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Possible Duplicate:
Index for multiple columns in ActiveRecord
I am using Ruby on Rails v3.2.2 and MySQL. I would like to know what are differences (at the database level) between these add_index method statements:
add_index :database_table_name, [:column_one_name]
add_index :database_table_name, [:column_one_name, :column_two_name]
What it happens when you specify more than one column name (:column_one_name, :column_two_name instead of :column_one_name)?
You create a composite index.