Hive - Extended Properties

In this article, you will learn how to list out the properties of a database or a table in Hive.

DATABASE LEVEL:

DESCRIBE DATABASE db_name;

DESCRIBE SCHEMA db_name;

Database or schema both are the same thing. These words can be used interchangeably.

DESCRIBE DATABASE EXTENDED db_name;

Use the above command to list all the database properties attached to a particular database in Hive.

TABLE LEVEL:

DESC TableName

DESCRIBE TableName

Use the above command to get the schema of the table.

DESC EXTENDED TableName

Use the above command to get detailed information about the table that includes comments, last modified date, etc along with the table's definition.

DESC FORMATTED TableName

Use the above command to get the summary, details, and formatted information about the specified table.

Hive Table Properties

The TBLPROPERTIES clause enables you to use your own metadata key/value pairs to tag the table definition.

There are also several predefined table properties, such as last-modified-user and last-modified-time, which Hive automatically adds and manages. 

To view the properties of a table use the below command in hive prompt.

SHOW TBLPROPERTIES tblname;

This lists all the properties of the table. 

If the table's input format is ORC (refer to the input file formats) then you'll see which compression (snappy or zlib) has opted. You'll see if the transactional property set to true or false. You'll also see the predefined table properties that managed by Hive.

Word Count in HiveQL - Explode and Split Usage

This article aims to explain the usage of the SPLIT function in HiveQL. If you are looking for a similar function in SQL Server, then please click here.

Let's create a staging table to load the data temporarily.
CREATE TABLE tempData (col1 STRING);

Load the data to the table.
LOAD DATA INPATH 'Desktop/DataFile' OVERWRITE INTO TABLE tempData;

To split the data from the above-created temp table 
SELECT word, count(1) AS count FROM
(SELECT explode(split(col1, '\s')) AS word FROM tempData) temp
GROUP BY word
ORDER BY word;

Split function splits the data based on the delimiter provided. The Explode function will further split the data into smaller chunks. Let's see what these explode and split functions are doing with another example.

Below is the patient's blood pressure variations information.

TableName: PatientsData
Systolic-Diastolic
122/80, 122/83, 130/83, 135/86, 140/95, 147/92

SELECT split(data,'\/') as split_data from PatientsData;

Result:
split_data
122,80
122,83
130,83
130,83
135,86
140,95
147,92

SELECT split(data,'\/')[0] AS Systolic, split(data,'\/')[1] AS Diastolic from PatientsData;

Result:
Systolic   Diastolic
122          80
122          83
130          83
130          83
135          86
140          95
147          92

SELECT explode(split(data,'\/')) as exploded_data from PatientsData;

Result:
exploded_data
122
80
122
83
130
83
130
83
135
86
140
95
147
92

Hope you understood the behavior of the function with the examples.

If you are looking for a word-count program using SQL Server, then click here.

If you are looking for a word-count program using Pig, then click here.

File formats and compression in Apache Hive

The purpose of this article is to address the different file formats and compression codecs in Apache Hive that are available for different data sets. We will also explore how to use them properly and when to use them.

HiveQL handles structured data only, much like SQL. In order to store the data in it, Hive has a derby database by default. The data will be stored as files in the backend framework while it shows the data in a structured format when it is retrieved. Some special file formats that Hive can handle are available, such as:

• Text File Format
  
  
• Sequence File Format
  
  
• RCFile (Row column file format)
  
  
• Avro Files
  
  
• ORC Files (Optimized Row Columnar file format)
  
  
• Parquet
  
  
• Custom INPUTFORMAT and OUTPUTFORMAT

1) TEXT FILE FORMAT:
 

Hive Text file format is a default storage format to load data from comma-separated values (CSV), tab-delimited, space-delimited, or text files that delimited by other special characters. You can use the text format to interchange the data with other client applications. The text file format is very common for most of the applications. Data is stored in lines, with each line being a record. Each line is terminated by a newline character (\n). 

The text file format storage option is defined by specifying "STORED AS TEXTFILE" at the end of the table creation.

2) SEQUENCE FILE FORMAT: 

Flat files consisting of binary key-value pairs are sequence files. When converting queries to MapReduce jobs, Hive chooses to use the necessary key-value pairs for a given record. The key advantages of using a sequence file are that it incorporates two or more files into one file.

