回复： bug report: some issues about pg_15_stable(8fa4a1ac61189efffb8b851ee77e1bc87360c445) - Mailing list pgsql-hackers

Hi，hackers

 

 I may have discovered another issue in the concurrency scenario of merge,  and I am currently not sure if this new issue is related to the previous one. 

 It seems that it may also be an issue with the EPQ mechanism in the merge scenario? 

 I will provide this test case, hoping it will be helpful for you to fix related issues in the future.

    DROP TABLE IF EXISTS src1, tgt;
    CREATE TABLE src1 (a int, b text);
    CREATE TABLE tgt (a int, b text);
    INSERT INTO src1 SELECT x, 'Src1 '||x FROM generate_series(1, 3) g(x);
    INSERT INTO tgt SELECT x, 'Tgt '||x FROM generate_series(1, 6, 2) g(x);
    insert into src1 values(3,'src1 33');

 If I only execute merge , I will get the following error:

    merge into tgt a using src1 c on  a.a = c.a when matched then update set b = c.b when not matched then insert (a,b) values(c.a,c.b);  -- excute fail
    ERROR:  MERGE command cannot affect row a second time
    HIINT:  Ensure that not more than one source row matches any one target row.

 But when I execute the update and merge concurrently, I will get the following result set.

  --session1

    begin;

    update tgt set b = 'tgt333' where a =3;

On Wed, Feb 28, 2024 at 8:11 PM Dean Rasheed <dean.a.rasheed@gmail.com> wrote:
>
> On Wed, 28 Feb 2024 at 09:16, jian he <jian.universality@gmail.com> wrote:
> >
> > + oldcontext = MemoryContextSwitchTo(estate->es_query_cxt);
> > +
> > + node->as_epq_tupdesc = lookup_rowtype_tupdesc_copy(tupType, tupTypmod);
> > +
> > + ExecAssignExprContext(estate, &node->ps);
> > +
> > + node->ps.ps_ProjInfo =
> > + ExecBuildProjectionInfo(castNode(Append, node->ps.plan)->epq_targetlist,
> > +
> > EvalPlanQualStart, EvalPlanQualNext will switch the memory context to
> > es_query_cxt.
> > so the memory context switch here is not necessary?
> >
>
> Yes it is necessary. The EvalPlanQual mechanism switches to the
> epqstate->recheckestate->es_query_cxt memory context, which is not the
> same as the main query's estate->es_query_cxt (they're different
> executor states). Most stuff allocated under EvalPlanQual() is
> intended to be short-lived (just for the duration of that specific EPQ
> check), whereas this stuff (the TupleDesc and Projection) is intended
> to last for the duration of the main query, so that it can be reused
> in later EPQ checks.
>
sorry for the noise. I understand it now.

Another small question:
for the Append case, we can set/initialize it at create_append_plan,
all other elements are initialized there,
why we set it at set_append_references.
just wondering.