Re: JIT performance bug/regression & JIT EXPLAIN

From: Andres Freund <andres(at)anarazel(dot)de>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: JIT performance bug/regression & JIT EXPLAIN
Date: 2019-11-13 20:03:18
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On 2019-11-13 14:29:07 -0500, Robert Haas wrote:
> On Mon, Oct 28, 2019 at 7:21 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > Because that's the normal way to represent something non-existing for
> > formats like json? There's a lot of information we show always for !text
> > format, even if not really applicable to the context (e.g. Triggers for
> > select statements). I think there's an argument to made to deviate in
> > this case, but I don't think it's obvious.
> I've consistently been of the view that anyone who thinks that the
> FORMAT option should affect what information gets displayed doesn't
> understand the meaning of the word "format." And I still feel that
> way.

Well, it's not been that way since the format option was added, so ...

> I also think that conditionally renaming "Output" to "Project" is a
> super-bad idea. The idea of a format like this is that the "keys" stay
> constant and the values change. If you need to tell people more, you
> add more keys.

Yea, I don't like the compat break either. But I'm not so convinced
that just continuing to collect cruft because of compatibility is worth
it - I just don't see an all that high reliance interest for explain

I think adding a new key is somewhat ok for !text, but for text that
doesn't seem like an easy solution?

I kind of like my idea somewhere downthread, in a reply to Maciek, of
simply not listing "Output" for nodes that don't project. While that's
still a format break, it seems that tools already need to deal with
"Output" not being present?

> I also think that making the Filter field a group conditionally is a
> bad idea, for similar reasons.

Oh, yea, it's utterly terrible (I called it crappy in my email :)).

> But making it always be a group doesn't necessarily seem like a bad
> idea. I think, though, that you could handle this in other ways, like
> by suffixing existing keys. e.g. if you've got Index-Qual and Filter,
> just do Index-Qual-JIT and Filter-JIT and call it good.

Maciek suggested the same. But to me it seems going down that way will
make the format harder and harder to understand? So I think I'd rather
break compat here, and go for a group.

Personally I think the group naming choice for explain makes the the
!text outputs much less useful than they could be - we basically force
every tool to understand all possible keys, to make sense of formatted
output. Instead of something like 'Filter: {"Qual":{"text" : "...",
"JIT": ...}' where a tool only needed to understand that everything that
has a "Qual" inside is a filtering expression, everything that has a
"Project" is a projecting type of expression, ... a tool needs to know
about "Inner Cond", "Order By", "Filter", "Recheck Cond", "TID Cond",
"Join Filter", "Merge Cond", "Hash Cond", "One-Time Filter", ...


Andres Freund

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