FAQ
Here are replies to a couple Sam,

If optimiser mode is chose and any table in a query has stats then CBO
will be used. If stats do not exist on any table Oracle has a hard-coded
default value which is 100 if I recall a note on here correctly.
If stats do exist but they are very old then they will still be used
possibly giving a poor path.
The best bet is to analyze the tables.

50 user running explain plan at the same time? Some system
However the best bet is to select something which you can identify with the
trace. Something like
'select this is sams session from dual;'
Then do a grep on the udump directory searching for that phrase.

Put a trace on that particular session and get the user to run the
application and then you can view the trace files afterwards. (see my notes
on www.hcresources.co.uk re tracing sessions)
Check that performance is bad all the time rather than just at a peak time
etc

HTH

John

-----Original Message-----
Sent: 11 June 2002 14:24
To: Multiple recipients of list ORACLE-L

Hi,

I don't have much onhand experience, I thought
list is the appropriate place to get information.
These are related to Oracle Performance Tuning and
Backup & Recovery.

I have a system of Oracle 8i under HP-Unix and with
ARCHIVELOG on. It has been running and stable since
last 2-3 years. This was a brief overview.

Que 1: Assuming that I am using Cost based Optimizer
(CBO); I have a query which is a join of three tables
out of which only one table have been analyzed and in
first case rest two were analyzed but long time back
and after that I have changed the structure of the
table and in second case the other two tables have not
been analyzed. Will Oracle use CBO ? I know it will
but then what will be the behavior of optimizer ? Will
it take the best guess for the other two tables in
both the cases and if yes then those best guesses will
be depending upon on what ?

Que 2: Suppose u have ran EXPLAIN PLAN for a
particular query at sqlplus prompt and there are
around 50 more users running EXPLAIN PLAN at the same
time from the UNIX bos itself. In this case the
OSUSER, TERMINAL, PROGRAM will be same (u know it
better). Now suppose I want to run TKPROF against the
trace file how will I identify which will be my trace
file in USER_DUMP_DEST directory ?

Que 3: Supposing if I have Oracle Report running on
Oracle. I passed 3 parameters first time it took me
around 3 seconds to execute and 4 parameters second
time its hogging the system taking almost 25 minutes
to run. There are no concatenated indexs on the
underlying tables. What might be the cause and how
would u go about identifying it ?

Que 4: A user starts complaining about a particular
part of an appln. What will be my first steps to
optimize it ?

Que 5: Let's say we have a situation where at point
"A" I took a complete db backup. Somewhere after point
"B" I lost an online redo log file. I recovered the db
using the bck taken at point "A" and archived redo log
files and started the new incarnation at point "B". I
took a complete logical backup at point "B" and taking
incremental backup after that. Now again at point "C"
I took the complete bck. If I have dropped a
tablespace somewhere in between point B and C which
are the possible ways to recover it and which one will
be the fastest ? (Of course if possible).

A---------------B---------------C---------------D

If I am not clear on any of the part please let me
know and I'll do the needful.

Thanks

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