Re: "SELECT ... FROM DUAL" is not quite as silly as it appears

On Tue, 15 Jan 2019 at 13:17, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> writes:
> > I also did a quick benchmark of v6 and found the slowdown to be
> > smaller after the change made in build_simple_rel()
>
> Thanks for confirming. I was not very sure that was worth the extra
> few bytes of code space, but if you see a difference too, then it's
> probably worthwhile.

It occurred to me that a common case where you'll hit the new code is
INSERT INTO ... VALUES.

I thought I'd better test this, so I carefully designed the following
table so it would have as little INSERT overhead as possible.

create table t();

With fsync=off and a truncate between each pgbench run.

insert.sql = insert into t default values;

Unpatched:

$ pgbench -n -f insert.sql -T 60 postgres
tps = 27986.757396 (excluding connections establishing)
tps = 28220.905728 (excluding connections establishing)
tps = 28234.331176 (excluding connections establishing)
tps = 28254.392421 (excluding connections establishing)
tps = 28691.946948 (excluding connections establishing)

Patched:

tps = 28426.183388 (excluding connections establishing)
tps = 28464.517261 (excluding connections establishing)
tps = 28505.178616 (excluding connections establishing)
tps = 28414.275662 (excluding connections establishing)
tps = 28648.103349 (excluding connections establishing)

The patch seems to average out slightly faster on those runs, but the
variance is around the noise level.

--
David Rowley http://www.2ndQuadrant.com/
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