Date: Mon, 18 Jan 2021 17:25:49 -0300
From: =?iso-8859-1?Q?=C1lvaro?= Herrera <alvherre@alvh.no-ip.org>
To: Matthias van de Meent <boekewurm+postgres@gmail.com>
Cc: Masahiko Sawada <sawada.mshk@gmail.com>,
	Hamid Akhtar <hamid.akhtar@gmail.com>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: {CREATE INDEX, REINDEX} CONCURRENTLY improvements
Message-ID: <20210118202549.GA25552@alvherre.pgsql>
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On 2021-Jan-18, Matthias van de Meent wrote:

> Example:
> 
> 1.) RI starts
> 2.) PHASE 2: filling the index:
> 2.1.) scanning the heap (live tuple is cached)
> < tuple is deleted
> < last transaction other than RI commits, only snapshot of RI exists
> < vacuum drops the tuple, and cannot remove it from the new index
> because this new index is not yet populated.
> 2.2.) sorting tuples
> 2.3.) index filled with tuples, incl. deleted tuple
> 3.) PHASE 3: wait for transactions
> 4.) PHASE 4: validate does not remove the tuple from the index,
> because it is not built to do so: it will only insert new tuples.
> Tuples that are marked for deletion are removed from the index only
> through VACUUM (and optimistic ALL_DEAD detection).
> 
> According to my limited knowledge of RI, it requires VACUUM to not run
> on the table during the initial index build process (which is
> currently guaranteed through the use of a snapshot).

VACUUM cannot run concurrently with CIC or RI in a table -- both acquire
ShareUpdateExclusiveLock, which conflicts with itself, so this cannot
occur.

I do wonder if the problem you suggest (or something similar) can occur
via HOT pruning, though.

-- 
Álvaro Herrera       Valdivia, Chile