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

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(at)gmail(dot)com>
Sent: Thursday, May 11, 2023 3:26 PM
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>; Xiaoran Wang <wxiaoran(at)vmware(dot)com>; pgsql-hackers(at)lists(dot)postgresql(dot)org <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: [PATCH] Use RelationClose rather than table_close in heap_create_with_catalog

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Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us<mailto:tgl(at)sss(dot)pgh(dot)pa(dot)us>> 于2023年5月11日周四 00:32写道：
Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com<mailto:bharath(dot)rupireddyforpostgres(at)gmail(dot)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

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