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Difference between ‘select * from table’ and ‘select column from table’ in hive

  • Table in Hive is stored as a directory in the HDFS.
  • Using select from table the Hive query processor simply goes directory that have one or more files in table schema.
  • You may do this if you have very small data like less than a Gigabyte.
  • In real clusters if you hit ‘select * from table’, it may have data in Terabytes and displaying that will run for long time.
  • Hive achieved sequence of map reduce programs that reads data from table stored on Hadoop Distributed File System.
  • Any data processing you do in Hive is achieved through sequence of map reduce programs that reads data from table stored on HDFS.
  • Hive map reduce based on query processing engine.
  • Tables have wide number of columns that representing different values.To perform select column the map reduce program will scan all rows and extract a column.
select-from-table-and-select-column-from-table-in-hive
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DDL statement in hive

webhcat server

DDL (Data Definition Language):

  • The outline of the database schema typically change by SQL statements they are creating, deleting, or modifying schema objects such as databases, tables, and views.
  • Most statements starts with the keywords CREATE, DROP, or ALTER.

Some of Data Definition Language statements are:

  • CREATE – To create an object.
  • ALTER – To alter the structure of the existing object·
  • DROP – It used to delete objects.
  • TRUNCATE – Remove all records from a table, including spaces.
  • MSCK REPAIR TABLE – Refresh metadata information.

The DML statement define as INSERT statement, it involves metadata that must be have nodes, and is also damage the SYNC_DDL query option.

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Data Hive

  • Data Hive is a data warehouse software project built on top of Apache Hadoop for providing query, and analysis.
  • Hive gives SQL like interface to query data stored in different databases and file systems that integrate with Hadoop.
Apache hive data model
Data Processing Task
  • Download the data
  • Upload the data
  • Start the hive view
    • Explore the hive user interface(UI)
    • Create table temp_drivers
    • Create query to populate hive table temp_drivers with drivers.csv data
    • Create table drivers
    • To create query for extract data from temp_drivers and store it to drivers.
    • Create temp_timesheet and timesheet tables.
    • For filter the data (driverid, hours_logged, miles_logged).
    • For join the data (driverid, name, hours_logged, miles_logged).
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Hive

  • Hive is a component of Hortonworks Data Platform(HDP).
  • Hive provides a SQL interface to data stored in HDP.
  • Hive has 3 main functions:
    • Data Summarization
    • Query
    • Analysis.
  • It supports queries expressed language called HiveQL, which automatically translates SQL like queries into MapReduce jobs executed on Hadoop.
  • It also enables data serialization and increases flexibility in schema architecture including a system catalog called Hive Metastore.

Architecture of hive :

what is hive

Features of hive:

  • Different storage such as plain text, RCFile, ORC, HBase, and others.
  • Pre-defined functions (UDFs) to manipulate dates, strings, and other data-mining tools.
  • Hive supports extending the UDF set to handle use-cases not supported by built-in functions.

Limitations of Hive:

  • Hive supports overwriting or hold data, but not updates and deletes.
  • In Hive, sub queries are not supported
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