Re: now() vs transaction_timestamp()

Konstantin Knizhnik <k(dot)knizhnik(at)postgrespro(dot)ru> writes:
> On 06.10.2018 00:25, Tom Lane wrote:
>> So maybe the right answer is to change the parallel mode infrastructure
>> so it transmits xactStartTimestamp, making transaction_timestamp()
>> retroactively safe, and then in HEAD only we could re-mark now() as
>> safe. We might as well do the same for statement_timestamp as well.

> Attached please find very small patch fixing the problem (propagating
> transaction and statement timestamps to workers).

That's a bit too small ;-) ... one demonstrable problem with it is
that the parallel worker will report the wrong xactStartTimestamp
to pgstat_report_xact_timestamp(), since you aren't jamming the
transmitted value in soon enough. Also, I found that ParallelWorkerMain
executes at least two transactions before it ever gets to the "main"
transaction that does real work, and I didn't much care for the fact
that those were running with worker-local values of xactStartTimestamp
and stmtStartTimestamp. So I rearranged things a bit to ensure that
parallel workers wouldn't generate their own values for either
timestamp, and pushed it.

regards, tom lane