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Fluctuating Cost with Range Scan

P: n/a
Dan
I am a relatively new user on Oracle 9.2.0.1 and I am having trouble
performance tuning this production database.

I am running a large query that joins two tables, document(3 mil) and
entity(9 mil). I have reorganized my tablespaces so that the two
tables are on different tablespaces, different disks. They both have
their indexes stored on a third tablespace. Before reconfiguring the
production database, I was getting a range scan on index ix_document_8
that had a cost of 25. Now that I have reconfigured the prod
database, the cost of the range scan has gone through the roof, 5933.

I have tried moving this one particular index to different tablespaces
on all three disks available to me to no avail. I run "analyze index
compute statistics;" on the index after every time I recreate it. It
appears that no matter where I create this index it still has trouble
accessing it. I have computed the statistics for the table before
computing the index statistics. It appears to me that there is an I/O
problem here. Why did it cost so little before and now it costs so
much to access it?

Should the index be on the same disk/different tablespace?
Different disk/different tablespace?

I have run out of my testing capabilities here to try and troubleshoot
why the cost is so high. Could this be a problem with my CBO?

Any help is greatly appreciated,
-Dan
Jul 19 '05 #1
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2 Replies


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do*****@amcad.com (Dan) wrote in message news:<bd**************************@posting.google. com>...
I am a relatively new user on Oracle 9.2.0.1 and I am having trouble
performance tuning this production database.

I am running a large query that joins two tables, document(3 mil) and
entity(9 mil). I have reorganized my tablespaces so that the two
tables are on different tablespaces, different disks. They both have
their indexes stored on a third tablespace. Before reconfiguring the
production database, I was getting a range scan on index ix_document_8
that had a cost of 25. Now that I have reconfigured the prod
database, the cost of the range scan has gone through the roof, 5933.

I have tried moving this one particular index to different tablespaces
on all three disks available to me to no avail. I run "analyze index
compute statistics;" on the index after every time I recreate it. It
appears that no matter where I create this index it still has trouble
accessing it. I have computed the statistics for the table before
computing the index statistics. It appears to me that there is an I/O
problem here. Why did it cost so little before and now it costs so
much to access it?

Should the index be on the same disk/different tablespace?
Different disk/different tablespace?

I have run out of my testing capabilities here to try and troubleshoot
why the cost is so high. Could this be a problem with my CBO?

Any help is greatly appreciated,
-Dan


Cost won't necessarily be influenced by relocating tablespaces.
The foremost factor in cost is
- selectivity
- selectivity
- selectivity

and adequate optimizer_cost_adj and optimizer_index_cache parameters
set.
You probably don't have a histogram on the affected column.
Analyze table compute statistics for all indexed columns is probably
in order.
Did you try to use hints in your statement (forcing the index)? If so,
what was the result?

Please also note : cost is a meaningless number. The only things that
counts is the number of consistent gets.

Sybrand Bakker
Senior Oracle DBA
Jul 19 '05 #2

P: n/a
Dan
Thanks for the response Mark.

I looked into the problem a little farther and found that the
difference comes down to two parts of the WHERE clause. When these
two values are NOT sent as bind variables, the cost goes way down. I
realize that cost is an arbitrary number. I looked at the number of
consistent gets and there was a large difference (factor of 10x) when
I was sending these two conditions as bind variables. My problem is,
since the database has to run with CURSOR_SHARING=SIMILIAR (I'll be
arm wrestling our developers over their lack of bind variable usage) I
don't see a way to get around this issue.

I have a couple of books that explicitly say that if you are not using
bind variables from the application side, the CBO can't be guarenteed
to produce predictable/consistent executions. Am I up the creek here
because of this? I am forcing index usage using hints, as well as
ORDERED to make sure the join is done in the proper order. Is there
any way that you know of that I could selectively turn the
CURSOR_SHARING off for part of the query? Some sort of escape
characters of some sort that tell Oracle that I don't want it to bind
these particular variables, but bind the rest of the query?

I know that this is all patchwork, but I have an angry customer and
there is no way the entire application can be changed for the
deadline. Any ideas?

Thanks for the help,
-Dan

sy******@yahoo.com wrote in message news:<a1**************************@posting.google. com>...
do*****@amcad.com (Dan) wrote in message news:<bd**************************@posting.google. com>...
I am a relatively new user on Oracle 9.2.0.1 and I am having trouble
performance tuning this production database.

I am running a large query that joins two tables, document(3 mil) and
entity(9 mil). I have reorganized my tablespaces so that the two
tables are on different tablespaces, different disks. They both have
their indexes stored on a third tablespace. Before reconfiguring the
production database, I was getting a range scan on index ix_document_8
that had a cost of 25. Now that I have reconfigured the prod
database, the cost of the range scan has gone through the roof, 5933.

I have tried moving this one particular index to different tablespaces
on all three disks available to me to no avail. I run "analyze index
compute statistics;" on the index after every time I recreate it. It
appears that no matter where I create this index it still has trouble
accessing it. I have computed the statistics for the table before
computing the index statistics. It appears to me that there is an I/O
problem here. Why did it cost so little before and now it costs so
much to access it?

Should the index be on the same disk/different tablespace?
Different disk/different tablespace?

I have run out of my testing capabilities here to try and troubleshoot
why the cost is so high. Could this be a problem with my CBO?

Any help is greatly appreciated,
-Dan


Cost won't necessarily be influenced by relocating tablespaces.
The foremost factor in cost is
- selectivity
- selectivity
- selectivity

and adequate optimizer_cost_adj and optimizer_index_cache parameters
set.
You probably don't have a histogram on the affected column.
Analyze table compute statistics for all indexed columns is probably
in order.
Did you try to use hints in your statement (forcing the index)? If so,
what was the result?

Please also note : cost is a meaningless number. The only things that
counts is the number of consistent gets.

Sybrand Bakker
Senior Oracle DBA

Jul 19 '05 #3

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