Explanation: osm2pgsql imports normally the data in a static database schema. The tags without a corresponding column are ignored. By adding the option -k or --hstore, osm2pgsql will add a new hstore column tags to each table and save there all tags without column.
Another way:
The default.style file https://github.com/openstreetmap/osm2pgsql/blob/master/default.style does not pick all fields into its own column,so edit default.style what you want.
参考: http://stackoverflow.com/questions/31264674/osm2pgsql-data-converting-lost-columns
http://gis.stackexchange.com/questions/135715/osm2pgsql-cant-find-maxspeed-tag-after-import