The sequence file format storage option is defined by specifying  "STORED AS SEQUENCEFILE" at the end of the table creation.

3) RCFILE FORMAT:

The row columnar file format is very much similar to the sequence file format. It is a data placement structure designed for MapReduce-based data warehouse systems. This also stores the data as key-value pairs and offers a high row-level compression rate. This will be used when there is a requirement to perform multiple rows at a time. RCFile format is supported by Hive version 0.6.0 and later.

The RC file format storage option is defined by specifying "STORED AS RCFILE" at the end of the table creation.

4) AVRO FILE FORMAT:

Hive version 0.14.0 and later versions support Avro files. It is a row-based storage format for Hadoop which is widely used as a serialization platform. It's a remote procedure call and data serialization framework that uses JSON for defining data types and protocols and serializes data in a compact binary format to make it compact and efficient. This file format can be used in any of the Hadoop’s tools like Pig and Hive.

Avro is one of the common file formats in applications based on Hadoop. The option to store the data in the RC file format is defined by specifying "STORED AS AVRO" at the end of the table creation.

5) ORC FILE FORMAT:

The Optimized Row Columnar (ORC) file format provides a highly efficient way to store data in the Hive table. This file system was actually designed to overcome limitations of the other Hive file formats. ORC reduces I/O overhead by accessing only the columns that are required for the current query. It requires significantly fewer seek operations because all columns within a single group of row data are stored together on disk.

The ORC file format storage option is defined by specifying "STORED AS ORC" at the end of the table creation.

6) PARQUET:

Parquet is a binary file format that is column driven. It is an open-source available to any project in the Hadoop ecosystem and is designed for data storage in an effective and efficient flat columnar format compared to row-based files such as CSV or TSV files. It only reads the necessary columns, which significantly reduces the IO and thus makes it highly efficient for large-scale query types. The Parquet table uses Snappy, which is a fast data compression and decompression library, as the default compression.

The parquet file format storage option is defined by specifying "STORED AS PARQUET" at the end of the table creation.

7) CUSTOMER INPUTFORMAT & OUTPUTFORMAT:

We can implement our own "inputformat" and "outputformat" incase the data comes in a different format. These "inputformat" and "outputformat" is similar to Hadoop MapReduce's input and output formats. 

In upcoming posts, we will explore more with examples for each and every file format. 

Have a nice day..!!

Commonly used Apache Hive non-SQL Statements

Commands are non-SQL statements such as setting a property or adding a resource. They can be used in HiveQL scripts or directly in the CLI or Beeline. The below commands are mostly used ones and are helpful while working with partitions, adding external jar files, and changing the configuration settings.

Show column names in the result:

SET hive.cli.print.header=true;

Show database name in the Hive prompt:

SET hive.cli.print.current.db=true;

Display only the column names and exclude the table name in the resultset.

SET hive.resultset.use.unique.column.names=false;

Set property while using with Static Partitions:

SET hive.mapred.mode=strict;

Set property while using Dynamic Partitions:

SET hive.exec.dynamic.partition=true;

SET hive.exec.dynamic.partition.mode=nonstrict;

While working with buckets, enable the property by using the following command.

SET hive.enforce.bucketing=true;

Set properties while using bucket-map-join and sorted merge.

SET hive.enforce.sortmergebucketmapjoin=false;

SET hive.auto.convert.sortmerge.join=false;

SET hive.optimize.bucketmapjoin=true;

SET hive.optimize.bucketmapjoin.sortedmerge=true;

Do let me know if you need any clarification on any of the property mentioned above.

HDFS Basic Commands

This article will explore some Hadoop basic commands that help in our day-to-day activities.

Hadoop file system shell commands are organized in a similar way to Unix/Linux environments. For people who work with Unix shell, it is easy to turn to Hadoop shell commands. Such commands communicate with HDFS and other Hadoop-supported file systems.

1) List-out the contents of the directory.

ls

is to list out the files from the current directory (local system)

hadoop fs -ls

will list HDFS home directory (/user/cloudera/) content of the current user

hadoop fs -ls /

will list sub-directories of the root directory.

hdfs dfs -ls

will list the contents of the root directory.

