Hi,
On 2018-07-18 10:56:31 -0400, Robert Haas wrote:
> Are you talking about HOT updates, or HOT pruning? Disabling the
> former wouldn't help, and disabling the latter would break VACUUM,
> which assumes that any tuple not removed by HOT pruning is not a dead
> tuple (cf. 1224383e85eee580a838ff1abf1fdb03ced973dc, which was caused
> by a case where that wasn't true).
I don't think this reasoning actually applies for making HOT pruning
weaker as necessary for decoding. The xmin horizon on catalog tables is
already pegged, which'd prevent similar problems.
There's already plenty cases where dead tuples, if they only recently
became so, are not removed by the time vacuumlazy.c processes the tuple.
I actually think the balance of all the solutions discussed in this
thread seem to make neutering pruning *a bit* by far the most palatable
solution. We don't need to fully prevent removal of such tuple chains,
it's sufficient that we can detect that a tuple has been removed. A
large-sledgehammer approach would be to just error out when attempting
to read such a tuple. The existing error handling logic can relatively
easily be made to work with that.
Greetings,
Andres Freund