On Sat, Nov 2, 2013 at 6:07 AM, Simon Riggs <si...@2ndquadrant.com> wrote:
> On 29 October 2013 16:10, Peter Geoghegan <p...@heroku.com> wrote:
>> On Tue, Oct 29, 2013 at 7:53 AM, Leonardo Francalanci <m_li...@yahoo.it> 
>> wrote:
>>> I don't see much interest in insert-efficient indexes.
>> Presumably someone will get around to implementing a btree index
>> insertion buffer one day. I think that would be a particularly
>> compelling optimization for us, because we could avoid ever inserting
>> index tuples that are already dead when the deferred insertion
>> actually occurs.
> That's pretty much what the LSM-tree is.

What is pretty cool about this sort of thing is that there's no
intrinsic reason the insertion buffer needs to be block-structured or
disk-backed.  In theory, you can structure the in-memory portion of
the tree any way you like, using pointers and arbitrary-size memory
allocations and all that fun stuff.  You need to log that there's a
deferred insert (or commit to flushing the insertion buffer before
every commit, which would seem to miss the point) so that recovery can
reconstruct the in-memory data structure and flush it, but that's it:
the WAL format need not know any other details of the in-memory
portion of the tree.  I think that, plus the ability to use pointers
and so forth, might lead to significant performance gains.

In practice, the topology of our shared memory segment makes this a
bit tricky.  The problem isn't so much that it's fixed size as that it
lacks a real allocator, and that all the space used for shared_buffers
is nailed down and can't be borrowed for other purposes.  I'm very
interested in solving the problem of getting a real allocator for
shared memory because I think I need it for parallel sort, and even if
there's a way to avoid needing it there I have a strong feeling that
we'll want it for other applications of dynamic shared memory.  But it
should be possible to write the allocator in such a way that you just
give it a chunk of memory to manage, and it does, remaining agnostic
about whether the memory is from the main shared memory segment or a
dynamic one.

Of course, it's possible that even we do get a shared memory
allocator, a hypothetical person working on this project might prefer
to make the data block-structured anyway and steal storage from
shared_buffers.  So my aspirations in this area may not even be
relevant.  But I wanted to mention them, just in case anyone else is
thinking about similar things, so that we can potentially coordinate.

Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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