Re: Runtime pruning problem

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>
Cc: Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp>, Yuzuko Hosoya <hosoya(dot)yuzuko(at)lab(dot)ntt(dot)co(dot)jp>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: Runtime pruning problem
Date: 2019-04-17 04:10:14
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David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> writes:
> On Wed, 17 Apr 2019 at 15:54, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)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

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