ngx_postgres | upstream module that allows nginx

Both echo_read_request_body and postgres_pass directives work in content phase. Only one module would work in this case.

The problem here that nginx is async in its nature. Nginx may initiate upstream connection before full request body is received.

Using OpenResty you can force nginx to read the whole request body by lua_need_request_body. Take care about client_body_buffer_size and client_max_body_size. Include empty rewrite_by_lua*.

Another possible solution is to write Lua code, for example within set_by_lua_block and read the full request body, keep in mind that it may be buffered into a file, use ngx.req.get_body_file to check it.