Hello all,
Since even before I saw Alex Gaynor's presentation "I hate the Django ORM"
(the one with the `Sum(F("end_time") - F("start_time"))` query), the problem
of complex aggregates and expressions has vexed me. So, I figured I would
try to solve it.
Originally, I started this trying to pursue a solution to ticket #14030,
but after
I took a couple of lousy shots at it, it dawned on me that the ticket would
be
better resolved as a result of solving the more general case. I realized
that
aggregates were just a special case of expressions, and that the best
solution
was going to take a refactoring of Aggregate into ExpressionNode.
I have uploaded my branch; it can be found here:
https://github.com/NateBragg/django/tree/14030
Since this is a non-trivial change, I was hoping to open the topic for
debate
here, and get some feedback before proposing my solution for inclusion.
Some particular points of note:
* I tried to preserve as much interface as possible; I didn't know how much
was considered to be more public, so generally I tried to add rather than
subtract. However, I did remove a couple things - if you see something
missing that shouldn't be, let me know.
* Currently, I'm getting the entire test suite passed on sqlite, oracle,
mysql,
postgres, and postgis. I was unable to test on oracle spatial - any help
with that would be appreciated.
* When fields are combined, they are coerced to a common type;
IntegerFields are coerced to FloatFields, which are coerced into
DecimalFields as needed. Any other kinds of combinations must be of
the same types. Also, when coerced to a DecimalField, the precision
is limited by the original DecimalField. If this is not correct, or other
coercions should be added, I'd like to correct that.
* When joins are required, they tend to be LEFT OUTER; I'd like some
feedback on this, as I'm not 100% sure its always the best behavior.
* As the aggregates are a little more complicated, on trivial cases there
is a minor reduction in performance; using djangobench, I measured
somewhere between a 3% and 8% increase in runtime.
* I don't have enough tests - mostly for a lack of creativity. What kind of
composed aggregates and expressions would you like to see? I'll add
tests for them.
Thank you all, and sorry for the above being a short book.
-- Nate Bragg
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