Re: is rebase the same as merging every commit?

From: Petr Baudis <pasky@suse.cz>
Date: Jun 27, 2008, 08:34 UTC
Message-ID: <20080627083419.GD12567@machine.or.cz>
In-Reply-To: <vpqfxqz5qzj.fsf@bauges.imag.fr>

On Fri, Jun 27, 2008 at 08:30:56AM +0200, Matthieu Moy wrote:

Show 36 quoted lines

> "David Jeske" <jeske@willowmail.com> writes:
> 
> > Rebasing is described in the docs I've read as turning this: (sorry for the
> > dots)
> >
> > ..........A---B---C topic
> > ........./
> > ....D---E---F---G master
> >
> > Into this:
> >
> > ...................A'--B'--C' topic
> > ................../
> > .....D---E---F---G master
> >
> > If I understand it right (and that's a BIG if), it's the same as doing a merge
> > of C into G where every individual commit in the C-line is individually
> > committed into the new C' line.
> >
> > ...........-------------A---B---C
> > ........../            /   /   /
> > ........./        /---A'--B'--C'  topic
> > ......../        /
> > ....D---E---F---G - master
> 
> I'd draw that the other way:
> 
>   ...........---------A---B---C
>   ........../          \   \   \
>   ........./        /---A'--B'--C'  topic
>   ......../        /
>   ....D---E---F---G - master
> 
> > (1) Is the above model a valid explanation?
> 
> Sounds correct to me.

I don't think you can call it correct since it assumes !(2) while (2) holds. Drawing the diagram this way is misleading; merging commits one-by-one implies preserving the merge information in the history graph; nothing like that is done by rebase.

Rebase is more like _cherry-picking_ all the patches on your branch on top of the upstream branch. You just essentially take each patch (commit message + diff to parent) growing on top of upstream's E and recommit it on top of G.

> > (2) From the documentation diagrams, it looks like the rebased A' has only (G)
> > as a parent, not (A,G). If this is the case, why?

..snip..

Show 5 quoted lines

> > (i.e. not connecting those nodes throws away useful information)
> 
> For the use-cases where this information is useful, "rebase" is not
> for you. Indeed, in these cases, a plain "merge" is usually what you
> want.

Indeed, noone forces you into the rebase workflow for your own projects. I personally never ever rebase (I do use StGIT though, but it records per-patch history and makes sure I'm always in some consistent state).

-- 
				Petr "Pasky" Baudis
The last good thing written in C++ was the Pachelbel Canon. -- J. Olson