Date: Sun, 16 Feb 2020 23:49:49 -0600
From: Justin Pryzby <pryzby@telsasoft.com>
To: Amit Langote <amitlangote09@gmail.com>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>, pgsql-hackers@postgresql.org
Subject: Re: ALTER tbl rewrite loses CLUSTER ON index (consider moving
 indisclustered to pg_class)
Message-ID: <20200217054949.GN31889@telsasoft.com>
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On Mon, Feb 17, 2020 at 02:31:42PM +0900, Amit Langote wrote:
> Hi Justin,
> 
> On Fri, Feb 7, 2020 at 11:39 PM Justin Pryzby <pryzby@telsasoft.com> wrote:
> > On Thu, Feb 06, 2020 at 02:24:47PM -0300, Alvaro Herrera wrote:
> > > On 2020-Feb-06, Justin Pryzby wrote:
> > >
> > > > I wondered if it wouldn't be better if CLUSTER ON was stored in pg_class as the
> > > > Oid of a clustered index, rather than a boolean in pg_index.
> > >
> > > Maybe.  Do you want to try a patch?
> >
> > I think the attached is 80% complete (I didn't touch pg_dump).
> >
> > One objection to this change would be that all relations (including indices)
> > end up with relclustered fields, and pg_index already has a number of bools, so
> > it's not like this one bool is wasting a byte.
> >
> > I think relisclustered was a's clever way of avoiding that overhead (c0ad5953).
> > So I would be -0.5 on moving it to pg_class..

In case there's any confusion: "a's" was probably me halfway changing
"someone's" to "a".

> Are you still for fixing ALTER TABLE losing relisclustered with the
> patch we were working on earlier [1], if not for moving relisclustered
> to pg_class anymore?

Thanks for remembering this one.

I think your patch is the correct fix.

I forgot to say it, but moving relisclustered to pg_class doesn't help to avoid
losting indisclustered: it still needs a fix just like this.  Anyway, I
withdrew my suggestion for moving to pg_class, since it has more overhead, even
for pg_class rows for relations which can't have indexes.

> I have read elsewhere [2] that forcing ALTER TABLE to rewrite in
> clustered order might not be a good option, but maybe that one is a
> more radical proposal than this.

Right; your fix seems uncontroversial.  I ran into this (indisclustered) bug
while starting to write that patch for "ALTER rewrite in clustered order".

-- 
Justin