Re: Poor performance of git describe in big repos

Alex Bennée <kernel-hacker@bennee.com> writes:

I think that is because you need to parse the objects at the tip of refs to see if they are on the DAG in the first place.

If there weren't any annotated tag, conceivably you could do without parsing these objects. You would:

 - First read the refs without parsing anything to learn the object
   name of the tips of refs;

 - Traverse the DAG, starting from the commit and notice when you
   see commits that are at the tips of refs you learned in the first
   step, arranging to stop when you found the "closest" tip.

But with annotated tags (and "git describe" is designed to be primarily used with them; you would need "--tags" option to make it notice unannotated tags), the object name you see sitting at the tip will never appear during the DAG traversal. You will only see commits from the latter, so you would need to parse the tips to learn what commits they refer to.

And of course, "then parse only annotated tags, without parsing commits" would not work, because you wouldn't know what the object is without looking at it ;-)