# Oracle metadata

Oracle Database provides information about all of the tables, views, columns, and procedures in a database. This information about information is known as metadata. It is stored in two locations: data dictionary tables (accessed via built-in views) and a metadata registry.
Other relational database management systems support an ANSI-standard equivalent called information schema.

## Views for metadata

The total number of these views depends on the Oracle version, but is in a 1000 range.
The main built-in views accessing Oracle RDBMS data dictionary tables are few, and are as follows:
* ALL_OBJECTS – list of all objects in the current database that are accessible to the current user;
* ALL_TABLES – list of all tables in the current database that are accessible to the current user;
* ALL_VIEWS – list of all views in the current database that are accessible to the current user;
* ALL_TAB_COLUMNS – list of all columns in the database that are accessible to the current user;
* ALL_ARGUMENTS – lists the arguments of functions and procedures that are accessible to the current user;
* ALL_ERRORS – lists descriptions of errors on all stored objects (views, procedures, functions, packages, and package bodies) that are accessible to the current user;
* ALL_OBJECT_SIZE – included for backward compatibility with Oracle version 5;
* ALL_PROCEDURES – (from Oracle 9 onwards) lists all functions and procedures (along with associated properties) that are accessible to the current user;
* ALL_SOURCE – describes the text (i.e. PL/SQL) source of the stored objects accessible to the current user;
* ALL_TRIGGERS – list all the triggers accessible to the current user.
In addition there are equivalent views prefixed "USER_" which show only the objects owned by the current user (i.e. a more restricted view of metadata) and prefixed "DBA_" which show all objects in the database (i.e. an unrestricted global view of metadata for the database instance). Naturally the access to "DBA_" metadata views requires specific privileges.

### Example 1: finding tables

Find all Tables that have PATTERN in the table name

### Example 2: finding columns

Find all tables that have at least one column that matches a specific PATTERN in the column name

### Example 3: counting rows of columns

Estimate a total number of rows in all tables containing a column name that matches PATTERN (this is SQL*Plus specific script)
Note that NUM_ROWS records the number of rows which were in a table when (and if) it was last analyzed. This will most likely deviate from the actual number of rows currently in the table.

### Example 4: finding view columns

Find view columns
Warning: This is incomplete with respect to multiple datatypes including char, varchar and timestamp and uses extremely old, deprecated dictionary views, back to oracle 5.

### Use of underscore in table and column names

The underscore is a special SQL pattern match to a single character and should be escaped if you are in fact looking for an underscore character in the LIKE clause of a query.
Just add the following after a LIKE statement:
And then each literal underscore should be a double underscore: __
Example

## Oracle Metadata Registry

The Oracle product Oracle Enterprise Metadata Manager (EMM) is an ISO/IEC 11179 compatible metadata registry. It stores administered metadata in a consistent format that can be used for metadata publishing. In January 2006, EMM was available only through Oracle consulting services.