Received: from malur.postgresql.org ([217.196.149.56])
	by arkaria.postgresql.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256)
	(Exim 4.89)
	(envelope-from <pgsql-hackers-owner+archive@lists.postgresql.org>)
	id 1iEORp-0002UK-Pf
	for pgsql-hackers@arkaria.postgresql.org; Sun, 29 Sep 2019 01:56:29 +0000
Received: from localhost ([127.0.0.1] helo=malur.postgresql.org)
	by malur.postgresql.org with esmtp (Exim 4.89)
	(envelope-from <pgsql-hackers-owner+archive@lists.postgresql.org>)
	id 1iEORo-0004ld-M9
	for pgsql-hackers@arkaria.postgresql.org; Sun, 29 Sep 2019 01:56:28 +0000
Received: from makus.postgresql.org ([2001:4800:3e1:1::229])
	by malur.postgresql.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256)
	(Exim 4.89)
	(envelope-from <bruce@momjian.us>)
	id 1iEORo-0004kp-9J
	for pgsql-hackers@lists.postgresql.org; Sun, 29 Sep 2019 01:56:28 +0000
Received: from momjian.us ([72.94.173.45])
	by makus.postgresql.org with esmtps (TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256)
	(Exim 4.92)
	(envelope-from <bruce@momjian.us>)
	id 1iEORl-00008w-Qr
	for pgsql-hackers@postgresql.org; Sun, 29 Sep 2019 01:56:27 +0000
Received: from bruce by momjian.us with local (Exim 4.92)
	(envelope-from <bruce@momjian.us>)
	id 1iEORk-0005GE-JW; Sat, 28 Sep 2019 21:56:24 -0400
Date: Sat, 28 Sep 2019 21:56:24 -0400
From: Bruce Momjian <bruce@momjian.us>
To: James Coleman <jtc331@gmail.com>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: [DOC] Document concurrent index builds waiting on each other
Message-ID: <20190929015624.GA19872@momjian.us>
References: <20190928161838.GD26853@momjian.us>
 <20190929012228.GA21412@alvherre.pgsql>
 <CAAaqYe-7n=eZU265QL_=4kueTggx4_Ewh_xgfRYEBrBpQDLyrA@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: 
 <CAAaqYe-7n=eZU265QL_=4kueTggx4_Ewh_xgfRYEBrBpQDLyrA@mail.gmail.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
List-Id: <pgsql-hackers.lists.postgresql.org>
List-Help: <https://lists.postgresql.org/manage/>
List-Subscribe: <https://lists.postgresql.org/manage/>
List-Post: <mailto:pgsql-hackers@lists.postgresql.org>
List-Owner: <mailto:pgsql-hackers-owner@lists.postgresql.org>
List-Archive: <https://www.postgresql.org/list/pgsql-hackers>
Precedence: bulk

On Sat, Sep 28, 2019 at 09:54:48PM -0400, James Coleman wrote:
> On Sat, Sep 28, 2019 at 9:22 PM Alvaro Herrera <alvherre@2ndquadrant.com> wrote:
> >
> > On 2019-Sep-28, Bruce Momjian wrote:
> >
> > > The CREATE INDEX docs already say:
> > >
> > >     In a concurrent index build, the index is actually entered into
> > >     the system catalogs in one transaction, then two table scans occur in
> > >     two more transactions.  Before each table scan, the index build must
> > >     wait for existing transactions that have modified the table to terminate.
> > >     After the second scan, the index build must wait for any transactions
> > > --> that have a snapshot (see <xref linkend="mvcc"/>) predating the second
> > > --> scan to terminate.  Then finally the index can be marked ready for use,
> > >
> > > So, having multiple concurrent index scans is just a special case of
> > > having to "wait for any transactions that have a snapshot", no?  I am
> > > not sure adding a doc mention of other index builds really is helpful.
> >
> > I always thought that create index concurrently was prevented from
> > running concurrently in a table by the ShareUpdateExclusive lock that's
> > held during the operation.
> 
> You mean multiple CICs on a single table at the same time? Yes, that
> (unfortunately) isn't possible, but I'm concerned in the patch with
> the fact that CIC on table X blocks CIC on table Y.

I think any open transaction will block CIC, which is my point.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

+ As you are, so once was I.  As I am, so you will be. +
+                      Ancient Roman grave inscription +