Date: Tue, 28 Apr 2015 12:38:45 +0200
From: Andres Freund <andres@anarazel.de>
To: Peter Geoghegan <pg@heroku.com>, Tom Lane <tgl@sss.pgh.pa.us>
Cc: hlinnaka <hlinnaka@iki.fi>, Jeff Janes <jeff.janes@gmail.com>,
 Stephen Frost <sfrost@snowman.net>,
 Dean Rasheed <dean.a.rasheed@gmail.com>, Bruce Momjian <bruce@momjian.us>,
 Pg Hackers <pgsql-hackers@postgresql.org>
Subject: Re: INSERT ... ON CONFLICT UPDATE/IGNORE 4.0,
 parser/executor stuff
Message-ID: <20150428103845.GC19123@awork2.anarazel.de>
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On 2015-04-27 23:52:58 +0200, Andres Freund wrote:
> On 2015-04-27 16:28:49 +0200, Andres Freund wrote:
> > On 2015-04-26 18:02:06 -0700, Peter Geoghegan wrote:
> > > * So far, there has been a lack of scrutiny about what the patch does
> > > in the rewriter (in particular, to support the EXCLUDED.* pseudo-alias
> > > expression) and optimizer (the whole concept of an "auxiliary"
> > > query/plan that share a target RTE, and later target ResultRelation).
> > > If someone took a close look at that, it would be most helpful.
> > > ruleutils.c is also modified for the benefit of EXPLAIN output. This
> > > all applies only to the ON CONFLICT UPDATE patch. A committer could
> > > push out the IGNORE patch before this was 100% firm.
> >
> > I'm far from an expert on the relevant regions; but I'm starting to look
> > nonetheless. I have to say that on a preliminary look it looks more
> > complicated than it has to.
> 
> So, I'm looking. And I've a few questions:
> * Why do we need to spread knowledge about speculative inserts that wide?
>   It's now in 1) Query, 2) ParseState 3) ModifyTable 4) InsertStmt. That
>   seems a bit wide - and as far as I see not really necessary. That e.g.
>   transformUpdateStmt() has if (!pstate->p_is_speculative) blocks seems
>   wrong.
> 
> * afaics 'auxiliary query' isn't something we have under that
>   name. This isn't really different to what wCTEs do, so I don't think
>   we need this term here.
> 
> * You refer to 'join like syntax' in a couple places. I don't really see
>   the current syntax being that join like? Is that just a different
>   perception of terminology or is that just remnants of earlier
>   approaches?
> 
> * I am rather unconvinced we need the whole 'ExcludedVar' mechanism. I
>   don't really see why we need it at all? Can't we 'just' make those
>   plain vars referring to the normal targetlist "entry"? I feel like I'm
>   missing something here.
> 
> * The whole dealing with the update clause doesn't seem super
>   clean. I'm not sure yet how to do it nicer though :(

The more I look at approach taken in the executor, the less I like it.
I think the fundamental structural problem is that you've chosen to
represent the ON CONFLICT UPDATE part as fully separate plan tree node;
planned nearly like a normal UPDATE. But it really isn't. That's what
then requires you to coordinate knowedge around with p_is_speculative,
have these auxiliary trees, have update nodes without a relation, and
other similar things.

My feeling is that this will look much less ugly if there's simply no
UpdateStmt built. I mean, all we need is the target list and the where
clause. As far as I can see so far that'll get rid of a lot of
structural uglyness. There'll still be some more localized stuff, but I
don't think it'll be that bad.

I'm actually even wondering if it'd not better off *not* to reuse
ExecUpdate(), but that's essentially a separate concern.

I'll try to rejigger things roughly in that directions. If you haven't
heard of me in three days, call the police.

Greetings,

Andres Freund


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