Re: Including Snapshot Info with Indexes

On Sat, Oct 20, 2007 at 09:24:07AM +0530, Gokulakannan Somasundaram wrote:
> Hi,
> I think i have a initial Implementation. It has some bugs and i am working
> on fixing it. But to show the advantages, I want to show the number of
> Logical I/Os on the screen. In order to show that, i tried enabling the
> log_statement option in PostgreSQL.conf. But it shows only the physical
> reads. What i wanted was a Logical reads count( No. of ReadBuffer calls,
> which is stored in ReadBufferCount variable). So i have added this stats to
> the bufmgr.c(function is BufferUsage, i suppose) to show Logical Reads and
> Physical Reads. Is this a acceptable change?

I'm not sure if the number of logical reads is really a useful
measurement. I can imagine there are places that deliberatly read the
block "logically" a few times but drop the pin in between to allow
others access. This will skew your results as in actual usage only the
first is likely to generate a real I/O.

If your problem is cache it seems to me you should test with a table
larger than your shared buffers and perhaps even larger than your total
memory, since this is the case we're actually interested in.

Have a ncie day,
--
Martijn van Oosterhout <kleptog(at)svana(dot)org> http://svana.org/kleptog/
> From each according to his ability. To each according to his ability to litigate.