[zaphar]

I'm certainly no expert but I can take a stab at it.

I think the author was talking less about the syntax and more about the semantics of SQL there. Specifically the query plan. For instance in a SQL statement there is no indication which fields are indexed. You don't even have to use a primary key to find the data. So the database has to take both the data you provide in your AST as well as data about the tables and fields you specify to figure out where to find your data in a distributed database.

Contrast that with the redis example where the client must specify a key that redis then knows how to map to a shard. The logic is much simpler and easier to implement.

Redis has shards builtin to the method you use to query it which allows Redis to simplify the logic of distributed queries.

SQL does not have a notion of sharding builtin in to it so this complicates the query plan logic when it comes to sharding.