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Monday, December 11, 2017

Bitmap index

Bitmapped indexes:

Oracle  bitmap index is generally used for columns of low Cardinality (Columns with lots of duplicate values) . Bitmap index structures is made of a two-dimensional array.

  • This two-dimensional array represents each value within the index multiplied by the number of rows in the table. 
  • During query Oracle decompresses the bitmap into the RAM data buffers so it can be rapidly scanned for matching values. 
  • These matching values are delivered to Oracle in the form of a Row-ID list, and these Row-ID values may directly access the required information.
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