Re: JIT compiling with LLVM v9.0

On Wed, Jan 31, 2018 at 10:22 AM, Peter Eisentraut
<peter(dot)eisentraut(at)2ndquadrant(dot)com> wrote:
> On 1/30/18 21:55, Andres Freund wrote:
>> I'm open to changing my mind on it, but it seems a bit weird that a
>> feature that relies on a shlib being installed magically turns itself on
>> if avaible. And leaving that angle aside, ISTM, that it's a complex
>> enough feature that it should be opt-in the first release... Think we
>> roughly did that right for e.g. parallellism.
>
> That sounds reasonable, for both of those reasons.

The first one is a problem that's not going to go away. If the
problem of JIT being enabled "magically" is something we're concerned
about, we need to figure out a good solution, not just disable the
feature by default.

As far as the second one, looking back at what happened with parallel
query, I found (on a quick read) 13 back-patched commits in
REL9_6_STABLE prior to the release of 10.0, 3 of which I would qualify
as low-importance (improving documentation, fixing something that's
not really a bug, improving a test case). A couple of those were
really stupid mistakes on my part. On the other hand, would it have
been overall worse for our users if that feature had been turned on in
9.6? I don't know. They would have had those bugs (at least until we
fixed them) but they would have had parallel query, too. It's hard
for me to judge whether that was a win or a loss, and so here. Like
parallel query, this is a feature which seems to have a low risk of
data corruption, but a fairly high risk of wrong answers to queries
and/or strange errors. Users don't like that. On the other hand,
also like parallel query, if you've got the right kind of queries, it
can make them go a lot faster. Users DO like that.

So I could go either way on whether to enable this in the first
release. I definitely would not like to see it stay disabled by
default for a second release unless we find a lot of problems with it.
There's no point in developing new features unless users are going to
get the benefit of them, and while SOME users will enable features
that aren't turned on by default, many will not.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company