There are currently 2 uses of org.hibernate.dialect.TypeNames from dialect:

1. mapping jdbcTypeCode+size -> database-specific datatype - this mapping is used in schema export as well as in handling CAST function calls.
2. mapping jdbcTypeCode+size -> Hibernate type - this mapping is used in native-sql auto-discovery

This information can mostly be gathered from the JDBC java.sql.DatabaseMetaData. The first form can completely, I believe. The second would need to leverage the org.hibernate.type.descriptor.sql.JdbcTypeJavaClassMappings#determineJavaClassForJdbcTypeCode code I think (essentially, resolve the recommended Java type for the JDBC type-code and then use the recommended Java type to look into BasicTypeRegsitry).