My idea is using new ArrayBuilder optimized for building multidimensional arrays with own State type. I think so casting to ArrayBuildState is base of our problems, so I don't would to do. Code in array_agg_* is simple, little bit more complex code is in nodeSubplan.c. Some schematic changes are in attachments.
Thanks! The structure looks clear, and thanks for the example on nodeSubplan.c. I will restructure the v10 of the patch to this structure.