Note: Use hadoop fs for older versions and hdfs dfs for newer versions of Hadoop. 

hadoop fs -ls /user/cloudera

/user/cloudera is default HDFS location in Cloudera VM where users files get copied.

hadoop fs -ls -R / 

recursively displays entries in all subdirectories of a path

2) Create or delete a directory

hadoop fs –mkdir /path/directory_name

mkdir is the command to create a folder/directory in a given path. 

Example:

hadoop fs -mkdir testdir1

hadoop fs –mkdir /user/cloudera/testdir2

hadoop fs -rm -r /user/cloudera/testdir2

-rm -r is the command to delete a folder/directory or a specific file.

Example:

hadoop fs -rm -r /user/cloudera/testdir2

hadoop fs -rmr /user/cloudera/testdir2/file1.txt

Note: If the OS is in safemode then you’ll not be able to create any directories in HDFS.

To check the status of safemode
hadoop dfsadmin -safemode get

To change the safemode to ON
hadoop dfsadmin -safemode enter

To change the safemode to OFF / or to leave the safemode

hadoop dfsadmin -safemode leave

3) Copy The File From Local System To Hadoop

hadoop fs -put <sourcefilepath> <destinationfilepath>

Examples:

hadoop fs -put Desktop/Documents/emp.txt /user/cloudera/empdir

hadoop fs -copyFromLocal Desktop/Documents/emp.txt /user/cloudera/emp.txt

To know more about "copyFromLocal", "put" "copyToLocal" and "get", please click here.  

4) Read the file

hadoop fs -cat /user/cloudera/emp.txt

The above command helps in reading the file however, one has to avoid using this command for large files since it can impact on I/O. This command is good for files with small data.

5) Copy the file from HDFS to Local System

hadoop fs -get /user/cloudera/emp.txt Desktop/Documents/emp1.txt

hadoop fs -copyToLocal /user/cloudera/emp.txt Desktop/Documents/emp2.txt

This is reverse scenario of Put & CopyFromLocal. For more information click here.

6) Move the file from one HDFS location to another (HDFS location)

Hadoop fs -mv emp.txt testDir

Hadoop fs -mv testDir tesDir2

Hadoop fs -mv testDir2/testDir /user/cloudera

Hadoop fs -mv testDir/emp.txt /user/cloudera

7) Admin Commands

sudo vi /etc/hadoop/conf/hdfs-site.xml 

Note: hdfs-site.xml is a configuration file where we can change.

To view the config settings
go to --> computer-browse folder-filesystem-->etc-->hadoop-->conf-->hdfs-site.xml

To change the default configuration values such as dfs.replication or dfs.blocksize from hdfs-site.xml, use the sudo commands

sudo vi /etc/hadoop/conf/hdfs-site.xml

Note: "vi" is the editor to edit such sudo files.

Click "I" for insert option or to bring it in edit mode.

Modify the values as per your requirement.

To save and exit :wq!

hadoop fs -tail [-f] <file>

The Hadoop fs shell tail command shows the last 1KB of a file on console or stdout.

File exists error in HDFS - CopyFromLocal

HDFS is a distributed file system designed to run on top of the local file system. Many times we may need to copy files from different sources i.e. from the internet, remote network, or from the local file system. There are  "CopyFromLocal" and "Put" commands to help us in performing the task. While copying a file from the local file system to HDFS, if the file exists in the destination, the execution will fail and we will receive 'the file exists' error.

Let's assume the file "emp.txt" already exists in the path /user/cloudera.

Hadoop fs -put Desktop/emp.txt /user/cloudera/emp.txt

This returned “the file already exists” error

Hadoop fs -copyFromLocal Desktop/emp.txt /user/cloudera/emp.txt

This also returned “the file already exists” error.

Hadoop fs -copyFromLocal -f Desktop/Documents/emp.txt /user/cloudera/emp.txt
This is succeeded. The file is copied to the destination without any errors.

The usage of the "-f" option with -copyFromLocal will overwrite the destination if it already exists.

Hope you find this article helpful.

Difference between CopyFromLocal, Put, CopyToLocal and Get

The purpose of this article is to let you know about few HDFS commands that are identical in behavior but distinct.

