Re: Comparison of PGSQL and DB2

Josh Berkus wrote:
> ?? I'm pretty sure that the SQL spec puts database at the top of the
> heirarchy tree, and treats them as inviolate.

Here are some relevant sections from SQL92.  (SQL99 rearranged this and
I can't quite see through it, but certainly they didn't add any restrictions.)

         4.12  Catalogs

         Catalogs are named collections of schemas in an SQL-environment. An
         SQL-environment contains zero or more catalogs. A catalog con-
         tains one or more schemas, but always contains a schema named
         INFORMATION_SCHEMA that contains the views and domains of the
         Information Schema. The method of creation and destruction of
         catalogs is implementation-defined. The set of catalogs that
         can be referenced in any SQL-statement, during any particular
         SQL-transaction, or during the course of an SQL-session is also
         implementation-defined. The default catalog for a <module> whose
         <module authorization clause> does not specify an explicit <cata-
         log name> to qualify the <schema name> is implementation-defined.
         The default catalog for <preparable statement>s that are dynami-
         cally prepared in the current SQL-session through the execution
         of <prepare statement>s and <execute immediate statement>s is ini-
         tially implementation-defined but may be changed by the use of <set
         catalog statement>s.


         4.13  Clusters of catalogs

         A cluster is an implementation-defined collection of catalogs.
         Exactly one cluster is associated with an SQL-session and it
         defines the totality of the SQL-data that is available to that
         SQL-session.

         An instance of a cluster is described by an instance of a defi-
         nition schema. Given some SQL-data object, such as a view, a con-
         straint, a domain, or a base table, the definition of that object,
         and of all the objects that it directly or indirectly references,
         are in the same cluster of catalogs. For example, no <referential
         constraint definition> and no <joined table> can "cross" a cluster
         boundary.

         Whether or not any catalog can occur simultaneously in more than
         one cluster is implementation-defined.

         Within a cluster, no two catalogs have the same name.