To expand a bit on Dan's explanation, you're right - the db-specific (Oracle, in this example) query in a .xql file overrides the query in the generic .xql file if it is present.
So in this case PG and any pther SQL92 compliant RDBMS will use the generic query present in the .xql file, while this SQL92 form will be overridden by the version in the *-oracle.xql file if you're running the Oracle version of OpenACS.
The only time we need queries to appear in both *-postgresql.xql and *-oracle.xql query files is when there's no SQL92 equivalent to the query or where neither RDBMS supports the SQL92 standard equivalent.
