time to stop tuning? - Mailing list pgsql-performance

We have a network application in which many clients will be executing a mix of select/insert/update/deletes on a
centralpostgres 7.4.5 database, running on Solaris 9 running on dual 2.3 ghz Xeons, with 2 gig of RAM and a RAID 10
disk.The test database is about 400 meg in size. 

We have tuned the postgresql.conf parameters to the point where we are confident we have enough memory for shared
buffersand for sorting. We are still tuning SQL statements, but we're pretty sure the big wins have been achieved. 

We are maxing out on the backend with 30 postmaster processes, each taking up about 2.5-3% of the CPU. We have tested
mountingthe whole database in /tmp, hence in memory, and it has made no difference in performance, so it seems we are
purelyCPU bound at this point. 

About 70% of our time is spent in selects, and another 25% spent in inserts/updates of a single table (about 10% out of
theselects % is against this table). 

Now, our application client is not doing nearly enough of it's own caching, so a lot the work the database is doing
currentlyis redundant, and we are working on the client, but in the meantime we have to squeeze as much as we can from
thebackend. 

After that long intro, I have a couple of questions:

1) Given that the data is all cached, what can we do to make sure that postgres is generating
the most efficient plans in this case? We have bumped up effective_cache_size, but it had no
effect. Also, what would the most efficient plan for in-memory data look like? I mean, does one
still look for the normal stuff - index usage, etc., or are seqscans what we should be looking for?
I've seen some stuff about updating statistics targets for specific tables, but I'm not sure I
understand it, and don't know if something like that applies in this case. I can supply some specific plans, if that
wouldhelp (this email is already too long...). 

2) We have SQL test environment where we just run the SQL statements executed by the clients (culled from the log file)
inpsql. In our test environment, the same set of SQL statements runs 4X faster that the times achieved in the test that
generatedour source log file. Obviously there was a bigger load on the machine in the full test, but I'm wondering if
thereare any particular diagnostics that I should be looking at to ferret out contention. I haven't seen anything that
lookedsuspicious in pg_locks, but it's difficult to interpret that data when the database is under load (at least for
someoneof my limited experience). 

I suspect the ultimate answer to our problem will be:

   1) aggressive client-side caching
   2) SQL tuning
   3) more backend hardware

But I would grateful to hear any tips/anecdotes/experiences that others might have from tuning similar applications.

Thanks!

- DAP
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David Parker    Tazz Networks    (401) 709-5130