Its concept is good to me. I think, the new combined function should be responsible to take a state data type as argument and update state object of the aggregate function. In other words, combined function performs like transition function but can update state object according to the summary of multiple rows. Right?
It also needs some enhancement around Aggref/AggrefExprState structure to inform which function shall be called on execution time. Combined functions are usually no-thank-you. AggrefExprState updates its internal state using transition function row-by-row. However, once someone push-down aggregate function across table joins, combined functions have to be called instead of transition functions. I'd like to suggest Aggref has a new flag to introduce this aggregate expects state object instead of scalar value.
Also, I'd like to suggest one other flag in Aggref not to generate final result, and returns state object instead.
So are you proposing not calling transfuncs at all and just use combined functions?
That sounds counterintuitive to me. I am not able to see why you would want to avoid transfns totally even for the case of pushing down aggregates that you mentioned.
finalfn wouldnt be called. Instead, combined function would be responsible for getting preaggregate results and combining them (unless of course, I am missing something).
Special casing transition state updating in Aggref seems like a bad idea to me. I would think that it would be better if we made it more explicit i.e. add a new node on top that does the combination (it would be primarily responsible for calling combined function).
Not a good source of inspiration, but seeing how SQL Server does it (Exchange operator + Stream Aggregate) seems intuitive to me, and having combination operation as a separate top node might be a cleaner way.
