If you want to test LLBLGenPro/Npgsql queries with LINQPad, and you really should, you have to follow these steps.
And voila, beautiful interactive queries at your disposal. You can even use ORM Profiler.
Sample Npgsql factory entry:
....
<system.data>
<DbProviderFactories>
<!-- PostgreSql -->
<add name="PostgreSql Client Data Provider" invariant="Npgsql" description=".Net Framework Data Provider for PostgreSql" type="Npgsql.NpgsqlFactory, Npgsql, Version=3.1.6.0, Culture=neutral, PublicKeyToken=5d8b90d52f46fda7" />
</DbProviderFactories>
</system.data>