What is the primary key in a Mnesia table?

The primary key of a Mnesia table is always its first declared attribute — there's no separate syntax to mark a different field as the key the way some databases let you designate an arbitrary column. Every lookup by mnesia:read/1 or mnesia:dirty_read/1 is keyed on this first field.

-record(person, {id, name, age}).   %% id is the primary key

mnesia:read({person, 1}).            %% looks up by id = 1

On a set or ordered_set table, this key must be unique — writing a new record with an existing key's value overwrites the old one. On a bag table, the same key value can appear across multiple distinct records, since uniqueness isn't enforced for that table type.

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