Re: oversight in EphemeralNamedRelation support

From: Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Julien Rouhaud <rjuju123(at)gmail(dot)com>, PG Hackers <pgsql-hackers(at)postgresql(dot)org>, Hugo Mercier <hugo(dot)mercier(at)oslandia(dot)com>, Kevin Grittner <kgrittn(at)gmail(dot)com>
Subject: Re: oversight in EphemeralNamedRelation support
Date: 2017-10-13 03:00:02
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On Fri, Oct 13, 2017 at 12:46 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com> writes:
>> On Fri, Oct 13, 2017 at 10:01 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> The CTE was simply not part of the available namespace for the INSERT's
>>> target, so it found the regular table instead. v10 has thus broken
>>> cases that used to work. I think that's a bug.
>> Hmm. Yeah. I have to say it's a bit surprising that the following
>> refers to two different objects:
>> with mytable as (select 1 as x) insert into mytable select * from mytable
> Yeah, I agree --- personally I'd never write a query like that. But
> the fact that somebody ran into it when v10 has been out for barely
> a week suggests that people are doing it.

Not exactly -- Julien's bug report was about a *qualified* reference
being incorrectly rejected.

>>> I think we need to either remove that call from setTargetTable entirely,
>>> or maybe adjust it so it can only find ENRs not CTEs.
>> I think it'd be better to find and reject ENRs only. The alternative
>> would be to make ENRs invisible to DML, which seems arbitrary and
>> weird (even though it might be more consistent with our traditional
>> treatment of CTEs). One handwavy reason I'd like them to remain
>> visible to DML (and be rejected) is that I think they're a bit like
>> table variables (see SQL Server), and someone might eventually want to
>> teach them, or something like them, how to be writable.
> Yeah, that would be the argument for making them visible. I'm not
> sure how likely it is that we'll ever actually have writable ENRs,
> but I won't stand in the way if you want to do it like that.

I hope so :-) I might be a nice way to get cheap locally scoped
temporary tables, among other things.

Okay, here's Julien's patch tweaked to reject just the ENR case. This
takes us back to the 9.6 behaviour where CTEs don't hide tables in
this context. I also removed the schema qualification in his
regression test so we don't break that again. This way, his query
from the first message in the thread works with the schema
qualification (the bug he reported) and without it (the bug or at
least incompatible change from 9.6 you discovered).

I considered testing for a NULL return from parserOpenTable() instead
of the way the patch has it, since parserOpenTable() already had an
explicit test for ENRs, but its coding would give preference to an
unqualified table of the same name. I considered moving the test for
an ENR match higher up in parserOpenTable(), and that might have been
OK, but then I realised no code in the tree actually tests for its
undocumented NULL return value anyway. I think that NULL-returning
branch is dead code, and all tests pass without it. Shouldn't we just
remove it, as in the attached?

I renamed the ENR used in plpgsql.sql's
transition_table_level2_bad_usage_func() and with.sql's "sane
response" test, because they both confused matters by using an ENR
with the name "d" which is also the name of an existing table. For
example, if you start with unpatched master, rename
transition_table_level2_bad_usage_func()'s ENR to "dx" and simply
remove the check for ENRs from setTargetTable() as you originally
suggested, you'll get a segfault because the NULL return from
parserOpenTable() wasn't checked. If you leave
transition_table_level2_bad_usage_func()'s ENR name as "d" it'll
quietly access the wrong table instead, which is misleading.

Thomas Munro

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