Re: [HACKERS] Restrict concurrent update/delete with UPDATE of partition key

From: Andres Freund <andres(at)anarazel(dot)de>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Pavan Deolasee <pavan(dot)deolasee(at)gmail(dot)com>, amul sul <sulamul(at)gmail(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Stephen Frost <sfrost(at)snowman(dot)net>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [HACKERS] Restrict concurrent update/delete with UPDATE of partition key
Date: 2018-03-28 16:35:41
Message-ID: 20180328163541.l5ksnqmpn6bl4rg2@alap3.anarazel.de
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Hi,

On 2018-03-08 13:46:53 -0500, Tom Lane wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> > On Thu, Mar 8, 2018 at 12:03 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >> FWIW, I would also vote for (1), especially if the only way to do (2)
> >> is stuff as outright scary as this. I would far rather have (3) than
> >> this, because IMO, what we are looking at right now is going to make
> >> the fallout from multixacts look like a pleasant day at the beach.
>
> > Whoa. Well, that would clearly be bad, but I don't understand why you
> > find this so scary. Can you explain further?
>
> Possibly I'm crying wolf; it's hard to be sure. But I recall that nobody
> was particularly afraid of multixacts when that went in, and look at all
> the trouble we've had with that. Breaking fundamental invariants like
> "ctid points to this tuple or its update successor" is going to cause
> trouble. There's a lot of code that knows that; more than knows the
> details of what's in xmax, I believe.

Given, as explained nearby, we already do store transient data in the
ctid for speculative insertions (i.e. ON CONFLICT), and it hasn't caused
even a whiff of trouble, I'm currently not inclined to see a huge issue
here. It'd be great if you could expand on your concerns here a bit, we
gotta figure out a way forward.

I think the proposed patch needs some polish (I'm e.g. unhappy with the
naming of the new macros etc), but I think otherwise it's a reasonable
attempt at solving the problem.

> I would've been happier about expending an infomask bit towards this
> purpose. Just eyeing what we've got, I can't help noticing that
> HEAP_MOVED_OFF/HEAP_MOVED_IN couldn't possibly be set in any tuple
> in a partitioned table. Perhaps making these tests depend on
> partitioned-ness would be unworkably messy, but it's worth thinking
> about.

They previously couldn't be set together IIRC, so we could just (mask &
(HEAP_MOVED_OFF |HEAP_MOVED_IN)) == (HEAP_MOVED_OFF |HEAP_MOVED_IN) but
that'd be permanently eating two infomask bits. For something that
doesn't in general have to be able to live on tuples, just on (at?) the
"deleted tuple at end of a chain".

Greetings,

Andres Freund

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