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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.
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