On 22/08/2018 08:20, Noah Misch wrote:
> Regardless of the choice of jit={on|off} default, these numbers tell me that
> some or all of jit_*_cost defaults are too low.
That was also my earlier analysis.
I'm suspicious that we haven't had much feedback about this. We've
heard of one or two cases where LLVM broke a query outright, and that
was fixed and that was a good result. But we haven't heard anything
about performance regressions. Surely there must be some. There hasn't
been any discussion or further analysis of the default cost settings
either. I feel that we don't have enough information.
Another problem is that LLVM is only enabled in some versions of
packages. For example, in the PGDG RPMs, it's enabled for RHEL 7 but
not RHEL 6. So you could be in for a surprise if you upgrade your
operating system at some point.
I would like, however, that we make a decision one way or the other
before the next beta. I've been handwaving a bit to users not to rely
on the current betas for performance testing because the defaults might
change later. That's bad either way.
FWIW; Our largest report-queries perform worse (then v10) with jit=on; https://www.postgresql.org/message-id/VisenaEmail.24.e60072a07f006130.162d95c3e17%40tc7-visena
Disabling JIT makes them perform slightly better than v10.
