Re: bad JIT decision

Hi,

On 2020-07-25 10:54:18 -0400, Tom Lane wrote:
> David Rowley <dgrowleyml(at)gmail(dot)com> writes:
> > ... nested at the bottom level join, about 6 joins deep. The lack of
> > any row being found results in upper level joins not having to do
> > anything, and the majority of the plan is (never executed).
>
> On re-reading this, that last point struck me forcibly. If most of
> the plan never gets executed, could we avoid compiling it? That is,
> maybe JIT isn't JIT enough, and we should make compilation happen
> at first use of an expression not during executor startup.

That unfortunately has its own downsides, in that there's significant
overhead of emitting code multiple times. I suspect that taking the
cost of all the JIT emissions together into account is the more
promising approach.

Greetings,

Andres Freund