Because database, in Postgres terms, doesn’t mean physical node. It’s more akin to a VM than anything else. The term for an installation of Postgres is database cluster, which can contain N databases, each of which can contain M schemas.
Thanks! Is there a good primer to this terminology that clarifies these terms on various popular database and cloud platforms?
It seems there is good potential for confusion unless we use the same terms consistently when discussing architecture and design across teams.
Even the term RDS (Relational Database Service) is sometimes said to be inaccurate since it is a "Relational Database SERVER as a Service"
A few related terms that cause confusion:
"Schema" could refer to a Database Schema or in some contexts, a single table's logical data structure. (Or a single data set's data structure -- or a standard for one, like JSON Schema)
Data Catalog products like "AWS Glue Data Catalog" which only store the metadata or schemas of the table they crawl ... refer to the entities they store as "Databases" and "Tables" (but no "Schemas") and documentation includes guides talk about "creating a database"[1] and "creating a table"[2] in AWS Glue. There has to be a better way to refer to all these entities without using the word schema with so many meanings -- or calling both physical tables and their metadata as "tables". Otherwise this is needlessly confusing and hard for beginners.
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EDIT: even more madness. This StackOverflow discussion [3] has more examples of confusing usage:
> On Oracle .... A Schema is effectively a user. More specifically it's a set of tables/procs/indexes etc owned by a user. Another user has a different schema (tables he/she owns) however user can also see any schemas they have select priviliedges on.
Not AFAIK. In MySQL, the only time “cluster” is used is to refer to NDB Cluster, which is a distributed DB product. Schema means a logical grouping of tables, the same (more or less) as Postgres.
As to schema itself, yes, it’s heavily overloaded, and you just to grok it from context. I can talk about a table’s schema or a DB’s schema, or as you mentioned, JSON schema. Although the latter is helped by simply not using JSON in a relational DB.
You must remember that SQL is an ancient language, and the relational model is even older. There are going to be oddities.