On Fri, Sep 19, 2014 at 2:35 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> Also, shouldn't you go back and fix up
> those abbreviated keys to point to datum1 again if you abort?
Probably not - it appears to make very little difference to
unoptimized pass-by-reference types whether or not datum1 can be used
(see my simulation of Kevin's worst case, for example [1]). Streaming
through a not inconsiderable proportion of memtuples again is probably
a lot worse. The datum1 optimization (which is not all that old) made
a lot of sense when initially introduced, because it avoided chasing
through a pointer for pass-by-value types. I think that's its sole
justification, though.
BTW, I think that if we ever get around to doing this for numeric, it
won't ever abort. The abbreviation strategy can be adaptive, to
maximize the number of comparisons successfully resolved with
abbreviated keys. This would probably use a streaming algorithm like
HyperLogLog too.
[1] http://www.postgresql.org/message-id/CAM3SWZQHjxiyrsqBs5w3u-vTJ_jT2hp8o02big5wYb4m9Lp1jg@mail.gmail.com
--
Peter Geoghegan