMPLE_ID, INST_ID, SESSION_ID and SESSION_SERIAL# together describe every single island. It can be userd later on to do specific analysis on this island. 
ISLAND_LENGTH show the total length of the island, and ACTIVE_COUNT is the amount of "land" on this island. The lower this number, the more rivers were found. 

This can be a total different approach to analyse ASH data.

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