As someone interested in PL theory, I've long found the query language exposed by Monarch surprisingly interesting (briefly discussed in section 5.1 but the description doesn't quite do it justice). It's a functional language, a breath of fresh air compared to "real programming languages" in use at Google like C++, Java, or Go.
The most interesting idea is that its native data types are time series of integers, doubles, distributions of doubles, booleans, and tuples of the above. This means that the data you operate on intrinsically consist of many timestamped data points. It's easy to apply an operation to each point of the data, and it's also easy to apply operations on a rolling window, or on successive points. This makes the language have the feel of an array-based language, but even better because the elements are timestamped and the array can be sparse.
Furthermore the presence of fields in each data point adds more dimensions to the level of aggregation (not just the inherent time-based). Now the language has the feel of a native multi-dimensional array language. It feels amazing to program in it. You can easily do sophisticated queries like figuring out how many standard deviations each task's RPC latency for a specific call is above or below all tasks' mean, for outlier detection.