Re: pgsql: Again fix initialization of auto-tuned effective_cache_size.

Andres Freund <andres@2ndquadrant.com> writes:
> On 2014-03-20 16:58:42 +0000, Tom Lane wrote:
>> Again fix initialization of auto-tuned effective_cache_size.

> This still is pretty ugly. Couldn't we just have a function in
> costsize.c that returns the actual value and computes it based on
> NBuffers if set to -1? Which is also called by a show_hook?

I agree that it's ugly, but what you're suggesting would have failure
modes of its own.  For example, reading out the value and then
re-assigning what that says would change the actual stored value from -1
to something else, resulting in the value ceasing to track the underlying
GUC.

In the particular example at hand, that might not matter, since
shared_buffers can't change anymore after the initial read of
postgresql.conf.  But it's certainly no good as a prototype solution for
other GUCs, which I think is the main criterion for ugliness here.

Basically, we do not have a good design for GUCs whose default is supposed
to change based on other GUCs.  I'd like to see that addressed in a more
general fashion, but I don't know what the solution will be.

Perhaps the real short-term question is whether we should revert this
whole change in effective_cache_size's behavior until we do have that
better design.

            regards, tom lane