I can think of two ways. The first would be to create a mapping table as described. For a relation like "links to reddit", the cardinality is such that it woul

I can think of two ways. The first would be to create a mapping table as described. For a relation like "links to reddit", the cardinality is such that it would probably break their sharding scheme. So the second approach is probably the one he mentions in the article: map reduce (more generally, separate computation). My guess is that for those sorts of "reports" they are using Hadoop. They could also be leveraging things like the HyperLogLog features of Redis.