The Exasol database has the NULL HIGH default ordering, whereas the Teiid engine works in the NULL LOW mode, which means that depending on whether the ordering is pushed down to Exasol or done by the engine, you may observe NULLs either at the beginning or at the end of returned results. If you need consistency, you can run Teiid with org.teiid.pushdownDefaultNullOrder=true and enforce the NULL LOW ordering (may impact performance).