A database connection handle. The parameter is the specific handle type for a database, for example
PGConnection for Postgres. The database-specific library will supply a function to create the handle.
In Kaya 0.4 and earlier this data type was not declared
abstract as the database connection libraries needed access to the constructors - users should not have modified the data.
In Kaya 0.5 and later, this is now a
abstract type. Database libraries can use the DBh constructor function to create handles, and should not need to modify the data.