Re: [PATCH v2 1/8] odb/source-packed: improve lookup when enumerating objects

[Patrick Steinhardt <ps@pks.im>]

On Mon, Jul 13, 2026 at 08:49:43PM -0700, Taylor Blau wrote:
> On Mon, Jul 13, 2026 at 11:54:43AM +0200, Patrick Steinhardt wrote:
> > On Fri, Jul 10, 2026 at 03:25:10PM -0700, Taylor Blau wrote:
> > > On Fri, Jul 10, 2026 at 10:48:53AM +0200, Patrick Steinhardt wrote:
> > > > Fix the issue by using `packed_object_info()` directly.
> > >
> > > What you wrote here makes sense to me insofar as I understand the
> > > pluggable ODB code.
> > >
> > > However, I am confused by the way this function is written in general.
> > > We use `bsearch_one_midx()` to locate the first possible MIDX position
> > > in which an object matching the given prefix may exist, which is
> > > sensible. However, we go from that position up to "num", where "num" is
> > > the total number of objects in the MIDX!
> > >
> > > Functionally this is not incorrect as we will happily discard objects
> > > that do not match the prefix. But it causes us to waste CPU cycles
> > > repeatedly calling `match_hash()` (at least for the first byte of the
> > > prefix) for objects that we know will match.
> >
> > That's not quite true though, as we abort iteration as soon as
> > `match_hash()` tells us that the prefix doesn't match anymore.
> 
> Right, we neither iterate through more objects than necessary once we
> know that `match_hash()` will stop returning true, nor do we emit
> objects that don't actually match the prefix.
> 
> What I was trying to say above is that in the special case where our
> prefix is a single byte long, we don't have to call `match_hash()` at
> *all*, since we can enumerate just the portion of the fanout for that
> specific byte, and we know that all such entries will match.

Oh, now that's what you're getting at. I don't think that this case ever
happens at all right now. I think the shortest prefix that we're ever
using should be at least 2 bytes, as we don't treat anything shorter
than 4 hex characters as an abbreviated object ID.

Thanks for clarifying!

Patrick