Re: What's the difference between `git show branch:file | diff -u - file` vs `git diff branch file`?

[Michael J Gruber <git@drmicha.warpmail.net>]

Marat Radchenko venit, vidit, dixit 23.08.2011 12:52:
>> Is that a very large tree or a very slow file system?
> Tree is large (500k files), file system is irrelevant since all time is spend on CPU.
> 
>> Do we enumerate all
>> differing files and only then limit diff output by path??
> 
> Dunno, that's why I am asking why it is so slow.

Well, we have to read the full tree before diffing. But I can't
reproduce the extreme difference which you observed (0.003s vs. 30s),
only a factor of ten or so for a repo with 100k files:

git init
seq 0 100000|while read n ; do echo a > a$n;done
git add .
git commit -m m
echo b > a0

On a ramdisk, I get:

time git diff  > /dev/null

real    0m0.160s
user    0m0.064s
sys     0m0.190s

time git diff -- a0 > /dev/null

real    0m0.070s
user    0m0.051s
sys     0m0.021s

time git diff HEAD > /dev/null

real    0m0.266s
user    0m0.145s
sys     0m0.212s

time git diff HEAD -- a0 > /dev/null

real    0m0.171s
user    0m0.136s
sys     0m0.033s

time git show HEAD:a0  > /dev/null

real    0m0.018s
user    0m0.009s
sys     0m0.007s

time git show HEAD:a0 | diff -u - a0 > /dev/null

real    0m0.019s
user    0m0.010s
sys     0m0.008s

Stumped.

In your case, do you have a lot of differing files besides the one you
are limitting to? Anyway, that does not seem to make a huge difference
in my timings. (Just tried.) Still Stumped.

Michael