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Re: Why our counters need to be time-based WAS: WIP: cross column correlation ...

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Josh Berkus <josh(at)agliodbs(dot)com>, pgsql-hackers Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Why our counters need to be time-based WAS: WIP: cross column correlation ...
Date: 2011-02-28 19:31:53
Message-ID: 28234.1298921513@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Mon, Feb 28, 2011 at 1:50 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Ultimately we need to think of a reporting mechanism that's a bit
>> smarter than "rewrite the whole file for any update" ...

> Well, we have these things called "tables".  Any chance of using those?

Having the stats collector write tables would violate the classical form
of the heisenberg principle (thou shalt avoid having thy measurement
tools affect that which is measured), not to mention assorted practical
problems like not wanting the stats collector to take locks or run
transactions.

The ideal solution would likely be for the stats collector to expose its
data structures as shared memory, but I don't think we get to do that
under SysV shmem --- it doesn't like variable-size shmem much.  Maybe
that's another argument for looking harder into mmap or POSIX shmem,
although it's not clear to me how well either of those fixes that.

			regards, tom lane

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