Tom Lane wrote:
> Hans-Jürgen Schönig <hs@cybertec.at> writes:
> > There is one nifty detail which seems VERY strange to me: If
> > serializable mode is set in postgresql.conf the system was 3 times
> > faster (~ 7.5 sec. vs. 2.5sec). If serializable mode was set for every
> > transaction (using set at the beginning of the transaction) serializable
> > mode was as fast as read committed.
>
> Seems pretty strange to me too. I can believe that taking a new
> snapshot for each command (as READ COMMITTED mode does) might take a
> significant amount of time, especially if you have a large number of
> backends connected. (I think the time to get the snapshot data is
> linear in the number of live backends; also there is the possibility
> of contention on the PROC array when multiple backends need to fetch
> snapshots at the same time.) But if that's where the performance
> difference is, it wouldn't matter whether you start in serializable
> mode by default or issue a SET command to select it.
But the snapshots only are grabbing the xids from each proc, right?
Doesn't seem that would take very long.
If this is the bottleneck, maybe we need a shared proc lock.
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