Oracle Database Startup Stages

This post explains the stages the oracle database goes with when we issue a STARTUP command to start an Oracle database instance:

1) The first thing is we have to export the ORACLE_SID
For e.g export ORACLE_SID=DB1

2) We need to logon as SYSDBA or SYSOPER priviledges:
    For e.g conn / as SYSDBA;
   
3) When we issue the STARTUP command Oracle database goes in three phases as follows:

a) Nomount Stage
b) Mount Stage
c) Open

a) Nomount Stage - In this stage oracle looks for the initialization parameter file (init.ora) and checks how the database is configured and also it checks the memory areas allocation. After the initialization file is accessed oracle the memory areas associated are allocated to the oracle instance.

b) Mount Stage - In this stage oracle open and reads the control file for the database related information like the location of the datafiles, the database name, etc. In this stage still the database is not opened.

c) Open - In this stage the database is opened and it accesses all the datafiles associated with the database. Once it is able to access all the database datafiles, it makes sure that all of the database datafiles are consistent then one can access the Oracle database by SQL*PLUS further ahead.