I've downloaded multiple metro extracts from openstreetmap as PBF files when i try to import them with osm2pgsql it works for the first and creates the tables. I then want to add a column in the planet_osm_ways with a cityID to know which "way id" belonged to which city after i then try to import another city it says 'ERROR: Missing data for column "city_id". is there a way to modify the planet_osm_ways table without breaking the script? I really need to know which id belonged to which metro extract.
You need to edit the style file (default.style, possibly in osm2pgsql-bin directory) used by osm2pgsql.
You can then add the instruction
#Add custom column
node,way citiid int4 linear
The column will be created, and - provided no tag has this name - will not be populated. You are then free to populate it as you want.