  As the need for machine learning (ML) increases rapidly across all industry
sectors, there is a significant interest among commercial database providers to
support "Query 2.0", which integrates model inference into SQL queries.
Debugging Query 2.0 is very challenging since an unexpected query result may be
caused by the bugs in training data (e.g., wrong labels, corrupted features).
In response, we propose Rain, a complaint-driven training data debugging
system. Rain allows users to specify complaints over the query's intermediate
or final output, and aims to return a minimum set of training examples so that
if they were removed, the complaints would be resolved. To the best of our
knowledge, we are the first to study this problem. A naive solution requires
retraining an exponential number of ML models. We propose two novel heuristic
approaches based on influence functions which both require linear retraining
steps. We provide an in-depth analytical and empirical analysis of the two
approaches and conduct extensive experiments to evaluate their effectiveness
using four real-world datasets. Results show that Rain achieves the highest
recall@k among all the baselines while still returns results interactively.
