Re: AW: AW: BUG #18147: ERROR: invalid perminfoindex 0 in RTE with relid xxxxx

I wrote:
> So the proximate problem is that we're trying to fetch an updatedCols
> bitmapset for a child table that has no permissions checks to make
> and thus doesn't need an RTEPermissionInfo, so its perminfoindex is
> zero.  It might be that this relinfo should have ri_RootResultRelInfo
> set, but it doesn't.

That seems to be a fairly good guess.  I was able to reduce the
test case to this:


drop table if exists t1 cascade;

create table t1 (f1 int, f2 int, f3 int
  , check (f1 < 10) no inherit
);

create table t2 () inherits(t1);

insert into t2 select i, i+1, 0 from generate_series(1,1000) i;

create index on t2(f1,f2);

update t1 set f3 = 11 where f1 = 12 and f2 = 13;


You don't see the failure without the check constraint on t1.
What is happening is that with that, we remove t1 from the plan
entirely, causing us to think that t2 is the root target relation,
and then things blow up because the root should have an
RTEPermissionInfo and doesn't.  The place where things start
to go off the rails is in ExecInitModifyTable:

     * If it's a partitioned table, the root partition doesn't appear
     * elsewhere in the plan and its RT index is given explicitly in
     * node->rootRelation.  Otherwise (i.e. table inheritance) the target
     * relation is the first relation in the node->resultRelations list.

node->resultRelations contains only t2, so boom.

Maybe we could fix this by setting ModifyTable.rootRelation to
the parent relation in the plain-inheritance case not only the
partitioned case; but I've not investigated what else might be
expecting it to be set only for partitions.

According to this understanding, the root cause is quite old
and the RTEPermissionInfo failure just exposes it in an obvious
way.  I wonder whether we can find related cases that misbehave
even before v16.

> (I haven't looked into exactly why it's so hard to reach this
> error.  Seems like any inherited UPDATE is at risk, so there
> must be additional gating factors to explain why we've not
> noticed this before.)

I haven't tracked that down yet, but I did find that without the
CHECK constraint we don't reach index_unchanged_by_update at
all in repeat executions of the problem query.  This is because
ExecUpdateEpilogue sees updateCxt->updateIndexes == TU_None
and thinks it doesn't have to do anything.  This seems a bit
suspicious in its own right.  Maybe it's okay, because the
updates would be HOT in this test case, but the first one
should be HOT too.  It smells like something is being cached
across queries that probably should not be.

            regards, tom lane