How do you add a secondary index to a Mnesia table?

mnesia:add_table_index/2 adds an index on a given attribute of an already-existing table, which can be done at runtime, even while the system is live and serving traffic.

mnesia:add_table_index(person, age).

Once added, queries using mnesia:index_read(Table, Value, Attribute) can look up records by that attribute's value directly. Building the index does require scanning the existing table once to populate it, so adding an index on a very large, already-populated table is a heavier one-time operation than adding it to an empty table up front as part of initial schema design.

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