On Thu, Aug 18, 2016 at 6:15 PM, Peter Geoghegan <p...@heroku.com> wrote: > On Thu, Aug 18, 2016 at 1:41 PM, Claudio Freire <klaussfre...@gmail.com> > wrote: >> In fact, that's why non-leaf index tuples need a different format, >> because while leaf index tuples contain the heap pointer already, >> non-leaf ones contain only the downlink, not the pointer into the >> heap. To be able to do comparisons and pick the right downlink, the >> original heap pointer in the leaf index tuple is copied into the >> downlink index tuple when splitting pages into an additional >> IndexTupleData header that is prepended only to non-leaf index tuples. > > I think that this is a bad idea. We need to implement suffix > truncation of internal page index tuples at some point, to make them > contain less information from the original leaf page index tuple. > That's an important optimization, because it increases fan-in. This > seems like a move in the opposite direction.
I see that. I could try to measure average depth to measure the impact this had on fan-in. While it should cut it in half for narrow indexes, half of very high is still high. Wide indexes, which are are the ones that would suffer from poor fan-in, would feel this far less, since the overhead is constant. Even if it does have an impact, I don't see an alternative, without also implementing suffix truncation. Perhaps I could try to avoid adding the leaf tid header if it isn't necessary, though I would have to use up the last available flag bit in t_info for that. > ISTM that the way to address this problem is with a duplicate list > and/or prefix compression in leaf pages. Prefix compression is another one I will be looking into eventually, but last time I tried it was far more invasive so I abandoned until I could find a less invasive way to do it. -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers