Re: Git's database structure

From: Reece Dunn <msclrhd@googlemail.com>
Date: Sep 4, 2007, 19:44 UTC
Message-ID: <3f4fd2640709041244s125f5988j1c2d28f06bfbe098@mail.gmail.com>
In-Reply-To: <20070904180429.GA626@glandium.org>

On 04/09/07, Mike Hommey <mh@glandium.org> wrote:

Show 10 quoted lines

> On Tue, Sep 04, 2007 at 01:44:47PM -0400, Jon Smirl <jonsmirl@gmail.com> wrote:
> > The reason databases don't encode the fields into the index is that
> > you can only have a single index on the table if you do that.
> > Databases do sometimes duplicate the field in both the index and the
> > table. Databases also have the property that indexes are just a cache
> > and can be dropped at any time.
>
> The big difference between a database and git is that a database is a
> general purpose tool. git has a much more restricted scope. As such, it
> doesn't need *that much* flexibility.

Databases are designed to be efficient at storing and accessing large amounts of data. The key thing about a database is that it does not track the *history* of the data it is storing. This is the main problem with using a database as a metadata storage facility.

Modern source control systems such as Perforce (and possibly Subversion), use a database to track metadata such as branch/merge history, user data and so on. This, IMHO is a huge weakness of these SCM systems. It is impossible to fully roll back to a given point in time, because that metadata is stored independently of the file content tracking.

Git *is not a database*. This is fundamental to understanding how git works. Git stores *all* of its data in a Directed Acyclic Graph (with the exception of the pointers to tag and the current head of each branch, that it stores locally in the .git directory). Read http://eagain.net/articles/git-for-computer-scientists/ for more information on this.

What this means is that for any commit, git has all the information it needs about the repository at that point in time. It doesn't need anything else. If you then store information in a database, you lose having the complete picture at any point in the history of the repository.

- Reece