Minor Description
SPARK-23711 implemented a nice graceful handling of allowing UnsafeProjection to fall back to an interpreter mode when codegen fails. That makes Spark much more usable even when codegen is unable to handle the given query.
But in its current form, the fallback handling can also be a mystery in terms of performance cliffs. Users may be left wondering why a query runs fine with some expressions, but then with just one extra expression the performance goes 2x, 3x (or more) slower.
It'd be nice to have optional logging of the fallback behavior, so that for users that care about monitoring performance cliffs, they can opt-in to log when a fallback to interpreter mode was taken. i.e. at
https://github.com/apache/spark/blob/a40ffc656d62372da85e0fa932b67207839e7fde/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Projection.scala#L183
