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Performance - moving from oracle to postgresql

From: "Greg Maples" <gregm(at)nimblefish(dot)com>
To: <pgsql-performance(at)postgresql(dot)org>
Subject: Performance - moving from oracle to postgresql
Date: 2005-06-24 19:49:51
Message-ID: 2F9239F5E316C54688FDA9814FF045D6AB0024@EXCHANGE.sf.nimblefish.com (view raw or flat)
Thread:
Lists: pgsql-performance
Hi:
	I'm beginning the push at our company to look at running 
postgreSQL in production here.  We have a dual CPU 2.8 GHZ Xeon 
Box running oracle.  Typical CPU load runs between 20% and 90%.
Raw DB size is about 200GB.  We hit the disk at roughly 15MB/s
read volume and 3MB/s write.
	At any given time we have from 2 to 70 sessions running
on the instance.  Sessions often persist for 24 hours or more.

                  Total 	 Free 	 Free
                  Mb           Mb          %
	
 IDXS_EXT10  	2000        290 	 14.5 	
 DATA_EXT100  	10000 	3200 	 32 	
 SYSTEM  	      220 	      95.2 	 43.3
 IDXS_EXT100  	20000 	9600 	 48
 DATA_EXT10  	6000 	      2990 	 49.8
 UNDOB  	      4000   	2561.1 64
 TEMP       	8000 	      5802.9 72.5	
 DATA_LOB_EXT20  	2000 	      1560 	 78
 IDXS_EXT1  	500 	      401 	 80.2
 DATA_EXT1  	4000 	      3758 	 94
Total Instance 	56720 	30258.2 	53.3 	 


There are some immediate questions from our engineers about performance

"- Oracle has one particular performance enhancement that Postgres is
missing.  If you do a select that returns 100,000 rows in a given order,
and all you want are rows 99101 to 99200, then Oracle can do that very
efficiently.  With Postgres, it has to read the first 99200 rows and
then discard the first 99100.  But...  If we really want to look at
performance, then we ought to put together a set of benchmarks of some
typical tasks."

Is this accurate:
accoring to
http://www.postgresql.org/docs/8.0/interactive/queries-limit.html
  -- " The rows skipped by an OFFSET clause still have to be computed 
inside the server; therefore a large OFFSET can be inefficient."


What are the key performance areas I should be looking at?
Where is psql not appropriate to replace Oracle?

Thanks in advance, apologies if this occurs as spam, please send
Replies to me off-list.  

Responses

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Subject: Re: max_connections / shared_buffers / effective_cache_size questions
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Subject: Re: max_connections / shared_buffers /

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