Exadata Tuning Question+
- From: "Ebadi, Abdul" <Abdul.Ebadi@xxxxxxxxxx>
- To: Oracle-L Freelists <oracle-l@xxxxxxxxxxxxx>
- Date: Fri, 7 Nov 2014 20:59:39 +0000
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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