Re: support for MERGE

From: Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
To: Daniel Gustafsson <daniel(at)yesql(dot)se>
Cc: Ranier Vilela <ranier(dot)vf(at)gmail(dot)com>, Peter Eisentraut <peter(dot)eisentraut(at)enterprisedb(dot)com>, Justin Pryzby <pryzby(at)telsasoft(dot)com>, Amit Langote <amitlangote09(at)gmail(dot)com>, Japin Li <japinli(at)hotmail(dot)com>, Zhihong Yu <zyu(at)yugabyte(dot)com>, simon(dot)riggs(at)enterprisedb(dot)co, Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>, Daniel Westermann <dwe(at)dbi-services(dot)com>, Erik Rijkers <er(at)xs4all(dot)nl>, Jaime Casanova <jcasanov(at)systemguards(dot)com(dot)ec>, Andres Freund <andres(at)anarazel(dot)de>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: support for MERGE
Date: 2022-04-02 15:02:01
Message-ID: 202204021502.hxkog726fulu@alvherre.pgsql
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On 2022-Mar-31, Daniel Gustafsson wrote:

> > On 31 Mar 2022, at 19:38, Ranier Vilela <ranier(dot)vf(at)gmail(dot)com> wrote:
> > I think that there is an oversight at 7103ebb
> > There is no chance of Assert preventing this bug.
> This seems reasonable from brief reading of the code, NULL is a legitimate
> value for the map and that should yield an empty list AFAICT.

There's no bug here and this is actually intentional: if the map is
NULL, this function should not be called.

In the code before this commit, there was an assert that this variable
was not null:

static List *
adjust_partition_colnos(List *colnos, ResultRelInfo *leaf_part_rri)
- List *new_colnos = NIL;
TupleConversionMap *map = ExecGetChildToRootMap(leaf_part_rri);
! AttrMap *attrMap;
ListCell *lc;

! Assert(map != NULL); /* else we shouldn't be here */
! attrMap = map->attrMap;

foreach(lc, colnos)

We could add an Assert that map is not null in the new function, but
really there's no point: if the map is null, we'll crash just fine in
the following line.

I would argue that we should *remove* the Assert() that I left in

Even if we wanted to make the function handle the case of a NULL map,
then the right fix is not to return NIL, but rather we should return the
original list.

Álvaro Herrera PostgreSQL Developer —

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