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

From: Michael J Gruber <git@drmicha.warpmail.net>
Date: Aug 23, 2011, 10:03 UTC
Message-ID: <4E537AF0.9070604@drmicha.warpmail.net>
In-Reply-To: <loom.20110823T091132-107@post.gmane.org>

Marat Radchenko venit, vidit, dixit 23.08.2011 09:25:

Show 18 quoted lines

> $ time git show branch:file | diff -u - file > /dev/null 
> 
> real    0m0.003s
> user    0m0.000s
> sys     0m0.000s
> 
> $ time git diff branch -- file > /dev/null 
> 
> real    0m31.442s
> user    0m31.040s
> sys     0m0.380s
> 
> What does git diff do so it takes that much time? And is there any flag to git 
> diff so that it will work as fast as show + diff? I thought these two are 
> equivalent but from run time it is obvious that they aren't.
> 
> gprof output: http://slonopotamus.org/git-diff/git-diff-branch.gprof.txt
>

Is that a very large tree or a very slow file system?

>From the gprof output, it would appear that we compare a lot of cache

entries (or rather: spent most time on...). Do we enumerate all differing files and only then limit diff output by path??

Michael