From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
cc: pgsql-hackers@postgresql.org
Subject: Re: Implement targetlist SRFs using ROWS FROM() (was Changed SRF in
 targetlist handling)
In-reply-to: <20160915203222.27fmxjme5gehluei@alap3.anarazel.de>
References: <20160912165403.vshr7tazhr6wrpp4@alap3.anarazel.de>
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Comments: In-reply-to Andres Freund <andres@anarazel.de>
	message dated "Thu, 15 Sep 2016 13:32:22 -0700"
Date: Thu, 15 Sep 2016 16:48:59 -0400
Message-ID: <14138.1473972539@sss.pgh.pa.us>
Precedence: bulk
Sender: pgsql-hackers-owner@postgresql.org

Andres Freund <andres@anarazel.de> writes:
> Hm. One thing I wonder about with this approach, is how we're going to
> handle something absurd like:
> SELECT generate_series(1, generate_series(1, 2)), generate_series(1, generate_series(2,4));

The patch that I posted would run both the generate_series(1, 2) and
generate_series(2,4) calls in the same SRF node, forcing them to run in
lockstep, after which their results would be fed to the SRF node doing
the top-level SRFs.  We could probably change it to run them in separate
nodes, but I don't see any principled way to decide which one goes first
(and in some variants of this example, it would matter).  I think the
LATERAL approach would face exactly the same issues: how many LATERAL
nodes do you use, and what's their join order?

I think we could get away with defining it like this (ie, SRFs at the same
SRF nesting level run in lockstep) as long as it's documented.  Whatever
the current behavior is for such cases would be pretty bizarre too.

			regards, tom lane


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