Re: Why is "git tag --contains" so slow?

From: Avery Pennarun <apenwarr@gmail.com>
Date: Jul 1, 2010, 01:00 UTC
Message-ID: <AANLkTikkLIKm3soF9agXnN34P7Xnq4AiVqGU_qFaaRmZ@mail.gmail.com>
In-Reply-To: <E1OU82h-0001xY-3b@closure.thunk.org>

On Wed, Jun 30, 2010 at 8:54 PM, Theodore Ts'o <tytso@mit.edu> wrote:

Show 10 quoted lines

> I'm trying to write a script that can determine the first kernel release
> (i.e., a tag of that matchs v2.6.*) that contains a particular commit.
>
> I can do this using "git tag --contains <commit-id>", but it's quite
> slow.  It takes something like 8-9 seconds.  (8.5 seconds of user time,
> 8.6 times of wall clock, to be precise).
>
> I can get the information much faster using "gitk -1 <commit-id>", which
> finishes painting the screen in under 2 seconds, but that throws up a
> GUI and then a human has to pull the information out using their eyes.

There's a big difference between the two: the gitk command you're using only works if the given commit is *itself* named by a tag, while 'git tag --contains' needs to search the entire history of every tag to see if the given commit is *inside* it somewhere.

If all you want is to see if a given commit exactly matches a tag, perhaps you want something like

   git describe --exact-match <commit-id>

Have fun,

Avery