Re: [PATCH] Use RelationClose rather than table_close in heap_create_with_catalog - Mailing list pgsql-hackers

From Xiaoran Wang
Subject Re: [PATCH] Use RelationClose rather than table_close in heap_create_with_catalog
Date
Msg-id DS0PR05MB9689BA4C4B3E4633DC0A6250BA7A9@DS0PR05MB9689.namprd05.prod.outlook.com
Whole thread Raw
In response to Re: [PATCH] Use RelationClose rather than table_close in heap_create_with_catalog  (tender wang <tndrwang@gmail.com>)
List pgsql-hackers
Thanks for all your responses.  It seems better to change the comments on the code
rather than call RelationClose here.

 table_close(new_rel_desc, NoLock);  /* do not unlock till end of xact */

Do I need to create another patch to fix the comments?

Best regards, xiaoran

From: tender wang <tndrwang@gmail.com>
Sent: Thursday, May 11, 2023 3:26 PM
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>; Xiaoran Wang <wxiaoran@vmware.com>; pgsql-hackers@lists.postgresql.org <pgsql-hackers@lists.postgresql.org>
Subject: Re: [PATCH] Use RelationClose rather than table_close in heap_create_with_catalog
 
!! External Email


Tom Lane <tgl@sss.pgh.pa.us> 于2023年5月11日周四 00:32写道:
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);
+1
 Personally, I prefer above code.

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

                        regards, tom lane



!! External Email: This email originated from outside of the organization. Do not click links or open attachments unless you recognize the sender.

pgsql-hackers by date:

Previous
From: Peter Geoghegan
Date:
Subject: Re: Overhauling "Routine Vacuuming" docs, particularly its handling of freezing
Next
From: John Naylor
Date:
Subject: Re: [PATCH] Clarify the behavior of the system when approaching XID wraparound