RE: Exadata Tuning Question+
- From: Iggy Fernandez <iggy_fernandez@xxxxxxxxxxx>
- To: Jonathan Lewis <jonathan@xxxxxxxxxxxxxxxxxx>, ORACLE-L <oracle-l@xxxxxxxxxxxxx>
- Date: Fri, 7 Nov 2014 15:24:28 -0800
Perhaps I'm stubborn as a mule that needs coffee but I don't see a problem. The
question was "However, when we display the output to the screen it takes hours
for it to finish and we see pauses in the display every second or two while it
is running. Trying to figure out what is causing these pauses?" The OP is
trying to bring the 7 million rows over the network to his laptop and scroll
them in a Putty screen or something similar, and should expect serious
buffering issues and time to completion.
Iggy
From: jonathan@xxxxxxxxxxxxxxxxxx
To: oracle-l@xxxxxxxxxxxxx
Subject: RE: Exadata Tuning Question+
Date: Fri, 7 Nov 2014 23:06:54 +0000
The 4 seconds you quoted is the time required to do the driving join, which is
(roughly) lines 26 - 36 of your plan.
The rest of the time would be the 35M parallel tablescans of
SERVICE_RELATIONSHIP which appear as the 5 groups of 5 lines in the earlier
part of the
plan.
Was the suggested index on SERVICE_RELATIONSHIP(relationship_level,
sid) ? This would seem to be the only choice - perhaps with hinting - to
change the efficiency of the query as it is currently written - since the size
of the service_relationship table is (probably) in the range of tens - or
perhaps low hundreds - of thousands
of blocks I would hope you could cache the entire data set to avoid millions
of single block random reads. The XML stuff could, of course, make all of my
guesstimates meaningless, so it would be nice to get some ideas of the typical
sizes of the XML component,
and the number of rows of service_relationship you expect to get (compared to
the optimizer's estimate of 25) for a typical (relationship_level, sid)
combination.
I have to admit that I don't understand how your inline scalar subqueries in
the select list guarantee to return a single value for every driving row
- in fact I don't understand why they don't raise a syntax error relating to
columns not in group by - but that's probably because I don't do much with XML.
If you can't optimise the work done by the inline subqueries, you'll have to
re-engineer them into a join (probably outer) of an extra tables.
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
@jloracle
From: oracle-l-bounce@xxxxxxxxxxxxx [oracle-l-bounce@xxxxxxxxxxxxx] on behalf
of Ebadi, Abdul [Abdul.Ebadi@xxxxxxxxxx]
Sent: 07 November 2014 22:02
To: tim@xxxxxxxxx; ORACLE-L
Subject: RE: Exadata Tuning Question+
Tim/others… thanks for your replies! Below is a slightly generalized version
of the query to give you an idea of the joins and an actual explain plan:
SELECT bunch of stuff.....,
(
SELECT RTRIM(XMLSERIALIZE(CONTENT EXTRACT(
XMLAGG(XMLELEMENT("e", sr1.RELATED_SID
||
',')
ORDER BY sr1.RELATED_SID), '//text()' ) ) , ',' )
FROM service_relationship sr1
WHERE sr1.SID = slv.SID
AND sr1.RELATIONSHIP_LEVEL = '1'
GROUP BY sr1.SID
) AS RELATEDSERVICEINSTANCEIDLEVEL1,
(
SELECT RTRIM(XMLSERIALIZE(CONTENT EXTRACT(
XMLAGG(XMLELEMENT("e", sr2.RELATED_SID
||
',')
ORDER BY sr2.RELATED_SID), '//text()' ) ) , ',' )
FROM service_relationship sr2
WHERE sr2.SID = slv.SID
AND sr2.RELATIONSHIP_LEVEL = '2'
GROUP BY sr2.SID
) AS RELATEDSERVICEINSTANCEIDLEVEL2,
(
SELECT RTRIM(XMLSERIALIZE(CONTENT EXTRACT(
XMLAGG(XMLELEMENT("e", sr3.RELATED_SID
||
',')
ORDER BY sr3.RELATED_SID), '//text()' ) ) , ',' )
