Date: Wed, 13 Jan 2021 14:33:43 -0300
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: James Coleman <jtc331@gmail.com>
Cc: Michael Paquier <michael@paquier.xyz>,
	David Johnston <david.g.johnston@gmail.com>,
	PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>,
	Andres Freund <andres@anarazel.de>
Subject: Re: [DOC] Document concurrent index builds waiting on each other
Message-ID: <20210113173343.GA31409@alvherre.pgsql>
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On 2021-Jan-13, James Coleman wrote:

> On Wed, Jan 13, 2021 at 12:58 AM Michael Paquier <michael@paquier.xyz> wrote:
> >
> > On Tue, Jan 12, 2021 at 04:51:39PM -0300, Alvaro Herrera wrote:
> > > I looked into this again, and I didn't like what I had added to
> > > maintenance.sgml at all.  It seems out of place where I put it; and I
> > > couldn't find any great spots.  Going back to your original proposal,
> > > what about something like this?  It's just one more para in the "notes"
> > > section in CREATE INDEX and REINDEX pages, without any additions to the
> > > VACUUM pages.
> >
> > +1.
> 
> I think one more para in the notes is good. But shouldn't we still
> clarify the issue is specific to CONCURRENTLY?

How is it specific to concurrent builds?  What we're documenting here is
the behavior of vacuum, and that one is identical in both regular builds
and concurrent builds (since vacuum has to avoid removing rows from
under either of them).  The only reason concurrent builds are
interesting is because they take longer.

What was specific to concurrent builds was the fact that you can't have
more than one at a time, and that one is what was added in 58ebe967f.

> Also that it's not just the table being indexed seems fairly significant.

This is true.  So I propose

    Like any long-running transaction, <command>REINDEX</command> can
    affect which tuples can be removed by concurrent <command>VACUUM</command>
    on any table.

-- 
Álvaro Herrera                            39°49'30"S 73°17'W