CopyFromLocal and Put: These two commands help in copying the file from one location to another. The difference between these two is that the "CopyFromLocal" command will help copy the file from local file system to HDFS, while the "Put" command will copy from anywhere (local or network) to anywhere (HDFS or local file system).

hadoop fs -put <Local system directory path or network path> <HDFS file path>

hadoop fs -copyFromLocal <Local system directory path>  <HDFS file path>

"Put" allows us to copy several file paths to HDFS at once (files or folders from 

local or remote locations), while copyFromLocal, on the other hand, is limited to a local file reference.

A choice exists to overwrite an existing file using -f when using copyFromLocal. However, an error is returned if the file persists when "put" is executed.

In short, anything you do with copyFromLocal, you can do with "put", but not vice-versa.

CopyToLocal and Get: These two commands are just opposite to "CopyFromLocal" and "Put".
The destination is restricted to a local file reference when we use copyToLocal. While using "Get" there are no such restrictions.

Anything you do with copyToLocal, you can do with "get" but not vice-versa.
hadoop fs -get <HDFS file path> <Local system directory path> hadoop fs -copyToLocal <HDFS file path> <Local system directory path>

For complete HDFS commands please click here. For complete Hive DDL commands please click here.

An alternative to ISNULL() and NVL() functions in Hive

The NVL() function enables you to substitute null for a more relevant alternative in the query results. This function accepts two arguments. If the first argument is null, then it returns the second argument. If the first argument is not null, it returns the first one and will ignore the second argument. This function is available in Oracle SQL*Plus, but not in MySQL, SQL Server, and Hive.

However, as an alternative, ISNULL() and COALESCE() functions can be used to achieve the same result in MySQL and SQL Server. Since ISNULL() is not available in Hive, COALESCE() function is the only option to achieve the desired output.

The difference between NVL() and COALESCE() is that COALESCE() will return the first non-null value from the list of expressions while NVL() only takes two parameters and returns the first if it is not null, otherwise, returns the second.

Let's see what these three functions will do.

Oracle:
SELECT first_name + middle_name + last_name As EmpName
FROM Employees;

Result:

Employees
---------------------------
Robert Finn Hill
Bruce M. Wills
Maria Andrew Brown
NULL

The last row is null because there is no middle name of the employee. NULL is returned when concatenated the null with first-name and last-name. There we use NVL() function. 

SELECT first_name + NVL(middle_name, ' ') + last_name As EmpName
FROM Employees;

Result:

Employees
----------------------------
Robert Finn Hill
Bruce M. Wills
Maria Andrew Brown
Ashley Miller

SQL Server:

SELECT first_name + ISNULL(middle_name,'') + last_name As EmpName
FROM Employees;

SELECT first_name + COALESCE(middle_name,'') + last_name As EmpName
FROM Employees;

Hive:

SELECT first_name + COALESCE(middle_name,'') + last_name As EmpName
FROM Employees;

Hope you find this article helpful.

Hive Internal vs External Tables

This article offers a summary of the situations in which 

you would need to create internal (managed) tables and external tables in Apache Hive.

Create "External" tables when:

• the data is being used outside the Hive. The data files are read and interpreted by an existing program that does not lock the files, for instance.
  
  
• data needs to stay in the underlying position even after a DROP TABLE. In other words, the data file always stays on the HDFS server even if you delete an external table. This also means that Metadata is maintained on the master node, and deleting an external table from HIVE only deletes the metadata not the data/file.
  
  
• you choose a custom place to be used, use external tables.
  
  
• Hive doesn't own the data.
  
  
• you are not creating a table based on an existing table (AS SELECT), use external tables.
  
  
• you are okay with the fact that External table files are accessible to anyone who has access to HDFS. Security needs to be handled at the HDFS folder level.

Create "Internal" tables when:

• the data is temporary.
  
  
• you want Hive to completely manage the lifecycle of the table and data.
  
  
• you want the data and metadata to be stored inside Hive's warehouse.
  
  
• You are okay with the fact that table deletion would also erase the master-node and HDFS metadata and actual data, respectively.
  
  
• you want the security of the data to be controlled solely via HIVE. 

Conclusion:

In "Internal" tables, the table is created first and data is loaded later.

In "External" tables, the data is already present in HDFS and the table is created on top of it.