From: Tom Lane <tgl@sss.pgh.pa.us>
To: Amit Kapila <amit.kapila16@gmail.com>
cc: konstantin knizhnik <k.knizhnik@postgrespro.ru>,
        pgsql-hackers <pgsql-hackers@postgresql.org>,
        Robert Haas <robertmhaas@gmail.com>
Subject: Re: now() vs transaction_timestamp()
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Comments: In-reply-to Amit Kapila <amit.kapila16@gmail.com>
	message dated "Sun, 07 Oct 2018 10:28:25 +0530"
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Date: Sun, 07 Oct 2018 01:06:39 -0400
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Amit Kapila <amit.kapila16@gmail.com> writes:
> On Sat, Oct 6, 2018 at 9:40 PM Tom Lane <tgl@sss.pgh.pa.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