What is sid in oracle

In Oracle, sid is the abbreviation of "System IDentifier", which is the unique identifier of the database. It is an initial ID automatically assigned by the system when establishing the database. It appears in the form of an environment variable and is used for Distinguish the forked process from other instances on the system.

The operating environment of this tutorial: Windows 10 system, Oracle 11g version, Dell G3 computer.

What is sid in oracle

SID is the abbreviation of System IDentifier, and ORACLE_SID is the abbreviation of Oracle System Identifier. In the Oracle system, ORACLE_SID appears in the form of an environment variable. In a specific version Under Oracle software installation (that is, ORACLE_HOME), when the Oracle instance starts, the fork process on the operating system must use this SID to distinguish the instance from other instances. This is the role of the SID.

SID means the unique identifier of the Oracle database is an initial ID automatically assigned by the system when you create a database.

SID is used in some DBA operations and interactions with the operating system. To access the instance name from the perspective of the operating system, you must pass ORACLE_SID (environment variable of the operating system), and it also exists in the registry. The database name is needed when installing the database, creating a new database, creating a database control file, modifying the data structure, backing up and restoring the database.

We know that an Oracle instance is composed of a shared memory area (SGA) and a set of background processes; the background process is the channel for interaction between the database and the operating system. These The name of the process is determined by ORACLE_SID.

Only one parameter file is required to start the instance. The name of this parameter file is determined by ORACLE_SID. For the init file, the default file name is init.ora. For the spfile file, the default file name is spfile.ora. Oracle determines and searches for the parameter file to start the instance based on ORACLE_SID. The default location of the parameter file is $ORACLE_HOME/ dbs ($ORACLE_HOME\database directory on Windows).

spfile was introduced from Oracle 9i and became the default parameter file. When Oracle starts the instance, it searches for the parameter file from the default directory in the following order: spfile.ora 'spfile.ora 'init.ora. If these three files do not exist, the Oracle instance will not start.

From this information, we can know that under the same ORACLE_HOME, Oracle can distinguish the instances based on ORACLE_SID; but if they are in different ORACLE_HOME , Oracle will not block ORACLE_SIDs with the same name. In other words, Oracle can create instances of the same ORACLE_SID under different ORACLE_HOMEs on the same host.

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