So, after hacking away a little bit yesterday, I came to the same
conclusion that Anssi did back when the first PR was sent. Namely, that the
ExpressionNode <-> SQLEvaluator structure seems overly complex, and makes
it difficult to create custom "ExpressionNodes".
To build a custom Expression, one needs to:
- Subclass ExpressionNode
- Build in SQL into the backend (sometimes)
- Modify SQLEvaluator (or create a new one)
- Modify sql/query to learn about the different types of
ExpressionNodes
I think it'd be better if SQLEvaluator was dropped altogether, and
ExpressionNode was given the evaluator responsibilities. In this world,
creating a custom Expression would look like:
- Subclass ExpressionNode
- The subclass contains SQL for each supported backend (and has
access to the 'connection' instance)
- sql/query asks ExpressionNode to evaluate itself
Provided that ExpressionNode (or one of its subclasses) had the appropriate
methods, it would allow library authors to create their own custom
functions. #11305 (Conditional Aggregates) could then be implemented quite
easily, without touching the rest of the stack.
So this is what I'm going to attempt first. I'll "merge" the functions of
SQLEvaluator and ExpressionNode, and see how far along that gets me. If
that turns out to simplify the handling of F(), then it should be equally
useful when refactoring Aggregate as an ExpressionNode. As a POC, I'll also
implement a version of Conditional Aggregates to confirm the API. Now, this
is definitely more complex than porting nateb's original changes, but it's
probably going to be a more simplified implementation. As such, my hopes
for inclusion in 1.7 are probably far-fetched.
Will report back when I've got something a little more concrete to talk
about.
Regards,
- Josh
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