Alvaro Herrera <alvherre@commandprompt.com> writes:
> Sure. The problem is that we are allowing updated rows to be locked (and
> locked rows to be updated). This means that we need to store extended
> Xmax information in tuples that goes beyond mere locks, which is what we
> were doing previously -- they may now have locks and updates simultaneously.
> (In the previous code, a multixact never meant an update, it always
> signified only shared locks. After a crash, all backends that could
> have been holding locks must necessarily be gone, so the multixact info
> is not interesting and can be treated like the tuple is simply live.)
Ugh. I had not been paying attention to what you were doing in this
patch, and now that I read this I wish I had objected earlier. This
seems like a horrid mess that's going to be unsustainable both from a
complexity and a performance standpoint. The only reason multixacts
were tolerable at all was that they had only one semantics. Changing
it so that maybe a multixact represents an actual updater and maybe
it doesn't is not sane.
regards, tom lane