Re: Not HOT enough

On Tue, Nov 22, 2011 at 7:25 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> On Tue, Nov 22, 2011 at 11:40 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>>> I think this is unsafe for shared catalogs.
>> I think so too. Thats why it uses IsMVCCSnapshot() to confirm when it
>> is safe to do so.
> Ah, you mean access to shared catalogs using MVCC snapshots.

Yeah. This change would have the disadvantage of disabling HOT
cleanup for shared catalogs; I'm not sure whether that's a good
decision.

But now that you mention it, something seems funky about the other bit
you mention, too:

+ /* MVCC snapshots ignore other databases */
+ if (!allDbs &&
+ proc->databaseId != MyDatabaseId &&
+ proc->databaseId != 0) /* always include WalSender */
+ continue;
+

It doesn't make sense for the RecentGlobalXmin calculation to depend
on whether or not the current snapshot is an MVCC snapshot, because
RecentGlobalXmin is a global variable not related to any particular
snapshot. I don't believe it's safe to assume that RecentGlobalXmin
will only ever be used in conjunction with the most-recently-taken
snapshot.

--
Robert Haas
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