FROM service_relationship sr3
WHERE sr3.SID = slv.SID
AND sr3.RELATIONSHIP_LEVEL = '3'
GROUP BY sr3.SID
) AS RELATEDSERVICEINSTANCEIDLEVEL3,
(
SELECT RTRIM(XMLSERIALIZE(CONTENT EXTRACT(
XMLAGG(XMLELEMENT("e", sr4.RELATED_SID
||
',')
ORDER BY sr4.RELATED_SID), '//text()' ) ) , ',' )
FROM service_relationship sr4
WHERE sr4.SID = slv.SID
AND sr4.RELATIONSHIP_LEVEL = '4'
GROUP BY sr4.SID
) AS RELATEDSERVICEINSTANCEIDLEVEL4,
(
SELECT RTRIM(XMLSERIALIZE(CONTENT EXTRACT(
XMLAGG(XMLELEMENT("e", sr5.RELATED_SID
||
',')
ORDER BY sr5.RELATED_SID), '//text()' ) ) , ',' )
FROM service_relationship sr5
WHERE sr5.SID = slv.SID
AND sr5.RELATIONSHIP_LEVEL = '5'
GROUP BY sr5.SID
) AS RELATEDSERVICEINSTANCEIDLEVEL5
FROM service_lookup slv
LEFT JOIN service_location sl
ON sl.service_location_id = slv.service_location_id;
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 1570133209
--------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes
| Cost (%CPU)| Time | TQ |IN-OUT| PQ Distrib |
--------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 7331K|
5593M| 1877 (5)| 00:00:01 | | | |
| 1 | SORT GROUP BY | | 1 | 22
| 368 (6)| 00:00:01 | | | |
| 2 | PX COORDINATOR | | |
| | | | | |
| 3 | PX SEND QC (RANDOM) | :TQ10000 | 25 | 550
| 368 (6)| 00:00:01 | Q1,00 | P->S | QC (RAND) |
| 4 | PX BLOCK ITERATOR | | 25 | 550
| 368 (6)| 00:00:01 | Q1,00 | PCWC | |
|* 5 | TABLE ACCESS STORAGE FULL | SERVICE_RELATIONSHIP | 25 | 550
| 368 (6)| 00:00:01 | Q1,00 | PCWP | |
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 6 | SORT GROUP BY | | 1 | 22
| 368 (6)| 00:00:01 | | | |
| 7 | PX COORDINATOR | | |
| | | | | |
| 8 | PX SEND QC (RANDOM) | :TQ20000 | 25 | 550
| 368 (6)| 00:00:01 | Q2,00 | P->S | QC (RAND) |
| 9 | PX BLOCK ITERATOR | | 25 | 550
| 368 (6)| 00:00:01 | Q2,00 | PCWC | |
|* 10 | TABLE ACCESS STORAGE FULL | SERVICE_RELATIONSHIP | 25 | 550
| 368 (6)| 00:00:01 | Q2,00 | PCWP | |
| 11 | SORT GROUP BY | | 1 | 22
| 368 (6)| 00:00:01 | | | |
| 12 | PX COORDINATOR | | |
| | | | | |
| 13 | PX SEND QC (RANDOM) | :TQ30000 | 25 | 550
| 368 (6)| 00:00:01 | Q3,00 | P->S | QC (RAND) |
| 14 | PX BLOCK ITERATOR | | 25 | 550
| 368 (6)| 00:00:01 | Q3,00 | PCWC | |
|* 15 | TABLE ACCESS STORAGE FULL | SERVICE_RELATIONSHIP | 25 | 550
| 368 (6)| 00:00:01 | Q3,00 | PCWP | |
| 16 | SORT GROUP BY | | 1 | 22
| 368 (6)| 00:00:01 | | | |
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 17 | PX COORDINATOR | | |
| | | | | |
| 18 | PX SEND QC (RANDOM) | :TQ40000 | 25 | 550
| 368 (6)| 00:00:01 | Q4,00 | P->S | QC (RAND) |
| 19 | PX BLOCK ITERATOR | | 25 | 550
| 368 (6)| 00:00:01 | Q4,00 | PCWC | |
|* 20 | TABLE ACCESS STORAGE FULL | SERVICE_RELATIONSHIP | 25 | 550
| 368 (6)| 00:00:01 | Q4,00 | PCWP | |
| 21 | SORT GROUP BY | | 1 | 22
| 368 (6)| 00:00:01 | | | |
| 22 | PX COORDINATOR | | |
| | | | | |
| 23 | PX SEND QC (RANDOM) | :TQ50000 | 25 | 550
| 368 (6)| 00:00:01 | Q5,00 | P->S | QC (RAND) |
| 24 | PX BLOCK ITERATOR | | 25 | 550
| 368 (6)| 00:00:01 | Q5,00 | PCWC | |
|* 25 | TABLE ACCESS STORAGE FULL | SERVICE_RELATIONSHIP | 25 | 550
| 368 (6)| 00:00:01 | Q5,00 | PCWP | |
| 26 | PX COORDINATOR | | |
| | | | | |
| 27 | PX SEND QC (RANDOM) | :TQ60002 | 7331K|
5593M| 1877 (5)| 00:00:01 | Q6,02 | P->S | QC (RAND) |
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|* 28 | HASH JOIN RIGHT OUTER BUFFERED| | 7331K|
5593M| 1877 (5)| 00:00:01 | Q6,02 | PCWP | |
| 29 | PX RECEIVE | | 3175K|
920M| 366 (3)| 00:00:01 | Q6,02 | PCWP | |
| 30 | PX SEND HASH | :TQ60000 | 3175K|
