Re: transction_timestamp() inside of procedures

On 2018-09-28 09:35:48 +0200, Peter Eisentraut wrote:
> On 26/09/2018 23:48, Peter Eisentraut wrote:
> > That's certainly a good argument.  Note that if we implemented that the
> > transaction timestamp is advanced inside procedures, that would also
> > mean that the transaction timestamp as observed in pg_stat_activity
> > would move during VACUUM, for example.  That might or might not be
> > desirable.

I think it doesn't hurt, although it's also not a huge advantage.


> diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
> index 9aa63c8792..245735420c 100644
> --- a/src/backend/access/transam/xact.c
> +++ b/src/backend/access/transam/xact.c
> @@ -1884,14 +1884,19 @@ StartTransaction(void)
>  	TRACE_POSTGRESQL_TRANSACTION_START(vxid.localTransactionId);
>  
>  	/*
> -	 * set transaction_timestamp() (a/k/a now()).  We want this to be the same
> -	 * as the first command's statement_timestamp(), so don't do a fresh
> -	 * GetCurrentTimestamp() call (which'd be expensive anyway).  Also, mark
> -	 * xactStopTimestamp as unset.
> -	 */
> -	xactStartTimestamp = stmtStartTimestamp;
> -	xactStopTimestamp = 0;
> +	 * set transaction_timestamp() (a/k/a now()).  Normally, we want this to
> +	 * be the same as the first command's statement_timestamp(), so don't do a
> +	 * fresh GetCurrentTimestamp() call (which'd be expensive anyway).  But
> +	 * for transactions started inside statements (e.g., procedure calls), we
> +	 * want to advance the timestamp.
> +	 */
> +	if (xactStartTimestamp < stmtStartTimestamp)
> +		xactStartTimestamp = stmtStartTimestamp;
> +	else
> +		xactStartTimestamp = GetCurrentTimestamp();
>  	pgstat_report_xact_timestamp(xactStartTimestamp);
> +	/* Mark xactStopTimestamp as unset. */
> +	xactStopTimestamp = 0;

It's a bit weird to make this decision based on these two timestamps
differing.  For one, it only indirectly seems to be guaranteed that
xactStartTimestamp is even set to anything here (to 0 by virtue of being
a global var).

Greetings,

Andres Freund