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From: Andres Freund <andres@anarazel.de>
To: pgsql-hackers@lists.postgresql.org,Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Pavan Deolasee <pavan.deolasee@gmail.com>
Cc: amul sul <sulamul@gmail.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>
Cc: Stephen Frost <sfrost@snowman.net>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Restrict concurrent update/delete with UPDATE of partition key
Date: Thu, 08 Mar 2018 10:48:52 -0800
Message-ID: <44DBC47E-36ED-4754-88EB-EBFC3FD8B45C@anarazel.de> (raw)
In-Reply-To: <21103.1520534813@sss.pgh.pa.us>
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	<10896.1520521635@sss.pgh.pa.us>
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	<8293.1520528639@sss.pgh.pa.us>
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	<21103.1520534813@sss.pgh.pa.us>



On March 8, 2018 10:46:53 AM PST, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>Robert Haas <robertmhaas@gmail.com> writes:
>> On Thu, Mar 8, 2018 at 12:03 PM, Tom Lane <tgl@sss.pgh.pa.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.
>
>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.

We're pretty much doing so for speculative lock IDs/upsert already. Which doesn't seem to have caused a lot of problems.

Andres 
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.




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