Oracle DataGuard switchiver vs. failover

Switchover is a planned role reversal between primary and standby databases in DataGuard configuration. it guarantees no data loss and is typically done for maintenance. There is no need to re-enable either database when the transition occurs.

Failover is not planned and is typically performed in event of primary database failure event, when there is no possibility of recovering primary database in a timely fashion. When failure occurs, standby database is taking over the primary role. Failover may result in data loss, depending on the protection mode enabled at the time failover occurs.

If a DataGuard broker is enabled, we can automate the failover, however if DataGuard broker is not enabled, we will have to perform manual failover.

If a DataGuard broker cannot detect the heartbeat of the primary DB, it will automatically perform the failover.

Some useful terminology

Term	Definition
Patch	Publicly released update to a known bug/issue
Hotfix	An update, not always publicly released, used to fix a very specific issue
Maintenance Release	An incremental update between software versions to fix multiple outstanding issues
Service Pack	A large update that fixes many outstanding issues, normally includes all Patches, Hotfixes, Maintenance releases that precede the service pack

What is the difference between restoring and recovering?

Restoring the database involves copying backup files from the backup location to disk, sometimes it involves replacing damaged files etc., while recovery is the process of applying redo logs to the database in order to roll it forward. Database can be rolled forward toa specific point-in-time, or until the last transaction in the log files.

During restoration, RMAN decides which backup sets, datafiles, and archive logs are necessary for the database restoration and verifies their contents. No files are actually restored during this process.

Restoration of a datafile or a control file from backup involves retrieving the file onto disk from a backup location and making it available to the database server. While recovering a datafile involved takinhg a restored copy of the datafile and applying changes recorded in the redo logs.

Undo data vs. Redo log

The main difference is that UNDO data can make the change go away, and REDO can make the change happen again if necessary. UNDO holds a copy of previous data untill the transaction commits and for some time afterwards (depending on undo retention period specified), thus providing read consistence, since the user can read the data as it is at the time a transaction started, while transaction is still in the process. Capturing the undo data enables Oracle to roll back the uncommitted data. Undo data is used for both read consistency and to recover from failed transactions.

UNDO segments store BEFORE images of a record, while REDO logs record redo entries.

For example the user adds a record to the database - an employee named John Doe with ID of 3. Currently database contains two records - with IDs of 1 and 2. So UNDO data captures those two records in case we need to rollback the insertion of record with ID 3.

Meanwhile REDO log records all the changes to the database as they happen. If the database crashes, Oracle will first read redo logs and apply all committed changes 9the ones that didn't end up in data files yet, due to the crash) before opening the database for regular use.

Here is a simple differences between the two:

Undo	Redo
Makes a change go away	Reproduces a change
Used for rollback and read consistency	Used for rolling forward the changes
Protects the database from inconsistent reads	Protects from data loss
Data stored in Undo segments in Undo tablespace	Logs stored in SGA, in memory

Oracle vs. SQL Server - Oracle equivalents for SQL Server DBAs

SQL Server	Oracle
SELECT GETDATE()	select sysdate from dual
SUBSTRING function does not take argument for starting position with negative value	SUBSTR can have negative starting position in Oracle
SELECT INTO Table1 FROM Table2	CREATE Table1 AS SELECT FROM Table2
master database	data dictionary
syslogins view	dba_users view
sys.objects view	dba_objects view
sys.tables	dba_tables view
tempdb	Temporary tablespace
IDENTITY field automatically incremented by 1 on INSERT	Creating a sequence using: CREATE SEQUENCE name_seq START WITH 1000 INCREMENT BY 1; and a trigger to activate the sequence on BEFORE INSERT: CREATE OR REPLACE TRIGGER name_bir BEFORE INSERT on TableName FOR EACH ROW WHEN (new.id IS NULL) BEGIN SELECT name_seq.NEXTVAL INTO :new.id FROM dual; END;
ISNULL() ISNULL(Field1, 0) returns 0 if Field1 is null	NVL() NVL(Field1, 0) returns 0 if Field1 is null

ALL_USERS vs. DBA_USERS vs. USER_USERS

There are 3 different sets of views ALL_, DBA_, and USER_.

ALL_ views display all the information accessible to the current user, i.e. it can looks at all the shcemas the user has permissions to, DBA_ views display infor for the entire database and is intended only for admins. Then USER_ views display info from the schema of the current user.

Tablespaces

One of the most basic concepts in Oracle is a tablespace. Tablespace is sort of a logical container used to group physical objects or datafiles. Each database contains at least one tablespace.

Tablespace is a logical entity because it is not visible in the filesystem. It consists of at least one datafile, which is physically located on the filesystem.. Oracle database contains at least SYSTEM tablespace, that contains most of the information about the structure and contents of the database.

Each table or index that is stored in the database, belongs to a tablespace. The tablespace acts like a bridge between Oracle database and filesystem.

Usually Oracle database also contains TEMP tablespace, that is used for sorting data on disk in case you run out of memory assigned for sorting data when you query the database.
Also usually there is USERS tablespace for storing user objects.

HTML5: Difference between article and section tags

An article is an independent, stand-alone piece of discrete content, sort of like a blogpost or a news item, a self-contained composition. It should be able to stand on its own as if it was independent of its surroundings and that can be independently distributable or reusable.

Section, on the other hand, is either a way of sectioning a page into different areas, or sectioning an article into sections. basically it is a thematic grouping of content, usually with a heading.

Section can be used to group related articles together. It helps to think of a web page as a newspaper that has different sections: sports, real estate, etc. with a number of articles in each section.

Each section should have a heading to identify that secion's content. If you don't use a heading in your section element, you most likely should not use the section element. In that case a div might be more appropriate.

Active Directory container vs. organizational unit

Active directory container is a built-in container that comes with AD and they cannot be altered to suit your organizational needs, without altering the AD schema. (Examples: Computers and Users containers).

An OU or Organizational Unit is a special purpose container created by the user and to which administrators can apply group policy. Group policy cannot be applied to a container. An organizational unit is a subdivision within an Active Directory into which you can place users, groups, computers, and other organizational units. You can create organizational units to suit your organization's functional or business structure. Each domain can implement its own organizational unit hierarchy.