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From: Pavel Borisov <pashkin.elfe@gmail.com>
Date: Thu, 12 Mar 2026 02:35:23 +0400
Message-ID: 
 <CALT9ZEFnE_aJ=nr5nMSxLYsNCfnKVOPFbiMZo5T_VoMb-sjO6Q@mail.gmail.com>
Subject: Re: Odd code around ginScanToDelete
To: Alexander Korotkov <aekorotkov@gmail.com>
Cc: Xuneng Zhou <xunengzhou@gmail.com>, Andres Freund <andres@anarazel.de>,
	pgsql-hackers@postgresql.org
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On Thu, 12 Mar 2026 at 02:22, Alexander Korotkov <aekorotkov@gmail.com> wro=
te:
>
> On Tue, Mar 10, 2026 at 11:29=E2=80=AFAM Pavel Borisov <pashkin.elfe@gmai=
l.com> wrote:
> > Hi, Xuneng
> >
> > > > Is it worth/possible in recursive calls of ginScanToDelete() to fre=
e
> > > > allocated myStackItem->child after processing all children of the
> > > > current level, when they are not needed anymore?
> > > > Previously to this patch, palloc-ed "me" variable also was't freed =
at
> > > > recursion levels.
> > >
> > > Freeing/reallocating it per subtree would add churn and make the
> > > lifetime rules harder to reason about without meaningful memory
> > > savings (the number of nodes is bounded by tree depth, not number of
> > > pages). We currently free the chain once after ginScanToDelete()
> > > returns in ginVacuumPostingTree(), which matches the natural lifetime
> > > boundary
> > I proposed not freeing child when child iteration is complete. They
> > indeed can be reused. I proposed cleaning children when "my" iteration
> > is complete. At that time all the children iterations are completed
> > and not needed when we return level up.
> This is not clear for me.  We need stack items to keep track of left
> pages until we scan the whole posting tree.  After scanning the whole
> posting tree we can free stack items as we do now.

Hi, Alexander!
You are right, that we can free all posting tree stack items after the
whole tree, as we do now. But I think we can also do it earlier. It
looks like all "children" items are needed and could be reused only
until iteration on "my" level ends. When function returns up the
recursion "my" level becomes "child" for a caller, and previous
"child" is not used anymore.

It's optional, maybe we can do freeing at the end of posting tree iteration=
.

Regards,
Pavel Borisov