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Home -> Community -> Mailing Lists -> Oracle-L -> RE: TC Enqueue waits
So what is the difference between parallel query and parallel execution?
-----Original Message-----
From: Riyaj Shamsudeen [mailto:rshamsud_at_jcpenney.com]
Sent: Saturday, May 01, 2004 7:53 PM
To: oracle-l_at_freelists.org
Subject: RE: TC Enqueue waits
Hi=20
Parallel Query reads the buffers in to PGA (except segment headers which are read in to SGA). But parallel execution seems to be reading the buffers in to SGA ( at least in 8.1.7.4, in my tests). TC locks are obtained whether it is a query or Execute though (gets in x$ksqst incremented twice each time the query/DML was executed. Don't know why this lock has gotten twice).=20
Direct reads maintains the data consistency by using the disk block
scn, after the object checkpoint. During parallel query reads, if the
disk block is more recent then the CR scn, then the SGA is accessed and
usual consistency mechanism is applied. So, it is still possible for the
parallel query to access the SGA.
Thanks
Riyaj "Re-yas" Shamsudeen
Certified Oracle DBA
-----Original Message-----
From: oracle-l-bounce_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Khedr, Waleed
Sent: Friday, April 30, 2004 9:58 PM
To: oracle-l_at_freelists.org
Subject: RE: TC Enqueue waits
I can't say that I'm 100% sure about the details, but I can say I was =
=3D
able to observe this behavior many times in environments that mix oltp =
=3D
(serial transactions) and Data Warehouses (reports that use PQO).
Also think about the fact, that direct reads go directly to files/TS =3D
where there might be a chance of having some dirty buffers that belong =
=3D
to that segment & Tablespace that could really affect the =3D
uniqueness/integrity of data being extracted by direct reads.
I'm sure many of the enqueue/X$/v$ experts on this list can correct me =
=3D
or add more details.
Regards,
Waleed
-----Original Message-----
From: Diego Cutrone [mailto:diegocutrone_at_yahoo.com.ar]
Sent: Friday, April 30, 2004 6:54 PM
To: oracle-l_at_freelists.org
Subject: Re: TC Enqueue waits
Thank you Waleed
Let me confirm what I understood.
So you're telling me that when an object needs to be accessed by using =
=3D
PQO, the session that required this access will (possible) have to wait
on a TC enqueue waiting for the dirty buffers to get into disk. did I =
=3D
get it correctly?
I thought the session used to waiting on the cross instance call (CI) enqueue while an object checkpoint was taking place (before a direct =3D read takes place)....
Please, let me know your thoughts.
Thanks!
Diego.
> PQ slaves uses direct reads bypassing the SGA to read the segment =3D
blocks =3D3D
> which requires waiting to flush all committed
> changes in the SGA (dirty buffers) of that object to make sure the PQ=20
> =3D
=3D3D
> slaves
> will access the most recent version of that object.
>
> Regards,
>
> Waleed
>
> -----Original Message-----
> From: Diego Cutrone [mailto:diegocutrone_at_yahoo.com.ar]
> Sent: Friday, April 30, 2004 4:25 PM
> To: Oracle List
> Subject: TC Enqueue waits
>
>
> Hi List.
> I have detected high enqueue waits in one instance.
> After quering x$ksqst I realized most of them were TX and TC enqueues.
>
> I'm on the TX enqueues by tracing some sessions. But I have never =3D
heard =3D3D
> of TC enqueues
> I have read that TC enqueue stands for "Thread Checkpoint =3D
enqueue".=3D3D20
>
> Anybody knows what are they used for? what kind of resources they =
=3D3D=20
> protect?
>
> And I have another quick question:
>
> I'm also seeing Parallel query events like:
>
> PX Deq: Execution Msg
> PX Idle Wait
> PX qref latch=3D3D20
> PX Deq: Execute Reply=3D3D20
> PX Deq: Table Q Normal=3D3D20
> PX Deq: Parse Reply=3D3D20
>
> Does anybody know where I can get a description for them?=3D3D20 I'd=20
> like to know which ones can be ignored and which ones can't.
>
> Thank you very much
> Regards
> Diego
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