Re: How to fix cache buffer chain issue

Jonathan wrote:
"
Looking at the predicate information for operation 15 you can see that it is an ACCESS predicate (that, since it came from memory) has "lost" the subquery bit.
This means the subquery runs one to generate a value that is effectively then used as a constant to drive the index range scan at operation 15. The index tad_ix2 is (presumably) on the column (date_cr), so this part of the query selects all data for the most recent date by index range scan. (And possibly will be doing this 17 times in the course of the query). "

According to the plan in the original attachment, the join to TAD is done via a nested loop. This would mean that this read of all rows in TAD where DT_CR=MAX(DT_CR) happens 18 * (rows in TNI with NE='XX'). This plan had notes but didn't indicate it was an adaptive plan, I reckon this difference would easily explain the difference in run times. Having to read rows which have already been recently read is usually going to be logical reads so the most likely waits *sampled* are going to be do with that, but most of the work will come under CPU.

Pap, what does

SELECT count(*)
FROM TNI TNI, TMTD TMTD
WHERE TNI.NE = 'XX'
AND TNI.NID = TMTD.DID Return? Somewhere in the region of 60?

(Obviously, Jonathan's advise is going to be sound, but this will probably explain your observation).
(Aside, AWR gets predicates in 21c so getting similar answers out of historic plans is going to be much easier)

Thanks,
Andrew

On Sun, 20 Jun 2021 at 20:27, Dominic Brooks <dombrooks_at_hotmail.com> wrote:

> I would think twice about materialize especially on Exadata when you can
> often query the data twice (or more) much quicker than the consequences of
> writing and reading back from temp.
>
> Sent from my iPhone
>
> > On 20 Jun 2021, at 18:41, Mladen Gogala <gogala.mladen_at_gmail.com> wrote:
> >
> > Pap, as a matter of principle, your SQL is too big. I have had a myriad
> of problems with gigantic SQL statements like yours. You are inserting into
> GTT using a statement with all kinds of hash group by, sorting and full
> table scans. Not even Exadata can help you with that. I would try slicing
> and dicing the SQL using CTE and /*+ MATERIALIZE */ hint.
> >
> > Regards
> >
> >> On 6/18/21 1:46 AM, Pap wrote:
> >> Hello Listers, Its version 11.2.0.4 of oracle exadata. And we are
> facing an issue in which a reporting query(part of plsql procedure) which
> normally finishes within ~1-2minutes runs for ~1-2hrs at times. This
> happens when the same query is submitted from 5-6 multiple sessions at the
> same time and is accessing the same customer data. When we kill them and
> rerun them in serial they run fine without any issue and finish in the same
> 1-2 minutes duration.
> >>
> >> Few things we observed is , when all the session submitted at same time
> and the query runs long , the event its showing for the session is "latch:
> cache buffers chains" but active session history is not showing up any
> significant activity for that session and also the sql monitor is not
> getting logged for that query. Which means it's not doing significant
> activity while this issue occurs but kind of stuck. Why is it so? And also
> due to that , I am not able to capture the current object on which it's
> actually holding that latch.
> >>
> >> The query is an INSERT query which inserts data into a global temporary
> table. It has ~17 UNION clauses of which most look similar. So i am
> wondering if by someway we can rewrite this query which will help us in
> fixing this issue or making the situation better?
> >>
> >> Attached is the sample INSERT query with UNION clauses(I have removed a
> few of the UNIONS to make it look simple) and its plan which suffers from
> "latch: cache buffers chains".
> >>
> >> Regards
> >> Pap
> >
> > --
> > Mladen Gogala
> > Database Consultant
> > Tel: (347) 321-1217
> >
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