Re: pgsql: Fix aboriginal mistake in lazy VACUUM's code for truncating away

On 9/15/2007 10:37 PM, Tom Lane wrote:
> Log Message:
> -----------
> Fix aboriginal mistake in lazy VACUUM's code for truncating away
> no-longer-needed pages at the end of a table.  We thought we could throw away
> pages containing HEAPTUPLE_DEAD tuples; but this is not so, because such
> tuples very likely have index entries pointing at them, and we wouldn't have
> removed the index entries.  The problem only emerges in a somewhat unlikely
> race condition: the dead tuples have to have been inserted by a transaction
> that later aborted, and this has to have happened between VACUUM's initial
> scan of the page and then rechecking it for empty in count_nondeletable_pages.
> But that timespan will include an index-cleaning pass, so it's not all that
> hard to hit.  This seems to explain a couple of previously unsolved bug
> reports.

You mean symptoms like multiple index tuples pointing to the same heap
tuple, thus presenting the same heap tuple twice during an index scan,
don't you?


Jan

>
> Tags:
> ----
> REL8_2_STABLE
>
> Modified Files:
> --------------
>     pgsql/src/backend/commands:
>         vacuumlazy.c (r1.81.2.2 -> r1.81.2.3)
>         (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuumlazy.c?r1=1.81.2.2&r2=1.81.2.3)
>
> ---------------------------(end of broadcast)---------------------------
> TIP 6: explain analyze is your friend


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