What is the Oracle Data Dictionary, and how can it be queried for metadata?

Oracle Data Dictionary is the core read-only structure of Oracle databases for storing metadata, providing information such as database objects, permissions, users and status. 1. The main views include USER_xxx (current user object), ALL_xxx (current user access object) and DBA_xxx (full library objects require DBA permission). 2. Metadata such as table column information, primary key constraints, table annotations, etc. can be obtained through SQL query. 3. Usage scenarios cover development structure review, debug permission analysis, query performance optimization and automated script generation. Mastering naming rules and common views can efficiently obtain database configuration and structure information.

Oracle Data Dictionary is a core component of Oracle databases. It is essentially a set of read-only tables and views used to store the database's metadata. These metadata include the structure of database objects (such as tables, indexes, views), permission information, user definitions, and the status of the system running time, etc. Simply put, it is the "instruction manual" of the database.

If you want to know which columns a table has, which user owns this view, or which fields a certain index uses, Oracle Data Dictionary is where you need to query.

1. Common Oracle Data Dictionary view types

Oracle provides three main data dictionary views:

• USER_xxx : Displays the object information that the current user has
  For example : USER_TABLES displays all tables under the current user; USER_CONSTRAINTS view the constraints of the current user.

• ALL_xxx : Displays all objects that the current user has permission to access
  For example : ALL_VIEWS can see all views that the current user can access.

• DBA_xxx : contains information about all objects in the entire database, but requires DBA permission to access it.
  For example : DBA_USERS lists all users in the database.

These prefixes allow you to quickly locate the range of objects you care about, avoiding too much irrelevant information.

2. How to use SQL to query data dictionary to obtain metadata

You can query these views using SELECT statements just like querying normal tables. Here are some examples of common uses:

View column information of a table:

 SELECT column_name, data_type, data_length
FROM user_tab_columns
WHERE table_name = 'EMPLOYEES';

This returns each field name, type, and length of the EMPLOYEES table.

View the primary key constraints of a table:

 SELECT cols.column_name
FROM user_constraints cons
JOIN user_cons_columns cols
ON cons.constraint_name = cols.constraint_name
WHERE cons.table_name = 'DEPARTMENTS'
AND cons.constraint_type = 'P';

This statement can find the primary key field of the DEPARTMENTS table.

Check the names and comments of all tables under the current user:

 SELECT table_name, comments
FROM user_tab_comments;

In this way, you can easily obtain information about the database structure without having to manually translate documents or rely on third-party tools.

3. Use scenarios and suggestions for data dictionary

• View structure in the development stage : When writing SQL or designing new features, check whether the existing table structure meets the needs.
• Analysis permissions when debugging problems : When you encounter problems with insufficient permissions, you can check ALL_TAB_PRIVS to see if you have permission to operate on a certain table.
• Optimize query performance : Analyzing index usage through DBA_INDEXES and DBA_CONS_COLUMNS will help optimize execution plans.
• Automated script reference : Some operation and maintenance scripts will dynamically read data dictionaries to generate table creation statements or migration scripts.

Tips: If you are not sure which view a certain information exists, you can start looking at it from USER_OBJECTS , and then find the corresponding specific view based on object_type.

Basically that's it. Mastering several commonly used views and combining them with simple SQL queries can quickly understand the internal structure and configuration information of the database. Although it looks a bit fragmented, once you get familiar with the naming rules and logic, you will find it very practical.

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