I agree with your proposal. I have a few comments to design:
1. patch doesn't hold documentation and regress tests, please append it.
OK, i'll add the documentation and regression test
2. this functionality (multidimensional aggregation) can be interesting more times, so maybe some interface like array builder should be preferred.
We already have accumArrayResult and makeArrayResult/makeMdArrayResult, haven't we?
Actually array_agg(anyarray) can be implemented by using accumArrayResult and makeMdArrayResult, but in the process we will deconstruct the array data and null bit-flags into ArrayBuildState->dvalues and dnulls. And we will reconstruct it through makeMdArrayResult. I think this way it's not efficient, so i decided to reimplement it and memcpy the array data and null flags as-is.
In other places, i think it's clearer if we just use accumArrayResult and makeArrayResult/makeMdArrayResult as API for building (multidimensional) arrays.
3. array_agg was consistent with array(subselect), so it should be fixed too
postgres=# select array_agg(a) from test; array_agg ----------------------- {{1,2,3,4},{1,2,3,4}} (1 row)
postgres=# select array(select a from test); ERROR: could not find array type for data type integer[]
I'm pretty new in postgresql development. Can you point out where is array(subselect) implemented?