From: Tom Lane <tgl@sss.pgh.pa.us>
To: David Rowley <david.rowley@2ndquadrant.com>
cc: Amit Langote <Langote_Amit_f8@lab.ntt.co.jp>,
        Yuzuko Hosoya <hosoya.yuzuko@lab.ntt.co.jp>,
        PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Runtime pruning problem
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Comments: In-reply-to David Rowley <david.rowley@2ndquadrant.com>
	message dated "Wed, 17 Apr 2019 15:58:52 +1200"
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Date: Wed, 17 Apr 2019 00:10:14 -0400
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David Rowley <david.rowley@2ndquadrant.com> writes:
> On Wed, 17 Apr 2019 at 15:54, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> What I'm more worried about is whether this breaks any internal behavior
>> of explain.c, as the comment David quoted upthread seems to think.
>> If we need to have a tlist to reference, can we make that code look
>> to the pre-pruning plan tree, rather than the planstate tree?

> I think most of the complexity is in what to do in
> set_deparse_planstate() given that there might be no outer plan to
> choose from for Append and MergeAppend. This controls what's done in
> resolve_special_varno() as this descends the plan tree down the outer
> side until it gets to the node that the outer var came from.

> We wouldn't need to do this if we just didn't show the targetlist in
> EXPLAIN VERBOSE, but there's also MergeAppend sort keys to worry about
> too.  Should we just skip on those as well?

No, the larger issue is that *any* plan node above the Append might
be recursing down to/through the Append to find out what to print for
a Var reference.  We have to be able to support that.

			regards, tom lane