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begin29/indexes.rb

Created December 23, 2016 16:57
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indexes, when should add index
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indexes.rb
# show indexes
show INDEXES from table_name
when I should add index?
#when selectivity is big
selectivity = count of variations/all rows * 100%
#selectivity for sex column is low: sel_of_sex = 2(male, female)/10000 *100 = 0.02%
#http://www.programmerinterview.com/index.php/database-sql/selectivity-in-sql-databases/
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