SQL Server: System Databases
In this article i will explain what are System Databases. And what is the role of each System Database.
In previous articles i explained SQL server Date formats, DDL, DML, DCL and TCL commands, SQL Server Split function, and other SQL Server articles. In this article i will explain what are System Databases. And what is the role of each System Database.
SQL Server includes following system databases.
- master Database
- msdb Database
- model Database
- Resource Database
- tempdb Database
The master database records all the system-level information for a SQL Server system. This includes instance-wide metadata such as logon accounts, endpoints, linked servers, and system configuration settings. Also, master is the database that records the existence of all other databases and the location of those database files and records the initialization information for SQL Server. Therefore, SQL Server cannot start if the master database is unavailable. In SQL Server, system objects are no longer stored in the master database; instead, they are stored in the Resource database.
The msdb database is used by SQL Server Agent for scheduling alerts and jobs and by other features such as Service Broker and Database Mail.
The model database is used as the template for all databases created on an instance of SQL Server. Because tempdb is created every time SQL Server is started, the model database must always exist on a SQL Server system.
When a CREATE DATABASE statement is issued, the first part of the database is created by copying in the contents of the model database. The rest of the new database is then filled with empty pages.
If you modify the model database, all databases created afterward will inherit those changes. For example, you could set permissions or database options, or add objects such as tables, functions, or stored procedures.
The Resource database is a read-only database that contains all the system objects that are included with SQL Server. SQL Server system objects, such as sys.objects, are physically persisted in the Resource database, but they logically appear in the sys schema of every database. The Resource database does not contain user data or user metadata.
The Resource database makes upgrading to a new version of SQL Server an easier and faster procedure. In earlier versions of SQL Server, upgrading required dropping and creating system objects. Because the Resource database file contains all system objects, an upgrade is now accomplished simply by copying the single Resource database file to the local server.
The tempdb system database is a global resource that is available to all users connected to the instance of SQL Server and is used to hold the following:
- Temporary user objects that are explicitly created, such as: global or local temporary tables, temporary stored procedures, table variables, or cursors.
- Internal objects that are created by the SQL Server Database Engine, for example, work tables to store intermediate results for spools or sorting.
- Row versions that are generated by data modification transactions in a database that uses read-committed using row versioning isolation or snapshot isolation transactions.
- Row versions that are generated by data modification transactions for features, such as: online index operations, Multiple Active Result Sets (MARS), and AFTER triggers.
Operations within tempdb are minimally logged. This enables transactions to be rolled back. tempdb is re-created every time SQL Server is started so that the system always starts with a clean copy of the database. Temporary tables and stored procedures are dropped automatically on disconnect, and no connections are active when the system is shut down. Therefore, there is never anything in tempdb to be saved from one session of SQL Server to another. Backup and restore operations are not allowed on tempdb.
CreatedSep 04, 2013
UpdatedSep 04, 2013