From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
cc: Michael Paquier <michael.paquier@gmail.com>,
        Tomas Vondra <tomas.vondra@2ndquadrant.com>,
        PostgreSQL mailing lists <pgsql-hackers@postgresql.org>
Subject: Re: Re: PANIC: invalid index offnum: 186 when processing BRIN indexes
 in VACUUM
In-reply-to: <20171031181053.xh7hzminpv6rvwr2@alvherre.pgsql>
References: <20171031181053.xh7hzminpv6rvwr2@alvherre.pgsql>
Comments: In-reply-to Alvaro Herrera <alvherre@alvh.no-ip.org>
	message dated "Tue, 31 Oct 2017 19:10:53 +0100"
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Date: Tue, 31 Oct 2017 15:09:11 -0400
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Alvaro Herrera <alvherre@alvh.no-ip.org> writes:
> Tom Lane wrote:
>> I really don't understand how any of this "let's release the buffer
>> lock and then take it back later" logic is supposed to work reliably.

> So summarize_range first inserts the placeholder tuple, which is there
> purposefully for other processes to update concurrently; then, without
> blocking any other process, scan the page range and update the
> placeholder tuple (this could take a long time, so it'd be a bad idea
> to hold buffer lock for that long).

Yeah, agreed, and your upthread point about avoiding deadlock when we
need to take two buffer locks at the same time is also something that
it's hard to see any other way around.  Maybe we just have to find a
way to make the existing structure work.  The sticking point is that
not only might the tuple we expected have been deleted, but someone
could have put an unrelated tuple in its place.  Are you confident
that you can detect that and recover from it?  If you're sure that
brin_tuples_equal() is trustworthy for this purpose, then maybe
we just have some run-of-the-mill bugs to find, like the off-the-end
bug I spotted in brin_doupdate.  There's apparently at least one
more, but given the error message it must be something like not
checking for a page to have turned into a revmap page.  Shouldn't
be too hard to find...

			regards, tom lane


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