Re: Aggregate Push Down - Performing aggregation on foreign server

This patch will need some changes to conversion_error_callback(). That
function reports an error in case there was an error converting the
result obtained from the foreign server into an internal datum e.g.
when the string returned by the foreign server is not acceptable by
local input function for the expected datatype. In such cases, the
input function will throw error and conversion_error_callback() will
provide appropriate context for that error. postgres_fdw.sql has tests
to test proper context
We need to fix the error context to provide meaningful information or
at least not crash. This has been discussed briefly in [1].