Re: [SQL] A bug in gistPageAddItem()/gist_tuple_replacekey() ???

[ Cc to hackers.]

I haven't see any comment on this. If no one replies, would you send
over a patch of fixes? Thanks.

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Dmitry Tkach wrote:
> I was trying to write a gist index extension, and, after some debugging,
> it looks like I found a bug somewhere in the gist.c code ...
> I can't be quite sure, because I am not familiar with the postgres
> code... but, here is what I see happenning (this is 7.1, but I compared
> the sources to 7.2, and did not see this fixed - although, I did not
> inspect it too carefully)...
>
> First of all, gistPageAddItem () calls gistdentryinit() with a pointer
> to what's stored in the tuple, so, 'by-value' types do not work (because
> gistcentryinit () would be passed the value itself, when called from
> gistinsert(), and then, in gistPageAddItem (), it is passed a pointer,
> coming from gistdentryinit () - so, it just doesn't know really how to
> treat the argument)...
>
> Secondly, gist_tuple_replacekey() seems to have incorrect logic figuring
> out if there is enough space in the tuple (it checks for '<', instead of
> '<=') - this causes a new tuple to get always created (this one, seems
> to be fixed in 7.2)
>
> Thirdly, gist_tuple_replace_key () sends a pointer to entry.pred (which
> is already a pointer to the actual value) to index_formtuple (), that
> looks at the tuple, sees that the type is 'pass-by-value', and puts that
> pointer directly into the tuple, so that, the resulting tuple now
> contains a pointer to a pointer to the actual value...
>
> Now, if more then one split is required, this sequence is repeated again
> and again and again, so that, by the time the tuple gets actually
> written, it contains something like a pointer to a pointer to a pointer
> to a pointer to the actual data :-(
>
> Once again, I've seen some comments in the 7.2 branch about gists and
> pass-by-value types, but brief looking at the differences in the source
> did not make me conveinced that it was indeed fixed...
>
> Anyone knows otherwise?
>
> Thanks a lot!
>
> Dima
>
>
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