EXISTS subquery shapes that do not match the EXISTS-over-UNNEST pattern (e.g. WHERE EXISTS (FROM OtherEntity e WHERE e.x = c.x), EXISTS with GROUP BY, EXISTS with UNION/QueryGroup) are not yet supported and currently throw FeatureNotSupportedException.

The most useful case is EXISTS over a related entity collection, which would require a different MQL translation strategy (e.g. $lookup-based correlated subquery).