example, I declare something like : "sequence Int!" in the datamodel, and I manually deal with the sequence inside postgres by altering the column with DEFAULT nextval('table_name_id_seq')

This topic came up some time ago with Graphcool. Most agreed that sequential numbering would be a useful feature, but I don't think it ever got moving. I don't think a postgres procedure for incrementing would cause any issues, but I'm curious to hear other thoughts.