Question

I used to think schema were the “upper wrapper” object before the database itself.
I mean DB.schema.<what_ever_object_name_under_schema>.

Well, the catalog “wrapper” is now quite confusing. Why should we need a catalog? For what purpose, precisely should the catalog be used?

Answer 1

From the relational point of view :

The catalog is the place where–among other things–all of the various schemas (external, conceptual, internal) and all of the corresponding mappings (external/conceptual, conceptual/internal) are kept.

In other words, the catalog contains detailed information (sometimes called descriptor information or metadata) regarding the various objects that are of interest to the system itself.

For example, the optimizer uses catalog information about indexes and other physical storage structures, as well as much other information, to help it decide how to implement user requests. Likewise, the security subsystem uses catalog information about users and security constraints to grant or deny such requests in the first place.

An Introduction to Database Systems, 7th ed., C.J. Date, p 69-70.

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From the SQL standard point of view :

Catalogs are named collections of schemas in an SQL-environment. An
SQL-environment contains zero or more catalogs. A catalog contains
one or more schemas, but always contains a schema named
INFORMATION_SCHEMA that contains the views and domains of the
Information Schema.

Database Language SQL, (Proposed revised text of DIS 9075), p 45

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From the SQL point of view :

A catalog is often synonymous with database. In most SQL dbms, if you query the information_schema views, you’ll find that values in the “table_catalog” column map to the name of a database.

If you find your platform using catalog in a broader way than any of these three definitions, it might be referring to something broader than a database–a database cluster, a server, or a server cluster. But I kind of doubt that, since you’d have found that easily in your platform’s documentation.