RE: Optimizer Mode......how to choose the right one?

From: Christopher Spence <cspence_at_FuelSpot.com>
Date: Tue, 07 Aug 2001 09:21:00 -0700
Message-ID: <F001.00362F3B.20010807093553@fatcity.com>

When using first rows, you force the cost based optimizer, in which the order of the tables does not matter.

Order of the tables only matters when using the ORDERED hint or using rbo AND there is a tie ranking.

"Do not criticize someone until you walked a mile in their shoes, that way when you criticize them, you are a mile a way and have their shoes."

Christopher R. Spence OCP MCSE MCP A+ RAPTOR CNA Oracle DBA
Phone: (978) 322-5744
Fax: (707) 885-2275

Fuelspot
73 Princeton Street
North, Chelmsford 01863

-----Original Message-----

Sent: Tuesday, August 07, 2001 12:37 PM
To: Multiple recipients of list ORACLE-L

Hi there,

Unfortunately you can't just throw things like FIRST_ROWS at a select statement to make it run faster. The person who wrote the statement should have taken into account such things as:

Size of tables, ordering, which is the driving table
Indexes, are the being used, running statements through explain plan will show you where the bottlenecks are...failing that TKPROF will identify potential problem areas with your database.
Size of queries, joins etc.

Too many developers do not write code to be efficient, as long as it returns the correct values it doesn't matter whether it takes 2 minutes or 2 hours...

I've spent a number of hours/days/weeks/months tidying up poorly optimised code, it's an absolute nightmare for the DBAs and it doesn't look good on the developers.

Cheers,
Kev.
(a cheesed off developer...not a DBA!)

Kevin Thomas
Technical Analyst
Deregulation Services
Calanais Ltd.
Tel: 0141 568 2314
Fax: 0141 568 2366
http://www.calanais.com

-----Original Message-----

Sent: 07 August 2001 17:06
To: Multiple recipients of list ORACLE-L

Morning listers!

I am having performance problems with this database, transactions ar running very slow and I am not sure if I have choose the right optimizer mode...

AIX 4.2.1, Oracle 7.3.4, 30 GB Database, 1 GB Real Memory, 500 MB SGA, 70 concurrent users, mostly OLTP transactions.

I have tunned init parameters the best I can we the resources I have:

db_files = 70
db_writers = 4
db_file_multiblock_read_count = 16
db_block_buffers = 57600
db_block_size = 8192

shared_pool_size = 157286400
processes = 200
dml_locks = 1500
log_buffer = 655360

sequence_cache_entries = 800
sequence_cache_hash_buckets = 89
log_checkpoint_interval = 80000

optimizer_mode=CHOOSE
session_cached_cursors = 300

sort_area_size=1048576
hash_area_size=262144
hash_multiblock_io_count=4
hash_join_enabled=TRUE

always_anti_join=HASH
job_queue_processes=8

35 rollback segments, inital 1MB, next 1MB, optimal 30 MB 8 multiplexed redologs, 30 MB each

I tried FIRST_ROWS, analyzing the tables, but users claimed that performance was getting worse, so I chaged it to Choose. Always analyzing the tables....

but, everytime I analyze the tables, performance gets worse. Is this a normal behavior?

Any advice will be welcome!

thanks is advance,

Saludos,
Veronica Levin Enriquez
Administrador AIX
Compañía Cervecera de Nicaragua
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Author: Veronica Levin
INET: vlevin_at_victoria.com.ni

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Author: Thomas, Kevin
INET: Kevin.Thomas_at_calanais.com

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