On Mon, Feb 25, 2019 at 1:27 AM Kyotaro HORIGUCHI
<horiguchi.kyotaro@lab.ntt.co.jp> wrote:
> > I'd like to see some evidence that catalog_cache_memory_target has any
> > value, vs. just always setting it to zero.
>
> It is artificial (or acutually wont't be repeatedly executed in a
> session) but anyway what can get benefit from
> catalog_cache_memory_target would be a kind of extreme.
I agree. So then let's not have it.
We shouldn't add more mechanism here than actually has value. It
seems pretty clear that keeping cache entries that go unused for long
periods can't be that important; even if we need them again
eventually, reloading them every 5 or 10 minutes can't hurt that much.
On the other hand, I think it's also pretty clear that evicting cache
entries that are being used frequently will have disastrous effects on
performance; as I noted in the other email I just sent, consider the
effects of CLOBBER_CACHE_ALWAYS. No reasonable user is going to want
to incur a massive slowdown to save a little bit of memory.
I see that *in theory* there is a value to
catalog_cache_memory_target, because *maybe* there is a workload where
tuning that GUC will lead to better performance at lower memory usage
than any competing proposal. But unless we can actually see an
example of such a workload, which so far I don't, we're adding a knob
that everybody has to think about how to tune when in fact we have no
idea how to tune it or whether it even needs to be tuned. That
doesn't make sense. We have to be able to document the parameters we
have and explain to users how they should be used. And as far as this
parameter is concerned I think we are not at that point.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company