The session 2231 had 49 samples in the observed time period:
select event,blocking_session,count(*)
from gv$active_session_history where session_id = 2231
and sample_time between
to_date('11.06.2019 07:08:30','dd.mm.yyyy hh24:mi:ss') and
to_date('11.06.2019 07:09:20','dd.mm.yyyy hh24:mi:ss')
group by event,blocking_session order by count(*) desc
;
EVENT
----------------------------------------------------------------
BLOCKING_SESSION COUNT(*)
---------------- ----------
log file switch (private strand flush incomplete)
736 49
As we can see, the session was solely waiting on "log file switch (private
strand flush incomplete)" and was blocked by the session 736.
But, only 12 samples were captured for the blocker during the same time period:
select count(*)
from gv$active_session_history where session_id = 736
and sample_time between
to_date('11.06.2019 07:08:30','dd.mm.yyyy hh24:mi:ss') and
to_date('11.06.2019 07:09:20','dd.mm.yyyy hh24:mi:ss')
;
COUNT(*)
----------
12
I'd like to offer some possible explanations for the blocker's "missing"
entries:
1. The session 2231 was indeed blocked by the session 736, but the session 736
was for the big part inactive.
2. The blocking_session information was obsolete when the sample was taken.
3. The blocker was active, but not sampled.
Interestingly, Tanel's ash_wait_chains.sql seems to operate under the
assumption 2, as only 24% ( round(12/49)*100 ) of the time the session appears
to be blocked:
SQL> @ash_wait_chains session_id||':'||event2 "session_id=2231"
"to_date('11.06.2019 07:08:00','dd.mm.yyyy hh24:mi:ss')" "to_date('11.06.2019
07:09:20','dd.mm.yyyy hh24:mi:ss')"
-- Display ASH Wait Chain Signatures script v0.2 BETA by Tanel Poder (
http://blog.tanelpoder.com ;)
old 22: AND sample_time BETWEEN &3 AND &4
new 22: AND sample_time BETWEEN to_date('11.06.2019
07:08:00','dd.mm.yyyy hh24:mi:ss') AND to_date('11.06.2019
07:09:20','dd.mm.yyyy hh24:mi:ss')
old 31: --, SYS_CONNECT_BY_PATH(&1, ' -> ')||CASE WHEN CONNECT_BY_ISLEAF
= 1 THEN '('||d.session_id||')' ELSE NULL END path
new 31: --, SYS_CONNECT_BY_PATH(session_id||':'||event2, ' -> ')||CASE
WHEN CONNECT_BY_ISLEAF = 1 THEN '('||d.session_id||')' ELSE NULL END path
old 32: , REPLACE(SYS_CONNECT_BY_PATH(&1, '->'), '->', ' -> ') path --
there's a reason why I'm doing this (ORA-30004 :)
new 32: , REPLACE(SYS_CONNECT_BY_PATH(session_id||':'||event2, '->'),
'->', ' -> ') path -- there's a reason why I'm doing this (ORA-30004 :)
old 42: AND d.sample_time BETWEEN &3 AND &4
new 42: AND d.sample_time BETWEEN to_date('11.06.2019
07:08:00','dd.mm.yyyy hh24:mi:ss') AND to_date('11.06.2019
07:09:20','dd.mm.yyyy hh24:mi:ss')
old 48: START WITH &2
new 48: START WITH session_id=2231
old 54: , ROUND(COUNT(*) / ((CAST(&4 AS DATE) - CAST(&3 AS DATE)) *
86400), 1) AAS
new 54: , ROUND(COUNT(*) / ((CAST(to_date('11.06.2019
07:09:20','dd.mm.yyyy hh24:mi:ss') AS DATE) - CAST(to_date('11.06.2019
07:08:00','dd.mm.yyyy hh24:mi:ss') AS DATE)) * 86400), 1) AAS
old 64: &1
new 64: session_id||':'||event2
%This SECONDS AAS
------ ---------- ----------
WAIT_CHAIN
--------------------------------------------------------------------------------
76% 37 .5
-> 2231:log file switch (private strand flush incomplete)
16% 8 .1
-> 2231:log file switch (private strand flush incomplete) -> 736:enq: CF -
contention -> 757:db file single write
4% 2 0
-> 2231:log file switch (private strand flush incomplete) -> 736:control file
%This SECONDS AAS
------ ---------- ----------
WAIT_CHAIN
--------------------------------------------------------------------------------
sequential read
2% 1 0
-> 2231:log file switch (private strand flush incomplete) -> 736:ON CPU
2% 1 0
-> 2231:log file switch (private strand flush incomplete) -> 736:LGWR all
worker groups
My question are:
1. Was the session 2231 really blocked by 736 the whole time?
2. If yes, why the samples from 736 are missing?
3. If no, when does blocking_session get cleared?
Best regards,
Nenad
https://nenadnoveljic.com/blog/
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