Re: tuptoaster.c must *not* use SnapshotAny - Mailing list pgsql-hackers

From Jan Wieck
Subject Re: tuptoaster.c must *not* use SnapshotAny
Date
Msg-id 200201161816.g0GIGe302180@saturn.janwieck.net
Whole thread Raw
In response to tuptoaster.c must *not* use SnapshotAny  (Tom Lane <tgl@sss.pgh.pa.us>)
Responses Re: tuptoaster.c must *not* use SnapshotAny  (Tom Lane <tgl@sss.pgh.pa.us>)
List pgsql-hackers
Tom Lane wrote:
> The tuple toaster currently fetches toast-table rows using SnapshotAny.
> This is quite uncool, because it will in fact find *any* row.  Including
> dead rows resulting from an aborted (or, in 7.2, still-in-progress)
> VACUUM.
   Really?  So  what  you're  saying  means that while vacuum is   running there's a window where one tuple can  exist
multiple  times  and  just  the  fact  that  other backends can see the   vacuum transaction alive prevents them from
showingup?  That   doesn't sound really good to me.
 


Jan

>
> I believe this is the explanation for a problem report I'm currently
> chasing from Joshua Drake:
>
> digivision2=# select * from change_log;
> ERROR:  chunk 1 for toast value 3388774 appears multiple times
>
> (there are some thousands of rows with similar problems)
>
> I suggest SnapshotSelf instead.  Comments anyone?
>
>              regards, tom lane
>
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