Re: BUG #18830: ExecInitMerge Segfault on MERGE

David Rowley <dgrowleyml@gmail.com> 于2025年3月4日周二 17:30写道：

> On Tue, 4 Mar 2025 at 19:58, Tender Wang <tndrwang@gmail.com> wrote:
> >  I found the partition_prune.sql does not cover merge into ... not match
> case,  and I found an easy reproduce step,  seeing below:
> >
> > postgres=#  merge into part_abc_view pt
> > using (select stable_one() + 2 as pid) as q join part_abc_1 pt1 on
> (true) on pt.a = stable_one() +2
> > when not matched then insert values(1, 'd', false);
> > server closed the connection unexpectedly
> >         This probably means the server terminated abnormally
> >         before or while processing the request.
> > The connection to the server was lost. Attempting reset: Succeeded.
>
> It looks like this is happening because ExecInitModifyTable() skips
> adding mergeActionsList items when bms_is_member(rti,
> estate->es_unpruned_relids) is false for all resultRelations. This
> results in an empty actions list.  Because the MERGE is performing a
> LEFT JOIN for the NOT MATCHED, ExecMerge() gets a row and runs
> ExecMergeNotMatched(), which crashes on "econtext->ecxt_scantuple =
> NULL;" because of a NULL econtext.  econtext is NULL because
> ExecInitMerge() skips calling ExecAssignExprContext() when
> mergeActionLists is empty.
>
> There are a couple of ways I can see to fix this, 1) would be to move
> the ExecAssignExprContext() above the "if (mergeActionLists == NIL)"
> in ExecInitMerge(), or 2) add code to return NULL in
> ExecMergeNotMatched() if actionStates is NULL.
>
> I think maybe #1 is the better option as #2 adds additional code that
> executes on every ExecMergeNotMatched() call.  The patch does #1. We
> should probably add your test case too.
>
>
Hmm,  apply your patch, I get different results when set
enable_partition_pruning = off, seeing below:
postgres=# merge into part_abc_view pt
using (select stable_one() + 2 as pid) as q join part_abc_1 pt1 on (true)
on pt.a = stable_one() +2
when not matched then insert values(1, 'd', false);
MERGE 0
postgres=# set enable_partition_pruning = off;
SET
postgres=# merge into part_abc_view pt
using (select stable_one() + 2 as pid) as q join part_abc_1 pt1 on (true)
on pt.a = stable_one() +2
when not matched then insert values(1, 'd', false);
MERGE 1

if all partitions are pruned, the resultRelation's mergeActions has no
chance to be set.


-- 
Thanks,
Tender Wang