Exposing ArrayBuildState to pl/pgsql

For many aggregate functions it is necessary to store the actual column values in the aggregate state and then do whatever calculation across them in the final function. In many examples this is done by using array_append as the sfunc and then a custom plpgsql function as the final function operating on the array. Unfortunately array_append allocates a new array on each call which makes it unusably slow for any large aggregates.

The speed of allocating an array is largely fixed by using array_agg_transfn as the state function and a custom C function as the final function, however this requires writing/compiling/packaging the aggregate as an extension written in C when often the final function does very simple operations on the array that could easily be expressed in a higher level language for convenience.

From what I can tell ArrayBuildState is defined as type internal because it is a pointer being passed by value. Is this in fact the limitation that requires it to be type 'internal' or is there another limitation? Is there someway to expose ArrayBuildState as a type or even another way to do a 'fast' array collection of an aggregate that is to be operated on by a final function written in sql or any of the pl-languages?

Thanks
Dave Byrne

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