Re: Really really slow select count(*)

On 02/07/2011 06:30 PM, Marti Raudsepp wrote:
> On Mon, Feb 7, 2011 at 05:03, Craig Ringer<craig(at)postnewspapers(dot)com(dot)au> wrote:
>> What would possibly help would be if Pg could fall back to lower
>> shared_buffers automatically, screaming about it in the logs but still
>> launching. OTOH, many people don't check the logs, so they'd think their
>> new setting had taken effect and it hadn't - you've traded one usability
>> problem for another. Even if Pg issued WARNING messages to each client
>> that connected, lots of (non-psql) clients don't display them, so many
>> users would never know.
>>
>> Do you have a suggestion about how to do this better? The current
>> approach is known to be rather unlovely, but nobody's come up with a
>> better one that works reasonably and doesn't trample on other System V
>> shared memory users that may exist on the system.
>
> We could do something similar to what Apache does -- provide distros
> with a binary to check the configuration file in advance. This check
> program is launched before the "restart" command, and if it fails, the
> server is not restarted.

That would work for config file errors (and would probably be a good
idea) but won't help with bad shared memory configuration. When Pg is
already running, it's usually not possible for a test program to claim
the amount of shared memory the config file says to allocate, because Pg
is already using it. Nonetheless, Pg will work fine when restarted.

--
Craig Ringer