At 10:36 9/11/00 -0500, Tom Lane wrote:
>Philip Warner <pjw@rhyme.com.au> writes:
>> Presumably this was raised before, but I'd love to see the consensus view,
>> if it is documented.
>
>AFAIR, the discussion trailed off without any specific decisions being
>made. One of the things that's still very open in my mind is whether
>we want to keep the existing notion of independent databases within an
>installation, and if so how that maps onto the SQL-defined concepts.
>To me, though, the point of independent databases is that they be
>*independent*,
I agree; it's a pain that one DB misbehaving kills an entire installation.
>and therefore if we keep them I'd vote for mapping them
>to the top-level SQL notion (catalog, you said?). Schemas ought to be
>substructure within a database.
I think the hierarchy goes:
Environment->Catalog->Schema
From what I can tell:
1. the environment contains truly general things like the SQL parser, the
tools for connecting to the DB etc - which I assume also contains the
user-authorization stuff.
2. The catalog contains multiple schemas (this is the top level as far as
data definitions go, I think). Some predefined schemas (eg. the
DEFINITION_SCHEMA) contain views that allow querying of all schema
definitions in the catalog.
3. Schemas are what we call databases. They contain tables, views wtc.
The SQL standard is careful to avoid using the term database in these
discussions, though at one point it does equate 'database' with the part of
the environment that contains the actual SQL data (as opposed to metadata).
It's a pretty broad definition, and contrary to most peoples expections, I
think. Perhaps I'm wrong, but I think most people will equate database with
a schema (ie. the thing in which you define tables).
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