count(*) is faster than count(1) revisited

Whoa, I didn't expect that much resonance to the last post. :-)

"Can you post the `explain analyze` output of both queries? I'm pretty sure this is normal "distribution" caused by other stuff going on in the system"

OK. Since my 'test' was on a VirtualBox VM with a self-compiled PostgreSQL and this might not be a reliable test setup, so I hijacked one of our real PostgreSQL servers:

uname -a
Linux  2.6.32-358.23.2.el6.x86_64 #1 SMP Sat Sep 14 05:32:37 EDT 2013 x86_64 x86_64 x86_64 GNU/Linux

Intel(R) Xeon(R) CPU E5-2670 v2 @ 2.50GHz ,16 GB RAM, storage on SAN, ext4.

select version();
"PostgreSQL 9.3.5 on x86_64-unknown-linux-gnu, compiled by gcc (GCC) 4.4.7 20120313 (Red Hat 4.4.7-4), 64-bit"

create table hmr(id serial, value real);

insert into hmr (value) select random()*10000000 from generate_series(1,10000000);

CREATE INDEX idx_id
  ON hmr
  USING btree
  (id);

vacuum analyze hmr;

-----------------------------------------------------------------------

explain analyze select count(1) from hmr;

"Aggregate  (cost=169248.00..169248.01 rows=1 width=0) (actual time=1439.521..1439.521 rows=1 loops=1)"
"  ->  Seq Scan on hmr  (cost=0.00..144248.00 rows=10000000 width=0) (actual time=0.006..642.276 rows=10000000 loops=1)"
"Total runtime: 1439.548 ms"

select count(1) from hmr; --811 msec.

-----------------------------------------------------------------------

explain analyze select count(*) from hmr;

"Aggregate  (cost=169248.00..169248.01 rows=1 width=0) (actual time=1299.034..1299.034 rows=1 loops=1)"
"  ->  Seq Scan on hmr  (cost=0.00..144248.00 rows=10000000 width=0) (actual time=0.006..644.912 rows=10000000 loops=1)"
"Total runtime: 1299.061 ms"

select count(*) from hmr; --670 msec 

-----------------------------------------------------------------------

explain analyze select count(id) from hmr;

"Aggregate  (cost=169248.00..169248.01 rows=1 width=4) (actual time=1576.046..1576.046 rows=1 loops=1)"
"  ->  Seq Scan on hmr  (cost=0.00..144248.00 rows=10000000 width=4) (actual time=0.004..636.076 rows=10000000 loops=1)"
"Total runtime: 1576.069 ms"

select count(id) from hmr; --920 msec

----------------------------------------------------------------------- 

And the winner is: still count(*) with count(1) second and count(id) third.

This could be an explanation for count(1) being slower:

"So count(1) is explicitly passing the 1 in and checking it for being NULL before incrementing a counter while count(*) is simply incrementing the counter."

And count(id) might be slower, because it takes additional time to consider using the index?
Or because it's actually reading data: width=4 vs. width=0?

I'm out of my league here.