[RFC] Extra-simple 'bzr file-log' command

[John Arbash Meinel]

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So at the moment 'bzr log file' is rather slow, and can consume huge amounts
of RAM. One of the primary factors in this, is because we project the per-file
graph back onto the whole tree graph, and then log both the revisions which
directly modify a file, *and* the revisions which then merged those changes.

For example, assume this graph:

 A   # create foo
 |\
 B C # C Modifies foo
 | |
 D E # E does nothing
 |/
 F   # B & C did not modify foo, and F simply merges it.

At the moment, doing "bzr log foo" will give you: A, E, F.

There are some nice advantages of this method, but it is expensive to compute
(at least in our current fashion).

I wrote a simple plugin available from:  lp:~jameinel/+junk/bzr-file-log

With this command, you can do:

  bzr file-log foo

And it will only display A, E.

It also doesn't do any work to compute merge sorting or revision numbers, so
it just displays a flat list of the revisions which modified the file. I
enable "show-ids" to work around this. (Without revnos, you need some sort of
handle to the revision to investigate further.)

I feel this command isn't really in the "bzr is beautiful" model, but it
*does* get around the "bzr is slow" issues.

For my specific timings on a mysql tree, doing
  "bzr log" is about 30s
  "bzr log sql/sql_yacc.yy" consumes all of my 2GB of RAM and I kill the
    process before it finishes, but others have claimed approx 3m for it to
    finish.
  "bzr file-log sql/sql_yacc.yy" shows 2.7k revisions in about 4.6s and starts
    displaying the first ones in 0.8s.

So I'd like to have a bit of discussion about how we would like to handle this
sort of "per file log is slow" issues. We have a very pretty command right
now, but it seems more people are tripped up by the performance than are
helped by the display.

Thoughts? (I'm including the plugin as a patch because it is really quite small.)

John
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