On Mon, Jan 10, 2022 at 11:25:08AM +0800, Julien Rouhaud wrote:
> On Fri, Jan 07, 2022 at 03:25:28PM +0100, Peter Eisentraut wrote:
> >
> > I tested this a bit. I used the following setup:
> >
> > create table t1 (a text);
> > insert into t1 select md5(generate_series(1, 10000000)::text);
> > select count(*) from t1 where a > '';
> >
> > And then I changed in varstr_cmp():
> >
> > if (collid != DEFAULT_COLLATION_OID)
> > mylocale = pg_newlocale_from_collation(collid);
> >
> > to just
> >
> > mylocale = pg_newlocale_from_collation(collid);
> >
> > I find that the \timing results are indistinguishable. (I used locale
> > "en_US.UTF-8" and made sure that that code path is actually hit.)
> >
> > Does anyone have other insights?
>
> Looking at the git history, you added this comment in 414c5a2ea65.
>
> After a bit a digging in the lists, I found that you introduced it to fix a
> reported 13% slowdown in varstr_cmp():
> https://www.postgresql.org/message-id/20110129075253.GA18784%40tornado.leadboat.com
> https://www.postgresql.org/message-id/1296748408.6442.1.camel%40vanquo.pezone.net
So I tried to run Noah's benchmark to see if I could reproduce the slowdown.
Unfortunately the results I'm getting don't really make sense as removing the
optimisation brings a 15% speedup, and with a few more runs I can see that I
have about 25% noise, so there isn't much I can do to help.