The 42 was meant to be a placeholder in the examples. Imagine it is a ? and bound at runtime instead.

In the second query, a composite index over (x, y) basically does the same job as an index over just x. The general query plan is to iterate over all values > x and keep the 10 smallest values around in memory to satisfy the ORDER BY.

The point is that there are many queries that seem deceptively simple but are actually extremely hard or even impossible to automatically index. Those queries which result in table scans, large index scans, and large sorts can easily dominate all the other queries which are easily indexable in terms of resource usage.