Обсуждение: [PATCH] Use RelationClose rather than table_close in heap_create_with_catalog

Hi hackers,

 In heap_create_with_catalog, the Relation new_rel_desc is created 

by RelationBuildLocalRelation, not table_open. So it's better to 

call RelationClose to release it.

What's more, the comment for it seems useless, just delete it.

Thanks!

On Sat, Mar 18, 2023 at 12:34 PM Xiaoran Wang <wxiaoran@vmware.com> wrote:
>
> Hi hackers,
>
>  In heap_create_with_catalog, the Relation new_rel_desc is created
> by RelationBuildLocalRelation, not table_open. So it's better to
> call RelationClose to release it.
>
> What's more, the comment for it seems useless, just delete it.

Essentially, all the close functions are the same with NoLock, IOW,
table_close(relation, NoLock) = relation_closerelation, NoLock) =
RelationClose(relation). Therefore, table_close(new_rel_desc, NoLock);
looks fine to me. 

And, the /* do not unlock till end of xact */, it looks like it's been
there from day 1. It may be indicating that the ref count fo the new
relation created in heap_create_with_catalog() will be decremented at
the end of xact, but I'm not sure what it means.

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com

Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com> writes:
> And, the /* do not unlock till end of xact */, it looks like it's been
> there from day 1. It may be indicating that the ref count fo the new
> relation created in heap_create_with_catalog() will be decremented at
> the end of xact, but I'm not sure what it means.

Hmm, I think it's been copied-and-pasted from somewhere.  It's quite
common for us to not release locks on modified tables until end of
transaction.  However, that's not what's happening here, because we
actually *don't have any such lock* at this point, as you can easily
prove by stepping through this code and watching the contents of
pg_locks from another session.  We do acquire AccessExclusiveLock
on the new table eventually, but not till control returns to
DefineRelation.

I'm not real sure that I like the proposed code change: it's unclear
to me whether it's an unwise piercing of a couple of abstraction
layers or an okay change because those abstraction layers haven't
yet been applied to the new relation at all.  However, I think the
existing comment is actively misleading and needs to be changed.
Maybe something like

    /*
     * Close the relcache entry, since we return only an OID not a
     * relcache reference.  Note that we do not yet hold any lockmanager
     * lock on the new rel, so there's nothing to release.
     */
    table_close(new_rel_desc, NoLock);

    /*
     * ok, the relation has been cataloged, so close catalogs and return
     * the OID of the newly created relation.
     */
    table_close(pg_class_desc, RowExclusiveLock);

Given these comments, maybe changing the first call to RelationClose
would be sensible, but I'm still not quite convinced.

                        regards, tom lane