[dustingetz]

indexes and chunks of data that are used often remain cached in each application instance, and new changes are streamed to the application cache.

It means that reads from a hot cache do not touch network. Reads are very fast and scale "out". You can write code that does a lot of reads without caring much about performance. (SQL reads only scale "up" and you care very much about their performance.)

Datomic is like Git (distributed reads, central writes); Postgres is like CVS/SVN (centralized reads and writes). This is made possible by immutable history.