920M| 366 (3)| 00:00:01 | Q6,00 | P->P | HASH |
| 31 | PX BLOCK ITERATOR | | 3175K|
920M| 366 (3)| 00:00:01 | Q6,00 | PCWC | |
| 32 | TABLE ACCESS STORAGE FULL | SERVICE_LOCATION | 3175K|
920M| 366 (3)| 00:00:01 | Q6,00 | PCWP | |
| 33 | PX RECEIVE | | 7331K|
3467M| 1507 (5)| 00:00:01 | Q6,02 | PCWP | |
| 34 | PX SEND HASH | :TQ60001 | 7331K|
3467M| 1507 (5)| 00:00:01 | Q6,01 | P->P | HASH |
| 35 | PX BLOCK ITERATOR | | 7331K|
3467M| 1507 (5)| 00:00:01 | Q6,01 | PCWC | |
| 36 | TABLE ACCESS STORAGE FULL | SERVICE_LOOKUP | 7331K|
3467M| 1507 (5)| 00:00:01 | Q6,01 | PCWP | |
--------------------------------------------------------------------------------------------------------------------------------------
From: Tim Gorman [mailto:tim@xxxxxxxxx]
Sent: Friday, November 07, 2014 2:36 PM
To: ORACLE-L; Ebadi, Abdul
Subject: Re: Exadata Tuning Question+
Abdul,
Although it might be a "best practice" to remove indexes to encourage full
scans in parallel, its important to remember that in some circumstances this
might cause a lot of SORT-MERGE or HASH joins, especially if the query involves
joins to lots of big tables.
Those SM or HA joins will spend a lot of time reading and writing to the
temporary tablespace, and that I/O on temporary tablespaces do not have any
special off-loading or smart-scan optimizations. Exadata only has off-loading
and smart-scan optimizations
for I/O operations taking place in datafiles.
For tables this size, you can't increase the PGA size enough to cache the
entire SM or HA join in memory, so that's probably why you haven't seen any
benefit.
Chances are good that SQL Plan Monitor or
DBMS_XPLAN.DISPLAY_CURSOR(option=>'ALLSTATS ALL') would show you that the time
spent scanning the row-sources is negligible, but the time spent on joins is
taking the majority of elapsed time.
How many and what type of join operations? Are they straight-forward INNER
joins, or OUTER joins? Are there any FULL OUTER JOINs or sub-queries?
Hope this helps...
-Tim
On 11/7/14 13:59, Ebadi, Abdul wrote:
We have a half rack 4-node Exadata (X2 high capacity) running several DW
databases for us. We have a query going against a 21 million row table with
several self-joins in it. This query returns 7 million rows takes way too
long too run (hrs).
We have made sure it is running in parallel using cell offloading (full storage
scans) and when we put a count(*) around the query it returns in only
4 seconds for 7 million rows returned.
However, when we display the output to the screen it takes hours for it to
finish and we see pauses in the display every second or two while it is
running.
Trying to figure out what is causing these pauses? The wait is PX Deq: type
waits when it runs with these pauses. SQL Monitor doesn’t tell us much
either except cell efficiency is negative 85%!
We have increased PGA size and didn’t make much difference. We are considering
putting TEMP tablespace on flash cache possibly. Another DBA added
an index to it just to see (bad idea on Exadata) and did’t improve it. Before
making any more change we would like to see some evidence for root cause.
We were told for best practices on Exadata it is better to remove indexes and
hints (if possible) and let the machine full scan in parallel using storage
offloading. Do you guys agree and are there other best practices on Exadata
also?
Any other suggestions on tuning this query and also general Exadata best
practices?
Thanks,
Abdul
Other related posts:
