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Re: Shared memory for large PostGIS operations



Hi Kevin, List, others...

On Mar 16, 2012, at 7:17 AM, Kevin Grittner wrote:

Andy Colson <andy@xxxxxxxxxxxxxxx> wrote:

I tried shared_buffers at both 2400M and 18000M, and it took 4.5
hours both times.  ... (weak attempts at humor omitted) ....

All else being the same, adjusting shared_buffers affects how much
of your cache is managed by PostgreSQL and how much of your cache is
managed by the OS; it doesn't exactly change how much you have
cached or necessarily affect disk waits.  (There's a lot more that
can be said about the fine points of this, but you don't seem to
have sorted out the big picture yet.)

Linux caching is aggressive already.. so I think this example points out that Postgres caching is not contributing here.. thats why I posted this short example to this list.. I thought ti was a useful data point.. that it might be
useful to others... and to the PostgreSQL project devs...


I heard of this program called vmstat that I'll read up on and
post some results for. -----ignore- I dont take advice with vinegar well...

That's a good way to get a handle on whether your bottleneck is
currently CPU or disk access.

(attempted insults omitted)

If you're looking to make things faster (a fact not yet exactly in
evidence), you might want to start with the query which runs the
longest, or perhaps the one which most surprises you with its run
time, and get the EXPLAIN ANALYZE output for that query.  There is
other information you should include; this page should help:

http://wiki.postgresql.org/wiki/SlowQueryQuestions


  some of the queries have been gone over fairly well, other not..
Its a complex sequence and we are in production mode here,
so I dont get a chance to do everything I might do with regard to
one particular query...



I just learned about http://explain.depesz.com/ and figure it
might help me.

It is a nice way to present EXPLAIN ANALYZE output from complex
queries.


  explain.depesz.com  definitely a good reference, thank you for that..


==
Brian Hamlin
GeoCal
OSGeo California Chapter
415-717-4462 cell




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