Re: Tuning to speed select

Michael,
Great suggestion. I've read about CLUSTER, but never had a chance to use
it. The only problem is that this table with 9 million records has 5 or
6 indexes. It is hard to pick the most used, but I'll bet CLUSTER will
make at least one of the queries run very fast, especially for an index
with a small number of distinct values.

The speed of the query is (as Michael implies) limited to the rate at
which the disk can seek and read. I have done experiments with views
and cursors; there was no improvement in speed. I've also tried only
pulling back primary keys in the hope that a smaller amount of data
would more quickly be read into memory. No speed increase. I have also
raised all the usual memory limits, with the expected results (slight
speed improvements).

I'll try CLUSTER (I'm looking forward to that test), but if we really
need speed, it will probably be necessary to create copies of the table,
or copy portions of the table elsewhere (essentially creating
materialized views, I suppose). I'm still trying to get my science
compatriot here to tell me which index he most wants to improve, then
I'll CLUSTER the table on that index.

Thanks!
Tom

Michael Fuhr wrote:

>On Wed, Aug 09, 2006 at 03:46:38PM +0200, Martijn van Oosterhout wrote:
>
>
>>On Wed, Aug 09, 2006 at 09:19:31AM -0400, Tom Laudeman wrote:
>>
>>
>>>Is there a tuning parameter I can change to increase speed of selects?
>>>Clearly, there's already some buffering going on since selecting an
>>>indexed ~50,000 records takes 17 seconds on the first try, and only 0.5
>>>seconds on the second try (from pgsql).
>>>
>>>
>>Your OS is probably buffering, 1GB of RAM holds a lot of data. You can
>>try increasing the shared_buffers parameter, but if the delay is
>>getting data from the disk, that won't really help you.
>>
>>
>
>If most of your queries use the same index then clustering on that
>index might speed up initial (i.e., not-cached) queries by reducing
>the number of disk pages that need to be read. See the documentation
>for more information.
>
>http://www.postgresql.org/docs/8.1/interactive/sql-cluster.html
>
>
>

--
Tom Laudeman
twl8n(at)virginia(dot)edu
(434) 924-2456
http://www.people.virginia.edu/~twl8n/
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