Re: Git's database structure

On Tue, 4 Sep 2007, Jon Smirl wrote:

Let's back up a little bit from "Caclulating tree node".  What are the
elements of git's data structures?

Right now we have an index structure (tree nodes) integrated in to a
base table. Integrating indexing into the data is not normally done in
a database. Doing a normalization analysis like this may expose flaws
in the way the data is structured. Of course we may also decide to
leave everything the way it is.

What about the special status of a rename? In the current model we
effectively have three tables.

commit - a set of all SHAs in the commit, previous commit, comment, author, etc
blob - a file, permissions, etc.
file names - name, SHA

The file name table is encoded as an index and it has been
intermingled with the commit table.

Looking at this from a set theory angle brings up the question, do we
really have three tables and file names are an independent variable
from the blobs, or should file names be an attribute of the blob?

There isn't a one-to-one mapping of file names to blobs.  The blob only 
describes the contents of the file.  In the extreme case you could have 
one blob for every single file in your tree.  For example:

# git ls-tree -r HEAD
100644 blob 05303ef858aeeb01ca40590dd6fe65928096ee6c    bar/foo
100644 blob 05303ef858aeeb01ca40590dd6fe65928096ee6c    foo
100644 blob 05303ef858aeeb01ca40590dd6fe65928096ee6c    foo2
100644 blob 05303ef858aeeb01ca40590dd6fe65928096ee6c    foo3
100644 blob 05303ef858aeeb01ca40590dd6fe65928096ee6c    foo4
100644 blob 05303ef858aeeb01ca40590dd6fe65928096ee6c    foo5
100644 blob 05303ef858aeeb01ca40590dd6fe65928096ee6c    foo6

How this gets structured in the db is an independent question about
how renames get detected on a commit. The current scheme for detecting
renames by comparing diffs is working fine. The question is, once we
detect a rename how should it be stored?

Ignoring the performance impacts and looking at the problem from the
set theory view point, should:
the pathnames be in their own table with a row for each alias
the pathnames be stored as an attribute of the blob

Both of these are the same information, we're just looking at how
things are normalized.

-- 
Julian

  ---
"You shouldn't make my toaster angry."
-- Household security explained in "Johnny Quest"