Alter is the command employed to make changes to an existing table. Using this command, you can change the maximum number of cells of a column family, set and delete table scope operators, and delete a column family from a table.

Changing the Maximum Number of Cells of a Column Family

Given below is the syntax to change the maximum number of cells of a column family.

hbase> alter ‘t1’, NAME ⇒ ‘f1′, VERSIONS ⇒ 5

In the subsequent example, the maximum number of cells is set to 5.

hbase(main):003:0> alter ’emp’, NAME ⇒ ‘personal data’, VERSIONS ⇒ 5

Updating all regions with the new schema…

0/1 regions updated.

1/1 regions updated.

Done.

0 row(s) in 2.3050 seconds

Table Scope Operators

Using alter, you can set and remove table scope operators such as MAX_FILESIZE, READONLY, MEMSTORE_FLUSHSIZE, DEFERRED_LOG_FLUSH, etc.

Setting Read Only

Below given is the syntax to make a table read only.

hbase>alter ‘t1’, READONLY(option)

In the subsequent example, we have made the emp table read only.

hbase(main):006:0> alter ’emp’, READONLY

Updating all regions with the new schema…

0/1 regions updated.

1/1 regions updated.

Done.

0 row(s) in 2.2140 seconds

Removing Table Scope Operators

We can also remove the table scope operators. Given below is the syntax to remove ‘MAX_FILESIZE’ from emp table.

hbase> alter ‘t1’, METHOD ⇒ ‘table_att_unset’, NAME ⇒ ‘MAX_FILESIZE’

Deleting a Column Family

Using alter, you can also delete a column family. Given below is the syntax to delete a column family using alter.

hbase> alter ‘ table name ’, ‘delete’ ⇒ ‘ column family ’

Given below is an example to delete a column family from the ‘emp’ table.

Assume there is a table named employee in HBase. It contains the subsequent data:

hbase(main):006:0> scan ’employee’

ROW COLUMN+CELL

row1 column = personal:city, timestamp = 1418193767, value = hyderabad

row1 column = personal:name, timestamp = 1418193806767, value = raju

row1 column = professional:designation, timestamp = 1418193767, value = manager

row1 column = professional:salary, timestamp = 1418193806767, value = 50000

1 row(s) in 0.0160 seconds

Now let us delete the column family named professional using the alter command.

hbase(main):007:0> alter ’employee’,’delete’⇒’professional’

Updating all regions with the new schema…

0/1 regions updated.

1/1 regions updated.

Done.

0 row(s) in 2.2380 seconds

Now verify the data in the table after alteration. Observe the column family ‘professional’ is no more, since we have deleted it.

hbase(main):003:0> scan ’employee’

ROW COLUMN + CELL

row1 column = personal:city, timestamp = 14181936767, value = hyderabad

row1 column = personal:name, timestamp = 1418193806767, value = raju

1 row(s) in 0.0830 seconds

Adding a Column Family Using Java API

You can add a column family to a table using the method addColumn() of HBAseAdmin class. Follow the steps given below to add a column family to a table.

Step 1

Instantiate the HBaseAdmin class.

// Instantiating configuration object

Configuration conf = HBaseConfiguration.create();

// Instantiating HBaseAdmin class

HBaseAdmin admin = new HBaseAdmin(conf);

Step 2

The addColumn() method requires a table name and an object of HColumnDescriptor class. Therefore instantiate the HColumnDescriptor class. The constructor of HColumnDescriptor in turn requires a column family name that is to be added. Here we are adding a column family named “contactDetails” to the existing “employee” table.

// Instantiating columnDescriptor object

HColumnDescriptor columnDescriptor = new

HColumnDescriptor(“contactDetails”);

Step 3

Add the column family using addColumn method. Pass the table name and the HColumnDescriptor class object as parameters to this method.

// Adding column family

admin.addColumn(“employee”, new HColumnDescriptor(“columnDescriptor”));

Given below is the complete program to add a column family to an existing table.

import java.io.IOException;

import org.apache.hadoop.conf.Configuration;

import org.apache.hadoop.hbase.HBaseConfiguration;

import org.apache.hadoop.hbase.HColumnDescriptor;

import org.apache.hadoop.hbase.MasterNotRunningException;

import org.apache.hadoop.hbase.client.HBaseAdmin;

public class AddColoumn{

public static void main(String args[]) throws MasterNotRunningException, IOException{

// Instantiating configuration class.

Configuration conf = HBaseConfiguration.create();

// Instantiating HBaseAdmin class.

HBaseAdmin admin = new HBaseAdmin(conf);

// Instantiating columnDescriptor class

HColumnDescriptor columnDescriptor = new HColumnDescriptor(“contactDetails”);

// Adding column family

admin.addColumn(“employee”, columnDescriptor);

System.out.println(“coloumn added”);

}

Compile and execute the above program as shown below.

$javac AddColumn.java

$java AddColumn

The above compilation works only if you have set the classpath in “ .bashrc ”. If you haven’t, follow the procedure given below to compile your .java file.

//if “/home/home/hadoop/hbase ” is your Hbase home folder then.

$javac -cp /home/hadoop/hbase/lib/*: Demo.java

If everything goes well, it will produce the subsequent output:

column added

Deleting a Column Family Using Java API

You can delete a column family from a table using the method deleteColumn() of HBAseAdmin class. Follow the steps given below to add a column family to a table.

Step1

Instantiate the HBaseAdmin class.

// Instantiating configuration object

Configuration conf = HBaseConfiguration.create();

// Instantiating HBaseAdmin class

HBaseAdmin admin = new HBaseAdmin(conf);

Step2

Add the column family using deleteColumn() method. Pass the table name and the column family name as parameters to this method.

// Deleting column family

admin.deleteColumn(“employee”, “contactDetails”);

Given below is the complete program to delete a column family from an existing table.

import java.io.IOException;

import org.apache.hadoop.conf.Configuration;

import org.apache.hadoop.hbase.HBaseConfiguration;

import org.apache.hadoop.hbase.MasterNotRunningException;

import org.apache.hadoop.hbase.client.HBaseAdmin;

public class DeleteColoumn{

public static void main(String args[]) throws MasterNotRunningException, IOException{

// Instantiating configuration class.

Configuration conf = HBaseConfiguration.create();

// Instantiating HBaseAdmin class.

HBaseAdmin admin = new HBaseAdmin(conf);

// Deleting a column family

admin.deleteColumn(“employee”,”contactDetails”);

System.out.println(“coloumn deleted”);

}

Compile and execute the above program as shown below.

$javac DeleteColumn.java

$java DeleteColumn

The subsequent should be the output:

column deleted

So, this brings us to the end of blog. This Tecklearn ‘How to make changes to an existing Table and describe it in HBase’ helps you with commonly asked questions if you are looking out for a job in HBase and No-SQL Database Domain.

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