RE: Exadata Tuning Question+
Date: Sat, 8 Nov 2014 13:46:01 +0000
Message-ID: <CE70217733273F49A8A162EE074F64D901E5CB27_at_exmbx06.thus.corp>
Good idea about the single extra join and pivot, potentially much better than extending by a five-table outer join.
A lot would then depend on whether the CPU required to do the sort associated with the aggregation and pivot of the large volume was less than the CPU required for implementing a 5 in-line scalar subqueries -- IF they turned into highly efficient indexed accesses with a high degree of caching.
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
_at_jloracle
From: Iggy Fernandez [iggy_fernandez_at_hotmail.com] Sent: 08 November 2014 04:59
To: Jonathan Lewis; ORACLE-L
Subject: RE: Exadata Tuning Question+
I take it back. With incomplete information to draw upon, it's a good guess that SERVICE_RELATIONSHIP is a big table. Perhaps SID is the primary key of SERVICE_LOOKUP and, since the query wants a comma-delimited list of RELATED_SID values, perhaps SERVICE_RELATIONSHIP has 25X more rows than SERVICE_LOOKUP. Hence we have 35 million full table scans of a monster table compared to the single scan that is actually needed.
Iggy
From: iggy_fernandez_at_hotmail.com
To: jonathan_at_jlcomp.demon.co.uk; oracle-l_at_freelists.org Subject: RE: Exadata Tuning Question+
Date: Fri, 7 Nov 2014 18:20:10 -0800
Jonathan,
Actually, I didn't read the comment that "we could expect the server to take a couple of seconds to populate the next array fetchsize of rows before returning them" before I got onto my ornery mule to deliver a sermon :-) But let me try and paint my way out of my corner.
I fell into a trap because the original posting did not include a query or query plan. But perhaps I can still argue that the cause of the pauses is equally likely to be screen buffering as anything else. The query plan is a hash join of service_lookup (a large table with 21 million rows out of which 7,331,000 rows satisfy the query filters) to service_location (a large lookup table with 3,175,000 rows satisfying the query filters) resulting in 7,331,000 rows. Each row of this result requires five scalar subqueries on service_relationship. We don't know the size of the service_relationship table but EXPLAIN PLAN thinks that the queries return 25 rows each so, in the absence of more data, I could reasonably assume that service_relationship is a small table. If I make such an assumption, I could reasonably argue that Oracle should be able to perform fetches at a sustained clip.
But now that we have the query in hand, we know that is grossly inefficient because it requires 35 million little subqueries--five for each of the 7 million rows in the result. It could be rewritten as three inner joins and a pivot.
SELECT * FROM
(
SELECT
slv.sid,
slv.service_location_id,
sr.relationship_level,
RTRIM(XMLSERIALIZE(CONTENT EXTRACT( XMLAGG(XMLELEMENT("e", sr.RELATED_SID || ',') ORDER BY sr.RELATED_SID), '//text()' ) AS VARCHAR2(4000)) , ',' ) AS related_sid,
FROM
service_lookup slv
LEFT JOIN service_location sl ON sl.service_location_id = slv.service_location_id
LEFT JOIN service_relationship sr ON sr.sid = slv.sid AND sr.relationship_level IN ('1', '2', '3', '4', '5')
)
PIVOT (MAX(related_sid) FOR relationship_level IN (
'1' AS RELATEDSERVICEINSTANCEIDLEVEL1, '2' AS RELATEDSERVICEINSTANCEIDLEVEL2, '3' AS RELATEDSERVICEINSTANCEIDLEVEL3, '4' AS RELATEDSERVICEINSTANCEIDLEVEL4, '5' AS RELATEDSERVICEINSTANCEIDLEVEL5
);
Instead of the hard-to-read series of XML functions, all that is needed is LISTAGG(sr.related_sid, ',') WITHIN GROUP (ORDER BY sr.related_sid) AS related_sid
Iggy
From: jonathan_at_jlcomp.demon.co.uk
To: iggy_fernandez_at_hotmail.com; oracle-l_at_freelists.org Subject: RE: Exadata Tuning Question+
Date: Fri, 7 Nov 2014 23:49:07 +0000
I take it that this is a response to the other email I sent a few minutes ago, saying:
"While it doesn't make sense to return 21M rows to the screen, the output (in general) shouldn't pause every couple of seconds, as this would (usually) be a measure of much work the database had to do to acquire the next batch of rows."
and then explaining why (probably) the OP is seeing those pauses, viz:
"each row from the sample code requires 5 parallel tablescans of a reasonably large table, so we could expect the server to take a couple of seconds to populate the next array fetchsize of rows before returning them."
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
_at_jloracle
From: Iggy Fernandez [iggy_fernandez_at_hotmail.com] Sent: 07 November 2014 23:24
To: Jonathan Lewis; ORACLE-L
Subject: RE: Exadata Tuning Question+
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_at_jlcomp.demon.co.uk
To: oracle-l_at_freelists.org
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
_at_jloracle
From: oracle-l-bounce_at_freelists.org [oracle-l-bounce_at_freelists.org] on behalf of Ebadi, Abdul [Abdul.Ebadi_at_Level3.com] Sent: 07 November 2014 22:02
To: tim_at_evdbt.com; 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_at_evdbt.com]
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
-- http://www.freelists.org/webpage/oracle-lReceived on Sat Nov 08 2014 - 14:46:01 CET