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Install Apache Hadoop – Single Node REHL 7

Hadoop is a Java-based programming framework that supports the processing and storage of extremely large datasets on a cluster of inexpensive machines. It was the first major open source project in the big data playing field and provides high throughput access to application data .

The main goal of this tutorial is to get a simple Hadoop installation up and running so that you can play around with the software and learn more about it.

Environment: This  blog has been tested in the following software version.

  • REHL ( Red hat Linux 7.4) on Virtual box 5.2
  • Hadoop 2.7.3 version
  • update /etc/hosts file with Hostname and IP address.

[root@cdhs ~]# cat /etc/hosts cdhs

Dedicated Hadoop system user:

After VM set up, please add a non sudo user dedicated to Hadoop which will be used to configure Hadoop. Following command will add the user hduser and the group hadoop to VM machine. Continue reading → Install Apache Hadoop – Single Node REHL 7


DBMS_LOCK.SLEEP is depreciated in Oracle 18c and DBMS_SESSION.SLEEP is introduced, and is available with no additional grants needed.

SLEEP procedure suspends the session for the specified number of seconds.

DBMS_LOCK procedure was  not accessible by default and user must be granted execute permission on the dbms_lock package. Continue reading → Oracle 18c – DBMS_SESSION.SLEEP

Oracle 12c – Delete Pluggable Database with DBCA

We can delete Pluggable database using Database Configuration Assistant (DBCA) OR manually from SQL prompt as sysdba privilege.

Here I have covered DBCA to delete Pluggable database in silent and GUI mode.

Continue reading → Oracle 12c – Delete Pluggable Database with DBCA

Oracle Database 18c – Schema only Accounts

From 18c onwards,  now you can create schema without a password. These are called “Schema Only Accounts”.  These account don’t allow direct connections but can proxy in a single session proxy. This new feature can  allow administrators to further secure their databases by not allowing direct connections to application schemas for any reason. There are few points to note:

  • Schema only accounts can be used for both administrator and non-administrator accounts.
  • You can grant system privileges ( create ant table)  and admin roles (like DBA) to schema only accounts.  But note that administrative privileges like sysdba/sysoper/sysasm   can’t be granted to schema only accounts.
  • Schema only accounts can’t connect through db links.
  • Schema only accounts can be created for database instance only. Same is not valid for ASM environment.

Continue reading → Oracle Database 18c – Schema only Accounts

Oracle 12R2 – Local Undo Switch

From 12.2 onwards,  each container in an instance can use its own undo table space and this is called local undo mode. In previous release, all containers in an instance shared the same undo table space in multitenant set up.

Today I upgraded, container database from 12.1 -> 12.2 via DBUA and looked at the UNDO setting. Found that we have to manually switch the undo modes for upgraded instances. So here are the steps. Continue reading → Oracle 12R2 – Local Undo Switch

Oracle 18c – Scalable Sequences

From Oracle 18c, Sequences can be made scalable by using SCALE clause in CREATE/ ALTER SEQUENCE statement. Scalable sequences optimize the sequence generation by using a unique combination of instance number and session number to reduce the impact of Index leaf block contention during massive loads. This is  one of the few features that is not automatically enabled as it requires some intervention by DBA’s to ensure this does not change their implemented business logic.

Here is the syntax for defining a scalable sequence.

Create /Alter  sequence sequence_name …… scale [extend | noextend] | noscale

You can check DBA_SEQUENCES/USER_SEQUENCE/ALL_SEQUENCE dictionaries to know whether sequence is scalable or not. By default sequences are not scalable as you can see in dictionary both columns scale_flag/extend_flag is set to N.


Now create the sequence with scale clause. When the scale clause is specified, a 6 digit numeric scalable number is prefixed to the digit of the sequence. Out of 6 digits, three digits  are instance numbers followed by a three digit session number. These are generated by: Instance number is [(instance id % 100) + 100] Session number is [session id % 1000]

Continue reading → Oracle 18c – Scalable Sequences