Stefan,
I had one too many: hardware changes could not affect execution plans.
I added that to pad the list without much thought just before pressing
SEND, and of course regretted it two seconds later.
There are certainly more items to add, but I started with the
easily-verifiable stuff.
Clearly a 10053 trace is the ultimate, but as the OP noted, it requires
prescience to be set before it is needed, and prescience isn't always
available.
10053 trace output is also not easy to read. Reading two such traces,
comprehending both, and then comparing and contrasting usually requires
intelligence and attention to detail approaching the level of Wolfgang
Breitling.
Thanks!
-Tim
On 7/18/18 21:03, Stefan Knecht wrote:
Tim, you forgot one:
7. The fact whether it rains Monday morning or not
The original anecdote referred to the fact that if it rained, a certain employee that normally arrives first on a sunny day, would get to the office later - which caused a different employee to first trigger execution plan creation, with different bind variables, leading to a different plan.
So the query would run fast all day on a sunny day, but slow all day when it rained.
Venky - try looking at the values of the bind variables of a good run vs a bad run.
On Thu, Jul 19, 2018 at 5:58 AM, Tim Gorman <tim.evdbt@xxxxxxxxx <mailto:tim.evdbt@xxxxxxxxx>> wrote:
Venky,
"Assuming there is not much change in the DB"
Let's narrow down the things that can change an execution plan...
1. hardware change (i.e. #-cpus, # GB of RAM, storage, etc)
2. application software change (i.e. change to the SQL text)
3. Oracle software change (i.e. patch, upgrade, etc)
4. initialization parameter change
5. gathering system statistics
6. gathering table, index, column statistics
When you state the assumption about "no much change in the DB", I
am assuming that you're discussing items #1-4.
How about item #5? Can you query the SYS.AUX_STATS$ table and
display the column PVAL1 where PNAME has the value "DSTART" or
"DSTOP"?
How about item #6? Can you display the contents of
DBA_TAB_STATS_HISTORY for the tables involved in the query?
Please refer to the useful blog posts by Uwe Hesse HERE
<https://uhesse.com/2012/04/23/diff_table_stats_in_history-example/>
and by Marcel-Jan Krijgsman HERE
<https://mjsoracleblog.wordpress.com/2013/02/19/more-statistics-history/>
for more information, if necessary?
Hope this helps?
Thanks!
-Tim
On 7/18/18 15:30, V Raman wrote:
List
We have a SQL that is performing intermittently bad in our prod
env. The good ones take 2 to 5 mins, the bad ones run for hours
we kill them. They run fine in the non prod env. I ran an awsqrpt
and based on that I see that there are a few executions with the
bad ones taking hours. Looking at the differences in the
execution plan, the good ones have lots of nested loops in them,
with the bad ones having lots of hash joins.
I am trying to figure out the cause(s). Assuming there is not
much change in the DB, the first thing that comes to mind is
statistics. Can the listers help with ideas? Thanks.
If anyone is interested is seeing the report, i can provide a
link to them by email.
Venky
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