Re: Intermitent slow queries

On 2-May-07, at 11:24 AM, Parks, Aaron B. wrote:

> My pg 8.1 install on an AMD-64 box (4 processors) with 9 gigs of
> ram running RHEL4 is acting kind of odd and I thought I would see
> if anybody has any hints.
>
>
>
> I have Java program using postgresql-8.1-409.jdbc3.jar to connect
> over the network. In general it works very well. I have run batch
> updates with several thousand records repeatedly that has worked fine.
>
>
>
> The Program pulls a summation of the DB and does some processing
> with it. It starts off wonderfully running a query every .5
> seconds. Unfortunately, after a while it will start running
> queries that take 20 to 30 seconds.
>
>
>
> Looking at the EXPLAIN for the query no sequential scans are going
> on and everything has an index that points directly at its search
> criteria.
>
>
>
> Example:
>
>
>
> Select sum(whatever) from a inner join b on a.something=b.something
> WHERE b.day=1 and b.hour=1
>
> Select sum(whatever) from a inner join b on a.something=b.something
> WHERE b.day=1 and b.hour=2
>
> Select sum(whatever) from a inner join b on a.something=b.something
> WHERE b.day=1 and b.hour=3
>
> .
>
> .
>
> Select sum(whatever) from a inner join b on a.something=b.something
> WHERE b.day=1 and b.hour=23
>
> Select sum(whatever) from a inner join b on a.something=b.something
> WHERE b.day=1 and b.hour=24
>
> Select sum(whatever) from a inner join b on a.something=b.something
> WHERE b.day=2 and b.hour=1
>
> Select sum(whatever) from a inner join b on a.something=b.something
> WHERE b.day=2 and b.hour=2
>
> .
>
> .
>
> .
>
>
>
>
>
> This query runs fine for a while (up to thousands of times). But
> what happens is that it starts to have really nasty pauses when you
> switch the day condition. After the first query with the day it
> runs like a charm for 24 iterations, then slows back down again
>
>
>
> My best guess was that an index never finished running, but REINDEX
> on the table (b in this case) didn’t seem to help.
I'd think it has more to do with caching data. The first query caches
the days data, then the next day's data has to be read from disk.
>
>
> Ideas?
>
>
>
> AP
>
>