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Unselective indexes and skewed data distribution


#1

The selectivity of an index is defined as the fraction of rows in a table having the same value for the indexed key. A highly selective index has few rows for each index entry and an unselective index has many rows for each index entry.


This is a companion discussion topic for the original entry at https://blog.toadworld.com/2018/10/05/unselective-indexes-and-skewed-data-distribution