On 11/29/2013 07:13 PM, Tom Lane wrote:
Andreas Karlsson <andr...@proxel.se> writes:
I decided to look into how much work implementing the todo item about
supporting amgettuple in GIN would be, since exclusion constraints on
GIN would be neat. Robert Haas suggested a solution[1], but to fix it we
also need to look into why the commit message mentions that it did not
work anyway with the partial matches.
...
This TIDBitmap becomes lossy if it too many TIDs are added to it, and
this case is what broke amgettuple for partial matches.


Right, see
https://urldefense.proofpoint.com/v1/url?u=http://www.postgresql.org/message-id/49AC300F.1050903%40enterprisedb.com&k=oIvRg1%2BdGAgOoM1BIlLLqw%3D%3D%0A&r=xGch4oNJbpD%2BKPJECmgw4SLBhytSZLBX7UnkZhtNcpw%3D%0A&m=OqhHlGFG81LH1EqJLzTW8HuXdXslGEL%2FPu1f27HxV%2Bs%3D%0A&s=9f3fad064e2845bd2b99c85f684d237fbe96e542081e4b2dc49b1fe51f91f144

Note that fixing the potential lossiness in scanning is not the only
roadblock to re-enabling amgettuple.  Fast updates also pose problems:
https://urldefense.proofpoint.com/v1/url?u=http://www.postgresql.org/message-id/4974B002.3040202%40sigaev.ru&k=oIvRg1%2BdGAgOoM1BIlLLqw%3D%3D%0A&r=xGch4oNJbpD%2BKPJECmgw4SLBhytSZLBX7UnkZhtNcpw%3D%0A&m=OqhHlGFG81LH1EqJLzTW8HuXdXslGEL%2FPu1f27HxV%2Bs%3D%0A&s=0e08a781fcc17a3d68ce247344a3499a23a9f545b937f254439dadfaf7b9b8ab

Half of that is basically the same lossiness problem, but the other
half is that we're relying on the bitmap to suppress duplicate reports
of the same TID.  It's fairly hard to see how you'd avoid that without
creating other problems.

Note that Robert's proposed solution is no solution, because it just
puts you right back in the bind of needing guaranteed non-lossy
storage of a TID set that might be too big to fit in memory.

You can always call amgetbitmap, and return the tuples from the bitmap one by one. For a lossy result, re-check all tuples on the page. IOW, do a bitmap index + heap scan. You could do that within indexam.c, and present the familiar index_getnext() interface for callers. Or you could modify the exclusion constraint code to do that if amgettuple is not available

- Heikki


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