Greg Sabino Mullane <htamfids@xxxxxxxxx> writes: > All we can guarantee > via pg_stat_activity is that if xact_start and query_start *are* identical, > no snapshot has been granted yet, Surely that's not true either. xact_start = query_start implies that the current statement is the first in its transaction (assuming sufficiently fine-grained clock timestamps, something I'm not sure is an entirely safe assumption). But if that statement is not simply a BEGIN, it's likely obtained its own transaction snapshot after a few microseconds. As long as "read the system clock" is a distinct operation from "read a snapshot", there are going to be skew issues here. We could maybe eliminate that by reading the clock while holding the lock that prevents commits while reading a snapshot, but I doubt that anybody is going to accept that on performance grounds. Adding a not-guaranteed-cheap syscall inside that extremely hot code path seems unsatisfactory. Also, we currently do guarantee that xact_start matches query_start for the first statement of the transaction (the converse of what I said above). Removing that guarantee in order to add some other one wouldn't necessarily please everybody. regards, tom lane
