Re: planner/optimizer question

Jochem van Dieten <jochemd(at)oli(dot)tudelft(dot)nl> writes:
> The moment the heap tuple is updated/deleted the visible-to-all
> flag needs to be set to false again in all indexes. This is
> critical,

Exactly. This gets you out of the hint-bit semantics and into a ton
of interesting problems, such as race conditions. (Process A determines
that tuple X is visible-to-all, and goes to update the index tuple.
Before it can reacquire lock on the index page, process B updates the
heap tuple and visits the index to clear the flag bit. Once A obtains
lock it will set the flag bit. Oops.)

Basically what you are buying into with such a thing is multiple copies
of critical state. It may be only one bit rather than several words,
but updating it is no less painful than if it were a full copy of the
tuple's commit status.

regards, tom lane