A schema is a collection of database objects. A schema is owned by a database user and has the same name as that user. Schema objects are the logical structures that directly refer to the database’s data. Objects in the same schema can be in different tablespaces, and a tablespace can hold objects from different schemas.
Tables:
basic unit of data storage in an Oracle database. Database tables hold all user-accessible data. each table has columns and rows.
Indexes
Indexes are optional structures associated with tables. An Oracle index provides an access path to table data. Indexes are useful when applications frequently query a table for a range of rows or a specific row.
Views
Views are customized presentations of data in one or more tables or other views. Views derive their data from the tables on which they are based, referred to as the base tables of the views. Like tables, views can be queried, updated, inserted into and deleted from, with some restrictions. All operations performed on a view actually affect the base tables of the view.
Clusters
clusters are groups of one or more tables physically stored together because they share common columns and are often used together. Because related rows are physically stored together, disk access time improves.
Synonyms
A synonym is an alias for any table, view, materialized view, sequence, procedure, function, package, type, Java class schema object, user-defined object type, or another synonym. A synonym requires no storage other than its definition in the data dictionary.
Oracle data dictionary
Each Oracle database has a data dictionary. An Oracle data dictionary is a set of tables and views that are used as a read-only reference about the database. A data dictionary stores information about both the logical and physical structure of the database. A data dictionary also stores the following information:
1. The valid users of an oracle database
2. Information about integrity constraints defined for tables in the database
3. The amount of space allocated for a schema object and how much of it is in use.
During database operation, Oracle reads the data dictionary to verify that schema objects exist and that users have proper access to them.