Re: [Monotone-devel] responses to some IRC discussion of 'automate'

[Thomas Keller]

Nathaniel Smith wrote:
> > SELECT ra1.child
> > FROM revision_ancestry ra1
> > LEFT JOIN revision_ancestry ra2 ON ra1.child=ra2.parent
> > INNER JOIN revision_certs rc ON ra1.child=rc.id AND rc.name="branch" 
> > AND rc.value LIKE "net.venge.monotone"
> > WHERE ra2.parent IS NULL
> > GROUP BY ra1.child
>
> Okay, I've spent a few minutes staring at this, and I give up :-).
> Can you break down what it does in English?

Okay, here we go:

a) we left join the revision_anchestry table with itself to find any 
child revision which is not parent revision of another record (thus 
ra2.parent IS NULL); simple speak we find all leaves
b) then we inner join with the certs table to ensure that we only get 
those leaves which have a particular branch cert attached, thus are 
heads of that branch

> Speed is a reason.  But why do you think this could be slower?
> Monotone can't use any smarter of an algorithm then the one you
> outline here -- it would run the code that 'automate branches' runs,
> then do a loop over that and run the code that 'automate heads' runs X
> times, etc.

Thats right, but one could think about optimizations here, which would
only be possible in mtn itself. If the heads of multiple branches should
be found, one could do that with one single SQL statement and later 
group the result accordingly, while separate the results from each 
other. With the above SQL, this would look like the following:

SELECT rc.value, ra1.child
FROM revision_ancestry ra1
LEFT JOIN revision_ancestry ra2 ON ra1.child=ra2.parent
INNER JOIN revision_certs rc ON ra1.child=rc.id AND rc.name="branch"
AND rc.value LIKE "first.branch" OR rc.value LIKE "second.branch" OR ...
WHERE ra2.parent IS NULL
GROUP BY ra1.child
ORDER BY rc.value ASC

This would return

first.branch  | 123abc...
first.branch  | 456def...
second.branch | ...

You can of course do that the "hard" way, but then you'd indeed make for 
every single branch one query for its head revisions. Retrieving the 
other certs on the appropriate revision would then still need another 
query, of course. This can hardly be done in one.

> There are only two advantages I see:
>   -- You save a _little_ bit of time not having to parse multiple
>      automate command requests.  But this is almost nothing.
>   -- The API for users is a little bit simpler... but the API for my
>      proposal is not at all complicated either.  In python, it would
>      look like:
>
> def get_lots_of_head_info(mtn):
>     branches = mtn.branches()
>     heads = {}
>     for branch in branches:
>         heads[branch] = mtn.heads(branch)
>     certs_for_head = {}
>     for head_set in heads.itervalues():
>         for head in head_set:
>             certs_for_head[head] = mtn.certs(head)
>     return heads, certs_for_head
>
> Easy to read and straightforward, and could actually be made even
> shorter with some judicious use of list or generator comprehensions,
> e.g.
>     heads = dict((branch, mtn.heads(branch)) for branch in mtn.branches())
> already gives you all branches and all heads, in one line, about as
> efficiently as anything can :-).

Well, its nice to see that its that easy to do in Python, but its not 
that easy in other languages. The amount of verbosity which is needed 
e.g. in C++ with Qt is quite big, and because of Qt's "superior" signal 
and slot handling one would need to couple the querying of each set with 
each other, and this is very messy. I won't give you an example here, 
but ask somebody else with Qt experience and he'll tell you the same 
thing. Of course you might say now "Qt was your choice", well, right, Qt 
was my choice, because its fast, consistent and otherwise very easy to 
code for different platforms, but retrieving tiny bits of information 
from different sources and/or different handlers even (and each mtn 
automate command would need to get its own handler) is a major headache.

Thomas.

--
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