Cardinality

tsinglee發表於2007-08-29
If the number of distinct values of a column is less than 1% of the
number of rows in the table, or if the values in a column are repeated more than 100 times, then the column is a candidate for a bitmap index.
a bitmap index on this column can out-perform a b-tree
index, particularly when this column is often queried in conjunction with other columns.[@more@]

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