org.postgresql.util.PSQLException: ERROR: relation "hibernate_sequence" does not exist
  Position: 17
   at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:2062)
   at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:1795)
......

@Entity
@Table(name = "users")
public class User {
private Long id;
private Set<String> roles = new HashSet<String>();
@Id
   @GeneratedValue()
   public Integer getId() {
      return id;
   }
// more getters & setters
@CollectionOfElements(targetElement = java.lang.String.class)
   @JoinTable(name = "user_roles", joinColumns = @JoinColumn(name = "user_id"))
   @Column(name = "role")   
   public Set<String> getRoles() {
      return roles;
   }
...

Configuration cfg = new Configuration().configure();
SchemaExport se = new SchemaExport(cfg);
se.setDelimiter(";");
se.setOutputFile("schema.sql");
se.create(true, false); // (script, export)

create sequence hibernate_sequence;