When using "extreme" values in the partition column (like having a randomly generated long number) overflow might happen, leading to the following warning message:

WARN JDBCRelation: The number of partitions is reduced because the specified number of partitions is less than the difference between upper bound and lower bound. Updated number of partitions: -1559072469251914524; Input number of partitions: 20; Lower bound: -7701345953623242445; Upper bound: 9186325650834394647.

When this happens, no data is read from the table.

This happens because of the following check in org/apache/spark/sql/execution/datasources/jdbc/JDBCRelation.scala :

if ((upperBound - lowerBound) >= partitioning.numPartitions)

Funny thing is that we worry about overflows a few lines later:

    // Overflow and silliness can happen if you subtract then divide.
    // Here we get a little roundoff, but that's (hopefully) OK.

A better check would be:

if ((upperBound - partitioning.numPartitions) >= lowerBound)