Re: Think I see a btree vacuuming bug - Mailing list pgsql-hackers

Any status on this?  I know we talked about it but never came to a
good solution.  Is it TODO?

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Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > Is this fixed, and if not, can I have some TODO text?
> 
> It's not fixed.  I'd like to fix it for 7.3, but I was hoping someone
> would think of a better way to fix it than I did ...
> 
>             regards, tom lane
> 
> > ---------------------------------------------------------------------------
> 
> > Tom Lane wrote:
> >> If a VACUUM running concurrently with someone else's indexscan were to
> >> delete the index tuple that the indexscan is currently stopped on, then
> >> we'd get a failure when the indexscan resumes and tries to re-find its
> >> place.  (This is the infamous "my bits moved right off the end of the
> >> world" error condition.)  What is supposed to prevent that from
> >> happening is that the indexscan retains a buffer pin (but not a read
> >> lock) on the index page containing the tuple it's stopped on.  VACUUM
> >> will not delete any tuple until it can get a "super exclusive" lock on
> >> the page (cf. LockBufferForCleanup), and the pin prevents it from doing
> >> so.
> >> 
> >> However: suppose that some other activity causes the index page to be
> >> split while the indexscan is stopped, and that the tuple it's stopped
> >> on gets relocated into the new righthand page of the pair.  Then the
> >> indexscan is holding a pin on the wrong page --- not the one its tuple
> >> is in.  It would then be possible for the VACUUM to arrive at the tuple
> >> and delete it before the indexscan is resumed.
> >> 
> >> This is a pretty low-probability scenario, especially given the new
> >> index-tuple-killing mechanism (which renders it less likely that an
> >> indexscan will stop on a vacuum-able tuple).  But it could happen.
> >> 
> >> The only solution I've thought of is to make btbulkdelete acquire
> >> "super exclusive" lock on *every* leaf page of the index as it scans,
> >> rather than only locking the pages it actually needs to delete something
> >> from.  And we'd need to tweak _bt_restscan to chain its pins (pin the
> >> next page to the right before releasing pin on the previous page).
> >> This would prevent a btbulkdelete scan from overtaking ordinary
> >> indexscans, and thereby ensure that it couldn't arrive at the tuple
> >> on which an indexscan is stopped, even with splitting.
> >> 
> >> I'm somewhat concerned that the more stringent locking will slow down
> >> VACUUM a good deal when there's lots of concurrent activity, but I don't
> >> see another answer.  Ideas anyone?
> >> 
> >> regards, tom lane
> 
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