Date: Wed, 14 Oct 2020 20:43:26 -0500
From: Justin Pryzby <pryzby@telsasoft.com>
To: David Rowley <dgrowleyml@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>,
	Andres Freund <andres@anarazel.de>
Subject: Re: jit and explain nontext
Message-ID: <20201015014326.GU9241@telsasoft.com>
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On Thu, Oct 15, 2020 at 02:23:01PM +1300, David Rowley wrote:
> On Thu, 15 Oct 2020 at 14:15, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > David Rowley <dgrowleyml@gmail.com> writes:
> > > Just for some reference. Some wisdom was shared in [1], which made a
> > > lot of sense to me.
> > > If we apply that, then we just need to decide if displaying any jit
> > > related fields without any jitted expressions is relevant.
> >
> > Hmm, I dunno if my opinion counts as "wisdom", but what I was arguing for
> > there was that we should print stuff if it's potentially invoked by a
> > run-time decision, but not if it was excluded at plan time.  I'm not
> > totally clear on whether jitting decisions are fixed by the plan tree
> > (including its cost values) or if the executor can make different
> > decisions in different executions of the identical plan tree.
> > If the latter, then I agree with Justin that this is a bug.
> 
> As far as I know, the only exception where the executor overwrites the
> planner's decision is in nodeValuesscan.c where it turns jit off
> because each VALUES will get evaluated just once, which would be a
> waste of effort to JIT.
> 
> Apart from that the choice is baked in by the planner and set in
> PlannedStmt.jitfFlags.

What about the GUCs themselves ?

They can change after planning, which means a given execution of a plan might
or might not use jit.

-- 
Justin