RE: Exadata Tuning Question+
Date: Mon, 10 Nov 2014 08:06:37 -0800
Message-ID: <00e501cffd00$4f03f340$ed0bd9c0$_at_comcast.net>
You would have to see if the plan changed for the count(*). I have never seen the Oracle optimizer use a previous result set the next row source result set and optimize away the full table scans for all the next row sources if they were going to happen. I have seen the Oracle optimizer switch to a full index scan to replace the full table scan when there is a pk or unique index to perform the count(*) against.
In my example of performing
Select count(*) from
(original query);
The original query will run completely through to result set in a cursor before the count (*) is resolved. Check the explain on the count(*).
On a side note is the plan output from a simple explain or from the actual cursor execution plan? They could be significantly different.
General rule of thumb
Explain Plan: 85% accurate
Cursor Plan: 99.95% accurate
Yes .05% of the time the plan in V$SQL_PLAN may not represent the real plan. Sometimes the explanation is simple, sometimes you can't derive why there was not a proper recording or branching of the plan.
This normally happens where odd full table scans appear to come into play.
From: Iggy Fernandez [mailto:iggy_fernandez_at_hotmail.com]
Sent: Monday, November 10, 2014 7:24 AM
To: dimensional.dba_at_comcast.net; Abdul.Ebadi_at_Level3.com; tim_at_evdbt.com;
'ORACLE-L'
Subject: RE: Exadata Tuning Question+
Apparently, select count(*) from (original query) completes in seconds instead of hours but that's because Oracle can optimize away the 35 million scalar subqueries (5 for each row) because they don't change the cardinality of the result. A full-table scan is being used for each subquery, so that's 35 million scans of a large table (21 million rows).
Iggy
From: dimensional.dba_at_comcast.net
To: dmarc-noreply_at_freelists.org; Abdul.Ebadi_at_Level3.com; tim_at_evdbt.com;
oracle-l_at_freelists.org
Subject: RE: Exadata Tuning Question+
Date: Mon, 10 Nov 2014 07:18:21 -0800
You can time
Select count(*) from
(original query);
And see fully with the rowset resolving in memory and disk. If a simple count(*) returns quickly even with rowset resolution unless the rowset is so large you are spending all your time writing to slow tempfile disk which again means not really the query.
Matthew Parker
Chief Technologist
425-891-7934 (cell)
Dimensional.dba_at_comcast.net
<http://www.linkedin.com/pub/matthew-parker/6/51b/944/> View Matthew Parker's profile on LinkedIn
From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org]
On Behalf Of David Fitzjarrell (Redacted sender "oratune_at_yahoo.com" for
DMARC)
Sent: Monday, November 10, 2014 7:03 AM
To: Abdul.Ebadi_at_Level3.com; tim_at_evdbt.com; ORACLE-L
Subject: Re: Exadata Tuning Question+
From what you've reported it appears to be the output buffering that's causing this problem; a count(*) returns in record time (probably because the output is a single line/column rather than a 7+ million row result set). As suggested in a prior response a CTAS may be of benefit; selecting from the 'temporary' table containing the current result set might be faster.
David Fitzjarrell
Principal author, "Oracle Exadata Survival Guide"
On Friday, November 7, 2014 3:04 PM, "Ebadi, Abdul" <Abdul.Ebadi_at_Level3.com> wrote:
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
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