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Oracle schema definition Tips

Oracle Database Tips by Donald BurlesonNovember 9, 2015

Question:  I want to understand the Oracle schema definition and how it relates to Oracle database.  What is an Oracle schema definition?

Answer:  A database many have many schemas, and each user ID is sometimes called a "schema owner" because a schema must be created under the auspices of a dingle user ID.  In general, a schema has many tables and indexes and serves as a working piece of Oracle to store and retrieve data.

The Oracle schema definition is stored as "metadata" within the Oracle data dictionary, which is a schema that collects the schema data definition language (DDL) for the schema  Once defined, a schema definition, can be extracted, (also known as "punching" a schema) into a flat text file. 

It's fairly easy to remember the relationships between Oracle schema definition components once you can visualize the architecture. Here are some simple concepts:

  • A "database" component can be thought-of as a set of xxx.dbf files

  • An "instance" component is a running set of programs that read the .dbf files (Oracle RAC allows many instances to connect to the database).

  • An "object" components is a table, index, cluster table, &c

  • The "user" component creates objects as the "schema owner" of those objects.

  • The "synonym" component allows you to "alias" an object to make the schema owner hidden:

    create synonym food for;

  • With "public synonym" components, the table owner name ("the user ID") is kept inside the synonym definition and appended to the table name at runtime to remove duplicates:

    create public synonym food for;

  • Here are details on Oracle File & Tablespace Management

Within a database we have schemas, each owned by a specific user_id. Hence, we can think of a USER and a OWNER as the same in the sense that a schema owner is a single, distinct user ID:

schema_owner = user_id

Also, we can refer to tables with their "fully-qualified" names, where we include the schema owner. Here we see two tables named "food", one created by the user "cat" and the other schema owner "dog":

select stuff from;
select stuff from;

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