Re: now() vs transaction_timestamp()

Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> writes:
> On Sat, Oct 6, 2018 at 9:40 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> 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.

> Currently, we serialize the other transaction related stuff via
> PARALLEL_KEY_TRANSACTION_STATE. However, this patch has serialized
> xact_ts via PARALLEL_KEY_FIXED which appears okay, but I think it
> would have been easier for future readers of the code if all the
> similar state variables have been serialized by using the same key.

That state is restored at least two transactions too late.

regards, tom lane