Reviewed/approved all changes, code looks super clean, almost nothing to comment on I think I understand how it works reading comments and docstrings. I'm not super happy that pipe_base is exposed to sqlalchemy stuff now, but it's probably not the last iteration of it, we can still improve things.

I've just pushed a few more changes.  In addition to addressing review comments (all pretty straightforward),

• The daf_butler changes broke a ci_hsc unit test, so there's now a ci_hsc branch to rewrite that test function.  Happily this also provides a bit more test coverage of the changes in daf_butler.
• Rewriting that test (and thinking about your comment about pipe_base being exposed to sqlalchemy) inspired me to add a whereDataId method to QueryBuilder to keep the sqlalchemy logic contained in daf_butler.
• Running that test uncovered a long-standing problem in how SqlRegistryDatabaseDict uses transactions.

So, upshot is that there are now three new commits on daf_butler (one a trivial doc fix), one more in pipe_base, and one in ci_hsc.  Andy Salnikov, could you take a quick look at those?

I checked new commits, looks good to me.

Merged to master.

This includes a branch for ci_hsc_gen2, which is not yet being advertised for general use but would have been broken by this change (the fix is just a cherry-pick of the commit for ci_hsc).

Hi folks — a reminder to please make sure completed tickets have appropriate epics and SP counts attached. Thanks!